DEC-Structure and Bonding Flashcards
What are cations?
ions with more protons than electrons. This means that cations are positively charged, as the combined positive charge of protons is greater than the combined negative charge of electrons. As sodium ions have 11 protons and 10 electrons, the overall charge is +1
What are anions?
negatively charged ions. They contain a greater number of electrons than protons. (Anions adopt a slightly different name: the first part corresponds to the name of their parent atom. This is followed by the suffix- ide.
How do the electron configurations of ionic transition metals vary?
A transition element is an element with partially filled d sublevel. In contrast to main group elements, a transition element can form multiple ions with different charges. e.g. iron commonly forms Fe2+ and Fe3+ ions
What is an ionic bond?
Cations and Anions are electrostatically attracted to each other because of their opposite charges. This attraction results in the formation of ionic bonds. Therefore, if a given element forms cations. This attraction results in the formation of ionic bonds. Therefore, if a given element forms cations, and the other forms anions, they can bond ionically and form an ionic compound.
How do you determine the ionic character of a compound, using the elements electronegativity?
The larger the difference in electronegativity between the two elements in a compound, the greater the ionic character of the bond between them. Ionic bonding is assumed to occur when the difference in the electronegativity is greater than 1.8. In reality, bonding occurs across a continuum, so above 1.8 the main type of bonding in the compound is ionic, but there may be other types of bonding present.
Why might some metal+non-metal bonds possess covalent character?
It is often incorrectly said that only ionic bonds form when a metallic and non metallic element bond together. There are substances, such as aluminium chloride, AlCl3, that do not fit this description. Aluminium is a metal and chlorine is a non metal, so you would expect them to bond ionically. But the compound has properties that are characteristic of covalent compounds, such as low melting point and high volatility. The electronegativity difference between these two elements (1.6) suggests they do not bond ionically.
what are lattices?
Lattices are continuous, three dimensional networks of repeating units of positive and negative ions. The exact arrangement of ions in a lattice depends on the size and charge ratio of the ions.
Ionic bonds are non-directional. This means that an ion will attract all oppositely charged species surrounding it, with the attraction being equal in all directions. Because of this non-directional quality, each cation in the ionic lattice attracts all the surrounding anions, and vice versa. This means the forces of attraction in ionic lattices are very strong.
How does lattice enthalpy increase/decrease?
Lattice enthalpy increases as the energy required to overcome the electrostatic forces of attraction between ions increases. Two factors affecting lattice enthalpy are ionic radius and ionic charge. The strength of the electrostatic forces of attraction between oppositely charged ions:
- increases with increasing ionic charge
- decreases with increasing ionic radius
NaF has a greater lattice enthalpy than KF because the cations in NaF are smaller, and therefore the electrostatic attraction between Na+ and F- ions is greater.
What is lattice enthalpy?
Lattice enthalpy values tell us how strong the ionic bonds are in particular ionic lattice. Lattice enthalpy ΔH^θlattice, is the standard enthalpy change that occurs on the formation of gaseous ions from one mole of the solid lattice. It is a measure of the strength of an ionic bond because, in order for the ions to become gaseous, all the electrostatic forces of attraction between cations and anions in the lattice need to be overcome.
Properties of Ionic compounds
Volatility:
Volatility refers to the tendency of a substance to vaporise (turn into a gas). For an ionic compound to turn into a gas, the strong electrostatic forces of attraction holding the ions together must be overcome. The volatility of ionic compounds is therefore very low: they are said to be “non-volatile”. This also means they have high boiling points.
Electrical conductivity:
In order to conduct electricity, substances must contain charged particles that are able to move. Ionic compounds contain charged particles, cations and anions. In a solid ionic lattice, cations and anions. In a solid ionic lattice cations and anions can vibrate around a fixed point, but they cannot change position. Solid ionic compounds do not conduct electricity because ions in a solid lattice aren’t mobile. When molten or aqueous, both cations and anions are free to move past one another, allowing them to conduct electricity when a potential difference is applied.
Solubility:
Ionic compounds are typically soluble in polar solvents such as water, and insoluble in non polar substances such as hexane.
What happens to ionic compounds in water?
When ionic compounds are added to water; the water molecules position themselves so that their partial negative charges point towards the cations, and their partial positive charges point towards the anions. As a result, individual ions are pulled out of the lattice and become surrounded by the water molecules. In the case of a non polar solvent, there is no attraction between the ions of the ionic compound and the solvent molecules, so the cations and anions remain within the lattice.
Why might ionic compounds
be insoluble in water?
Not all ionic compounds dissolve in water. This is because there are two competing forces of attraction present:
- ionic bonds between cations and anions in the lattice
- the association between the ions and the partial charges of the water molecules
Ionic compounds are insoluble when the electrostatic attractions between the cations and anions in the lattice are stronger than the association between the ions and water molecules. Ions that are insoluble in water include; calcium carbonate and silver chloride.
What are covalent bonds?
Covalent bonds are formed when atoms share pairs of valence electrons. A covalent bond results from the electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the atoms involved in the bond.
How does electronegativity affect covalent character?
Covalent bonds generally form between atoms of relatively high electronegativity, typically non-metals. When the difference in electronegativity between the atoms is less than 1.8, the bond between them is predominantly covalent.
Unlike ionic substances, which are always compounds. Covalent substances can be elements or compounds
What is lattice enthalpy?
Lattice enthalpy values tell us how strong the ionic bonds are in particular ionic lattice. Lattice enthalpy ΔH^θlattice, is the standard enthalpy change that occurs on the formation of gaseous ions from one mole of the solid lattice. It is a measure of the strength of an ionic bond because, in order for the ions to become gaseous, all the electrostatic forces of attraction between cations and anions in the lattice need to be overcome.
How does lattice enthalpy vary with other factors?
Lattice enthalpy increases as the energy required to overcome the electrostatic forces of attraction between ions increases. Two factors affecting lattice enthalpy are ionic radius and ionic charge. The strength of the electrostatic forces of attraction between oppositely charged ions:
- increases with increasing ionic charge
- decreases with increasing ionic radius
Why does NaF have a greater lattice enthalpy than KF
NaF has a greater lattice enthalpy than KF because the cations in NaF are smaller, and therefore the electrostatic attraction between Na+ and F- ions is greater.
What are the properties of ionic compounds?
- volatility
- electrical conductivity
- solubility
Properties of ionic compounds- volatility
Volatility refers to the tendency of a substance to vaporise (turn into a gas). For an ionic compound to turn into a gas, the strong electrostatic forces of attraction holding the ions together must be overcome. The volatility of ionic compounds is therefore very low: they are said to be “non-volatile”. This also means they have high boiling points.
Ionic compounds typically have high melting points too. The melting point of sodium chloride is approximately 1075K. Magnesium oxide, frequently used in furnaces due to its ability to withstand high temperatures, melts at around 3098K
Properties of ionic compounds- electrical conductivity
In order to conduct electricity, substances must contain charged particles that are able to move. Ionic compounds contain charged particles, cations and anions. In a solid ionic lattice, cations and anions. In a solid ionic lattice cations and anions can vibrate around a fixed point, but they cannot change position. Solid ionic compounds do not conduct electricity because ions in a solid lattice aren’t mobile. When molten or aqueous, both cations and anions are free to move past one another, allowing them to conduct electricity when a potential difference is applied.
Properties of ionic compounds- solubility
Ionic compounds are typically soluble in polar solvents such as water, and insoluble in non polar substances such as hexane.
Water is a polar solvent. The difference in electronegativity between the oxygen and hydrogen atoms, combined with the bent geometry of the water molecule, result in the water molecule having a partial negative charge on the oxygen atom and partial positive charge on the hydrogen atoms.
What happens to ionic compounds when they are added to water
When ionic compounds are added to water; the water molecules position themselves so that their partial negative charges point towards the cations, and their partial positive charges point towards the anions. As a result, individual ions are pulled out of the lattice and become surrounded by the water molecules. In the case of a non polar solvent, there is no attraction between the ions of the ionic compound and the solvent molecules, so the cations and anions remain within the lattice.
Why might some ionic compounds not dissolve in water?
Not all ionic compounds dissolve in water. This is because there are two competing forces of attraction present:
- ionic bonds between cations and anions in the lattice
- the association between the ions and the partial charges of the water molecules
Ionic compounds are insoluble when the electrostatic attractions between the cations and anions in the lattice are stronger than the association between the ions and water molecules. Ions that are insoluble in water include; calcium carbonate and silver chloride.
What is a covalent bond?
Covalent bonds are formed when atoms share pairs of valence electrons. A covalent bond results from the electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the atoms involved in the bond.
How does the electronegativity difference of compounds effect covalent character?
Covalent bonds generally form between atoms of relatively high electronegativity, typically non-metals. When the difference in electronegativity between the atoms is less than 1.8, the bond between them is predominantly covalent.
Unlike ionic substances, which are always compounds, covalent substances can be either elements or compounds. When two non-metal atoms of the same element bond together, the electronegativity difference is zero, so they form a covalent bond between them.
How does electronegativity vary across periods and up groups?
Electronegativity increases across periods and up groups. If two atoms of relatively high electronegativity are found close to each other in the periodic table, the bonds they form are likely to be covalent. For example, oxygen and fluorine form covalent bonds together.
How do you draw lewis formulas?
- work out the total number of the valence electrons for each atom in the molecule
- divide the total number of valence electrons by two to work out how many pairs of electrons there are
- arrange the atoms by drawing their symbols. The element with the least number of atoms is usually found in the centre. (hydrogen atoms always surround the central atom(s)when present)
- bond the central and peripheral atoms together by drawing single bonds between them. Each single bond represents an electron pair
- assign non bonding pairs of electrons to the peripheral atoms, and keep going until they achieve noble gas configurations
- assign any remaining electron pairs to the central atom(s)
- check that the central atom has a full octet. if it does not
- reassign non bonding pairs on the peripheral atoms to become additional bonds to the central atom
- check that the molecule you are looking at is not an exception to the octet rule
Exceptions to the octet rule
In boron tetrafluoride, the boron atom has only three pairs of electrons around it. the boron atom is electron deficient.
Similar to boron, many other elements of group 2 and 13 form stable electron deficient molecules. These include beryllium, magnesium and aluminium. In lewis formulas of such molecules, group 2 elements (Be and Mg) have only two bonding electron pairs while group 13 elements (B and Al) have three bonding electron pairs.
Bond order
The number of bonded electron pairs between two atoms is referred to as bond order. Single bonds (bond order 1), double bonds (bond order 2), and triple bonds (bond order 3) differ in strength and length. Double bonds are stronger than single bonds, and triple bonds are stronger still. Triple bonds hold atoms closer together than double bonds, and hence triple bonds are shorter than double bonds. Double bonds are in turn, shorter than single bonds.
What are dative covalent (co-ordination) bonds?
Sometimes both the electrons in the covalent bond come from the same atom. The resulting bonds are called coordination bonds. For example when a hydrogen cation encounters a water molecule, a coordination bond is formed, leading to the formation of a hydronium ion.
What is the VSEPR model and how does it explain molecular geometry?
- Electron pairs repel each other, and therefore arrange themselves as far apart from each other as possible
- non bonding electron pairs (lone pairs) occupy more space than bonding pairs (single bonds)
- double and triple bonds occupy more space than single bonds
Two electron domains: Linear Geometry
If there are two electron domains the electron pairs in those domains repel each other. They therefore adopt positions at 180°. The angle between bonding pairs in a molecule is known as the bond angle. This electron domain geometry is called linear to illustrate that the central atom and both domains on a straight line.
Molecules with this electron domain geometry also have a linear molecular geometry
Three electron domains: Trigonal Planar geometry
If there are three bonding domains, the electron pairs adopt positions at 120° from each other. This electron domain geometry is called trigonal planar. Trigonal because the domains form a triangle, and planar, because the atoms lie flat on a plane.
A trigonal planar geometry can have two possible molecular geometries, depending on the presence of non-bonding domains:
- when all three domains are bonding domains the molecule has trigonal planar geometry
- when only two of the three domains are bonding domains and one is a non bonding domain the molecule has an angular or V-shaped geometry (bent)
Four electron domains: tetrahedral geometry
If there are four bonding domains, the electron pairs adopt positions at 109.5° from each other. This electron geometry is called tetrahedral because the ends of the domains form the corners of a tetrahedron.
A tetrahedral electron domain gives rise to three possible molecular geometries, depending on the presence of non-bonding domains:
- when all four domains are bonding domains, the molecule has tetrahedral geometry
- when three of the four domains are bonding domains and one is a non bonding domain, the molecule has trigonal pyramidal geometry
- when only two of the four domains are bonding domains and two are non bonding domains, the molecule has angular or V-shaped geometry (bent)
How do lone pairs and multiple bonds influence molecular geometry and bond angles?
Non bonding pairs occupy more space than bonding domains, which leads to decreased bond angles. Methane, CH4, has no non bonding pairs, and therefore the bond angle corresponds to the predicted 109.5°. Ammonia, NH3, has one non-bonding pair, and therefore has a smaller bond angle (107°). Water, H2O, has two non bonding pairs and therefore has an even smaller bond angle of 105.5°.
Multiple bonds
Multiple bonds (double and triple bonds) count as one domain. A double bond represents one domain, but it contains two electron pairs. Similarly, a triple bond is one domain composed of three electron pairs. Since multiple bonds contain more than one pair of electrons, they exert a greater repulsion than single bonds. The increased repulsion causes the bond angles in the molecule to deviate from the predicted values.
Bond angles:
Two electron domains: linear
- 2BP 0LP linear: 180 degrees
Three electron domains: trigonal planar
- 3BP 0LP trigonal planar: 120 degrees
- 2BP 1LP bent: <120 degrees
Four electron domains: tetrahedral
- 4BP 0LP tetrahedral: 109.5 degrees
- 3BP 1LP trigonal pyramidal: 107 degrees
- 2BP 2LP bent: 104.5 degrees
Five electron domains: trigonal bipyramidal
- 5BP 0LP trigonal bipyramidal: 90, 120 degrees
-4BP 1LP see-saw: <90,<120 degrees
-3BP 2LP T- structure: <90 degrees
- 2BP 3LP linear: 180 degrees
Six electron domains: octahedral
- 6BP 0LP octahedral: 90, 90 degrees
-5BP 1LP square pyramidal: 90, <90 degrees
- 4BP 2LP square planar: 90 degrees
What is bond polarity?
Polarity is related to the way electrons are distributed within bonds and molecules. The shared pair of electrons in a covalent bond is not necessarily shared equally between the two atoms in the bond. Bond polarity results from the difference in the electronegativities of the bonded atoms.
In the case of identical atoms, such as the two fluorine atoms in F2, there is an equal sharing of the electrons in the bonding pair. This is not the case, however, in HF. The fluorine atom has a much greater electronegativity than the hydrogen atom, so it pulls the shared electron pair more strongly than the hydrogen does. This leads to what we describe as a polar covalent bond, with one atom (fluorine) adopting a partial negative charge, δ-, and the other atom (hydrogen) adopting a partial positive charge, δ+.
What is molecular polarity?
Molecular polarity is similar to bond polarity, but it describes the electron distribution throughout the whole molecule. A molecule is polar when the electron distribution leads to a partial negative charge on one end of the molecule and a partial positive charge on the other. This results in a dipole moment. The value of the dipole moment is often reported in the unit debye, D. Polarity is an important trait of molecules because it gives rise to many characteristic properties such as volatility, solubility and boiling point.
When are molecules polar?
Molecules are polar when their bond dipoles don’t cancel each other out.
This may occur due to the geometry of the molecule, or because the bonds have different polarities. Both water and trichloromethane are polar because they contain polar bonds that do not cancel out. This results in a net dipole across the whole molecule.
Asymmetrical Molecular geometry
When are molecules non polar? (1)
Molecules are non polar when their bond dipoles cancel each other out.
This happens when the bond dipoles are equal and their arrangement results in no net dipole. Examples include boron trifluoride and carbon dioxide. Both molecules contain polar bonds because of the difference in the electronegativity of the atoms involved. These polar bonds are positioned in such a way that they cancel each other out. This means boron trifluoride and carbon dioxide are both non-polar, despite the presence of polar bonds.
When are molecules non polar? (2)
Molecules are non polar when all their bonds are non polar.
Hydrocarbons, such as ethane contain two types of bonds: carbon-carbon bonds and carbon-hydrogen bonds. The carbon-carbon bonds are non polar because both atoms have the same electronegativity. The carbon-hydrogen bonds are virtually non polar because of the small electronegativity difference between the two atoms. In addition, the tetrahedral geometry around each carbon atom results in a net dipole of zero, or very close to zero. Therefore hydrocarbons are generally non-polar
Long molecules can have a polar region and a non polar region (amphipathic)
Allotropes of carbon
Some elements have different structural forms known as allotropes. Carbon is one such element, and its allotropes include diamond, graphite, graphene and a group of substances known as fullerenes. They are all composed of carbon atoms, but have different chemical and physical properties due to their different structural arrangements.
Diamond
In diamond, each carbon atom is bonded to four other atoms in a tetrahedral arrangement (and is therefore sp^3 hybridised). It is one of the hardest substances known. For this reason it is often used in heavy duty cutting tools such as saws, polishing tools and dental drills. Diamonds high refractive index and durability means that it is also used to make jewelry. Diamond is a poor electrical conductor because it has no mobile charged particles- the electrons are all localised in the bonds. It is however an excellent thermal conductor: vibrational energy carriers called phonons travel well through highly regular lattice and strong covalent bonds.
Graphite
Graphite is composed of layers and sheets made of carbon atoms. Each carbon atom is bonded to three other carbon atoms and is therefore sp^2 hybridised in a hexagonal arrangement where the geometry around each carbon atom is trigonal planar. The carbon atoms in graphite are bonded such that one electron per carbon atom is delocalised. These delocalised electrons are free to move in the planes above and below each sheet and therefore graphite is a good electrical conductor. While the covalent bonds between the carbon atoms of the sheets are strong, the forces of attraction (London dispersion forces) between the sheets are weak. This means that the sheets can be separated easily, making graphite a good lubricant, as well as an ideal material for pencil leads. When you use a pencil to write the force applied causes parts of the graphite sheets to come off, leaving a mark on the paper.
Graphene
Graphene is essentially a single sheet of graphite. Graphene is thus one-atom thick and is therefore said to be two dimensional. Like graphite, it is an excellent conductor. It is also flexible, lightweight, transparent and very strong. ( N.B. It was first isolated in 2004 and has a vast range of applications, from desalination technology, to bendable electronic displays)
Buckminsterfullerene
Buckminsterfullerenes, or buckyballs, have a covalent molecular structure. With the formula C₆₀, the atoms in buckminsterfullerene are arranged in hexagons and pentagons to suggest the familiar shape of a football. Along with other spherical fullerenes, buckyballs could have exciting new roles in medicine as drug carries. Because of its molecular structure, C₆₀ has a low boiling point: overcoming the weak intermolecular forces of attraction does not require much thermal energy.
They have sp^2 hybridisation
Silicon and silicon dioxide
Silicon forms three-dimensional lattice where each silicon atom is bonded to four other silicon atoms in a tetrahedral arrangement (and is therefore sp^3 hybridised). This arrangement is similar to that of the carbon atoms in diamond.
The extensive covalent bonds in the lattice result in high strength, as well as high melting and boiling point. The Si-Si bond is weaker than its C-C counterpart as silicon has a larger atomic radius. Therefore, the Si-Si bond is more reactive than the C-C bond. This difference in strength also helps explain the different melting and boiling points.
Intermolecular forces
Intermolecular forces apply to molecular substances. The type and strength of intermolecular forces depends on the size and polarity of the molecules. Types of intermolecular forces include, London (dispersion) forces, dipole-induced dipole forces, dipole-dipole forces and hydrogen bonding. Collectively the first three intermolecular forces are termed van der Waals forces.
Intermolecular forces are weaker than chemical bonds, but they affect physical properties such as volatility, solubility and boiling point of molecular substances.
Intermolecular forces are weak electrostatic forces of attraction that occur between molecules.
When molecular forces melt, boil or sublime, the intermolecular forces between the molecules are overcome. Therefore, melting and boiling points are often used as indicators of intermolecular force strength. The covalent bonds in the molecule do not break during phase changes.
London dispersion forces (LDFs)
All molecules experience LDFs, which are intermolecular forces resulting from temporary instantaneous dipoles. A molecule has a dipole when one side carries a partial positive charge, and the other a partial negative charge. LDFs involve induced dipoles, which means that one molecule causes (or induces) a temporary dipole in another molecule. These induced dipoles occur due to the random movement in electrons around the molecule. In a simple non-polar molecule such as hydrogen, H2, the electron distribution is on average symmetrical. However, electrons are constantly moving around within the molecule. If we could somehow freeze time, the electron distribution would very unlikely be perfectly symmetrical. Instead, we would see a somewhat unequal electron density, with one region of the molecule having more electrons, rendering it slightly negative(δ-). There would also be a region of lower electron density elsewhere in the molecule with a partial positive charge (δ+).
What increases the strength of LDFs
the number of electrons there are, and the molecular shape. They affect the polarisability of the molecule: how easily the electron distribution is distorted by an electron field. The greater the polarisability, the stronger the dispersion forces
What are dipole-induced dipole forces
LDFs are forces of attraction between temporary, or instantaneous, dipoles. Dipole induced dipole forces are a type of related intermolecular force occurring between a polar molecule and a nearby non-polar molecule. The presence of a permeant dipole in the polar molecule induces the formation of a temporary dipole in the neighbouring non-polar molecule. This type of intermolecular force attracts non polar oxygen molecules, to polar water molecules. Dipole-induced dipole forces are weak, which explains why the aqueous solubility of oxygen is relatively low.
What are dipole-dipole forces
Dipole-dipole forces involve permanent dipoles, whereas LDFs result from temporary dipoles. When a molecule is polar, it has a permanent dipole and therefore experiences dipole-dipole forces of attraction with neighbouring polar molecules.
Hydrogen chloride, HCl, and diatomic fluorine, F2, have similar sizes and comparable masses and so they experience LDFs of similar strength. However HCl molecules are polar and therefore experience dipole-dipole forces in addition to LDFs. The intermolecular forces between HCl molecules are stronger than those between F2 molecules, so HCl has a higher boiling point
Hydrogen bonding
Hydrogen bonds are strong intermolecular forces that form when a molecule contains a strong dipole involving hydrogen. When a hydrogen atom is covalently bonded to a highly electronegative atom such as oxygen, nitrogen or fluorine, the bond between them is very polar. The electrons in the covalent bond are drawn towards the more electronegative atom, resulting in a considerable partial positive charge (δ+) on the hydrogen. This hydrogen atom can then form a strong electrostatic interaction- a hydrogen bond- with the electrons pf another electronegative atom. This is usually found on a different molecule, but intramolecular hydrogen bonds can also exist in certain situations.
Physical properties of covalent substances (volatility)
Substances with covalent network structures are solids at room temperature and pressure. Vaporising them requires a lot of energy because of the strong covalent bonds holding the structure together. They are therefore non-volatile and have very high melting and boiling points
In order to vaporise molecular substances, the intermolecular forces between the molecules need to be overcome. Since intermolecular forces are relatively weak, the energy required to overcome them is low and therefore molecular substances are generally volatile.
Due to the variety of sizes in intermolecular forces, there is, however, a large variation in the volatility of molecular substances.
However, substances that consist of much larger molecules have lower volatility and higher melting and boiling points. This is because larger molecules have stronger LDFs between each other.
Physical properties of covalent substances (Electrical Conductivity)
Covalent substances, both network and molecular, are usually not electrical conductors. To conduct electricity, a substance needs to contain certain charged particles that are free to move. Covalent substances generally do not contain such particles, as their electrons are “locked up” in localised covalent bonds, and they do not contain ions.
Exceptions include; graphite, which is a good electrical conductor due to the presence of delocalised electrons. Silicon is a semiconductor, meaning that its intermediate conductivity places it between conductors and insulators.
Solar panels consist of photovoltaic cells, which convert solar energy into electricity. The cells contain semiconductors, such as silicon.
Physical properties of covalent substances (solubility)
When a substance dissolves, forces of attraction are formed between the substance (solute) and the solvent. Substances with covalent network structures are insoluble in most solvents because of the strong covalent bonds holding their atoms together
A molecular substance is likely to dissolve in a solvent if the intermolecular forces between the solute and the solvent are stronger than the attraction between the solute molecules.
Non-polar solutes are likely to dissolve in non-polar solvents. Similarly, polar solutes are likely to dissolve in polar solvents. Dissolving is unlikely to occur if the solute is polar and the solvent is non polar.
How can you calculate retardation factor (Rf)
The results of a chromatography experiment can be quantified by calculating the retardation factor. The Rf value for a spot on a chromatogram is the ratio of the distance travelled by the spot (b) to the distance travelled by the solvent (a)
Rf= (b/a)
why does Rf value vary
The distance travelled by a spot can vary depending on factors such as the composition of the solvent, temperature, pH and type of paper used (in paper chromatography). Rf values are reproducible and therefore can be used to identify substances by comparing experimental values to accepted values, provided conditions are the same.
Why and when are resonance structures used?
Sometimes a molecule cannot be described by a single lewis formula. Instead, there are two or more possible resonance structures that collectively represent the molecule. Resonance often happens when there is more than one position for a double or triple bond in a molecule
What does the resonance between ozone molecules show about resonant structures?
The two bonds in ozone are:
- identical to each other
- intermediate between a single O-O bond and a double O=O bond in terms of bond strength and length
- have a bond order of 1.5 because there are 3 bonding electron pairs distributed across two domains
How does the resonance in benzene work?
In benzene, one electron in each of the six carbon atoms occupies a p orbital. The six p orbitals overlap, forming a ring of electron density above and below the plane of the molecule. The electrons in this ring are delocalised. The carbon atoms are sp2 hybridised. (electrons are only delocalised in the ring, so it is not a good conductor of electricity)
Physical evidence for the structure of benzene
Once it was established that C6H6 was the molecular formula for benzene, the next step was to deduce its structure. The 1:1 ratio of carbon to hydrogen suggested the presence of multiple bonds. Eventually a cyclic arrangement of carbon atoms with alternating single and double bonds was proposed. X-ray diffraction patterns showed that a regular hexagonal arrangement of carbon atoms in benzene where all carbon atoms had the same length suggesting they were all equivalent. The carbon- carbon bond length in benzene is intermediate between that of a single and a double carbon-carbon double bond. They are also of intermediate strength.
chemical evidence for the structure of benzene
The chemical behaviour of benzene differs from what would normally be expected of alkenes. Like alkenes, benzene is unsaturated, but unlike alkenes, it does not readily undergo addition reactions. Alkenes undergo addition reactions, where the double bond breaks and the carbon atoms on either side form bonds with new species. In benzene, addition would disrupt the stabilising effect of the delocalised electron ring, which would be energetically unfavourable. Instead, benzene tends to undergo substitution reactions.
what is formal charge?
Formal charge is the charge an atom would have if all the bonding electrons in the molecule were shared equally, and if its non-bonding electrons were not shared at all. We can assign formal charge to atoms in a molecule to select the best lewis formula when several are possible. Formal charge is calculated by looking at the electrons an atom has before and after bonding
How do you calculate formal charge?
FC=VE-(NBE+1/2BE)
where:
FC= formal charge
VE= number of valence electrons
NBE= number of non-bonding electrons assigned to the atom in the lewis formula
BE= number of bonding electrons assigned to the atom in the lewis formula
what are sigma bonds?
Imagine an invisible line between the nuclei of two bonding atoms. This line is known as the bond axis or internuclear axis. The two 1s orbitals will overlap along this axis to form a sigma bond. This means that there is a region of high electron density along the bond axis
what are pi bonds?
If p orbitals are present in two neighbouring atoms, they can overlap sideways, above and below the bond axis. This type of overlap forms a pi bond. In pi bonds, the electron density is concentrated at opposite sides of the bond axis
what are single bonds?
one sigma bond
what are double bonds?
one sigma bond and one pi bond
what are triple bonds
one sigma bond and two pi bonds
hybridisation in methane
methane, CH4, has a tetrahedral structure. However the ground state of carbon, 1s2 2s2 2p2, contradicts these observations. Carbon contains two unpaired 2p electrons and therefore should form two bonds (one involving each unpaired electron), not four. Furthermore, the two occupied 2p orbitals are at 90° from one another, not 109.5°. This means that the atomic orbitals must undergo certain changes when they form bonds.
what is hybridisation?
Hybridisation is the concept of mixing atomic orbitals to form two hybrid orbitals for bonding
What is metallic bonding?
the electrostatic attraction between metal cations and delocalised electrons
properties of metallic structures
metals are typically:
- good electrical conductors because they contain mobile delocalised electrons
- good thermal conductors because they contain mobile delocalised electrons and closely packed cations
- malleable and ductile, because the layers of cations can slide past each other without breaking the metallic bonds.
How does the strength of metallic bonds vary?
The stronger the attractions between the cations and the delocalised electrons, the stronger the metallic bond.
Metallic bond strength decreases with greater ionic radius, and increases with greater ionic charge
A third factor, related to ionic charge, is the electron density of the sea of delocalised electrons.
what are alloys?
Alloys are mixtures. The ratio of the components in an alloy can vary without changing the density of the substance. For example, the proportion of carbon in steel ranges from traces to about 2%. In addition, the components of alloys retain many of their original properties.
properties of alloys
Alloying involves the addition of atoms or ions with a different radius to the cations of the pure metal, which disrupts the regular structure of the lattice. When an alloy is struck with a force, the layers of cations do not slide past each other as easily. Therefore, alloys are usually stronger than pure metals.
