DEC-Structure and Bonding Flashcards

Question

What is a covalent bond?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

1. work out the total number of the valence electrons for each atom in the molecule 2. divide the total number of valence electrons by two to work out how many pairs of electrons there are 3. 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) 4. bond the central and peripheral atoms together by drawing single bonds between them. Each single bond represents an electron pair 5. assign non bonding pairs of electrons to the peripheral atoms, and keep going until they achieve noble gas configurations 6. assign any remaining electron pairs to the central atom(s) 7. 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

Answer 5

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.

Answer 6

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.

Answer 7

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.

Answer 8

1. Electron pairs repel each other, and therefore arrange themselves as far apart from each other as possible 2. non bonding electron pairs (lone pairs) occupy more space than bonding pairs (single bonds) 3. double and triple bonds occupy more space than single bonds

Answer 9

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

Answer 10

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)

Answer 11

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)

Answer 12

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.

Answer 13

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

Answer 14

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+.

Answer 15

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.

Answer 16

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

Answer 17

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.

Answer 18

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)

Answer 19

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.

Answer 20

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.

Answer 21

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.

Answer 22

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)

Answer 23

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

Answer 24

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.

Answer 25

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.

Answer 26

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+).

Answer 27

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

Answer 28

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.

Answer 29

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

Answer 30

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.

Answer 31

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.

Answer 32

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.

Answer 33

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.

Answer 34

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)

Answer 35

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.

Answer 36

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

Answer 37

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

Answer 38

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)

Answer 39

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.

Answer 40

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.

Answer 41

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

Answer 42

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

Answer 43

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

Answer 44

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

Answer 45

one sigma bond

Answer 46

one sigma bond and one pi bond

Answer 47

one sigma bond and two pi bonds

Answer 48

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.

Answer 49

Hybridisation is the concept of mixing atomic orbitals to form two hybrid orbitals for bonding

Answer 50

the electrostatic attraction between metal cations and delocalised electrons

Answer 51

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.

Answer 52

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.

Answer 53

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.

Answer 54

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.