Topic 4: Chemical Bonding and Structure Flashcards
Ionic vs covalent
Both two main classes of compounds
Ionic: NM + M
Covalent: NM + NM
Elements far away from each other tend to form ionic bonds
Elements close together tend to form covalent bonds
How are positive and negative ions usually formed?
Positive ions are usually formed by metallic elements by the loss of valence electrons.
Ex. Mg - 2e- = Mg2+
Negative ions are usually formed by non metallic elements by the gain of electrons
Ex. O+2e- = O2-
Why do elements in the main groups of the periodic table (group 1, 2, 13 (to a certain extent), 15, 16, 17) form ions?
Electrons gained or lost electrons configuration of the nearest noble gas. That is, electrons are gained or lost to make an ion that is isoelectronic (same number of electrons) with the nearest noble gas.
Another way of saying: To achieve full outer shell of electrons.
Common ions to remember:
NH4+
OH-
O2-
NO3-
HCO3 2-
CO3 2-
SO4 2-
PO4 3-
Ammonium
Hydroxide
Oxide
Nitrate
Hydrogen carbonate
Carbonate
Sulfate
Phosphate
What in the periodic table can form more than one ion?
Transition metals
ex. Iron an form iron(II), Fe2+, and iron (III), Fe3+
What determines the number of electrons lost or gained?
The electron configuration of an atom
Electrons cannot be created or destroyed, therefore…
The total number of electrons lost must also equal the total number gained.
What is an ionic bond?
An ionic bond is an electrostatic attraction between oppositely charged ions
what is an electrostatic attraction?
Attraction between oppositely charged ions
What holds a crystal of sodium chloride together?
Sodium chloride consists of a giant lattice of NA+ and CL- ions. The electrostatic attraction between the oppositely charged ions hold crystals together. These electrostatic forces are so strong, so it is difficult to break apart the lattice structure.
Do ionic compounds have high or low melting and boiling points?
Ionic compounds usually have high melting point and boiling points. This means that ionic substances are usually solids at room temperature.
Why do ionic compounds have high melting and boiling points?
It is due to the strong electrostatic forces between the oppositely charged ions. When an ionic solid is melted, the electrostatic forces throughout the giant lattice must be broken, because they are so strong, a lot of energy is required.
Why does magnesium oxide has a much higher melting point than sodium chloride?
Because the electrostatic attraction between the 2+2 minus ions in magnesium oxide lattice are much stronger than those between the 1+ and 1- ions in sodium chloride.
What are examples of physical properties?
Examples include melting boiling point, solubility, electrical conductivity, etc. Chemical properties are how substances reacts.
What is meant by volatility and how is it for ionic solids?
Volatility refers to how readily a substance evaporates. Ionic solids have low lower volatility because the electrostatic forces between the ions are strong.
Can ionic substances conduct electricity?
Solid and liquid
Ionic substances do not conduct electricity when solid. In the solid-state, the ions are held tightly in position in the lattice structure so that they are not free to move around (other than vibrate).
When an ionic substance is melted the isles were able to move freely throughout the liquid. Positive ions can move towards a negative electrode and negative ions towards a positive electrode so allowing the conduction of electricity.
What is ionic compound solubility in water?
Ionic substances are often soluble in water. Water is a polar solvent, and energy is released when the ions are hydrated by being surrounded (ion-dipole attractions) by water molecule. This energy pays back the energy required to break apart the ionic lattice.
What is the electrical conductivity in ionic solutions?
Aqueous solutions (solutions made with water) Ionic substances conduct electricity. This is because the ions are free to move around.
Are ionic solids soluble in non-polar solvents? Why?
Ionic solids are not usually soluble in non-polar solvent such as hexane. This is because a great deal of energy is required to break apart the ionic lattice and this is not paid back by the energy released when the non-polar solvent form interaction with the ions (London forces).
Where is the electrostatic attraction between covalent bonds?
There’s electrostatic attraction between a shared pair of electrons and the nuclei of the atom that are bonded
What are the pairs of electrons in outer shell of atoms that are not involved in covalent bonds called?
Lone pairs
What holds atoms together in a covalent bond?
The covalent bond is the electrostatic attraction between positively charged nuclei of both atoms and the shared power of electrons.
why are triple bonds shorter than double bonds which are shorter than single bonds?
Because there is a stronger attraction between the bonding electrons and the nuclei when there are more electrons in a bond.
What happens to the bond length and energy as it goes down the periodic table?
The bond between the elements get weaker as a bond gets longer. This is because, that the atoms get bigger, the electron pair and a covalent bond is further away from the nuclei of the atom making up the bond. The electron pairs further away from the nuclei, it is less strongly attracted and a covalent bond is weaker.
What are the most electronegative elements in the periodic table?
Nitrogen, oxygen, fluorine, chlorine
What are molecules called when the atoms attract electron pair unequally in a covalent bond?
Polar molecules
What determines the polarity of a molecule?
The difference in electronegativity of the atoms.
What is electornegativity a measure of?
of the attraction of an atom in a molecule for the electorn pair in the covalent bond of which it is a part
what determines how strongly the electrons are attracted?
Depends on the size of the individual atoms and the nuclear charge
What does it mean when the fluorine is more electronegative than hydrogen in HF?
It means that fluorine attracts electrons in the H-F bond more strongly than a hydrogen atom does. The electrons in a bond like closer to the fluorine down to the hydrogen.
Why don’t noble gases have electronegativity values?
Because they do not form compounds
Which has higher electronegativities: non-metals or metals?
non-metals
What’s the trend in electronegativity in the periodic table?
Electronegativity generally decreases down a group and increases across a period
What’s the pattern between electronegativity with covalent and ionic bonds?
Atoms with similar electronegativities will form covalent bonds.
Atoms with widely different electronegativities will form ionic bonds.
The difference in electronegativity can be taken as a guide to how ironic or how covalent bond between two atoms as likely to be.
Do nonmetals have a higher or lower electronegative than metals?
Nonmetals have a higher electronegativity than metals
What value of electronegativity represent a covalent and ionic bond?
If the difference more than 1.7, the bonding between two elements is ironic.
If the difference is less than 1.7, the bonding between two elements is covalent.
What is the difference between electrostatic attraction and electronegativity?
Electrostatic attraction is the force between two oppositely charged ions and electronegativity is the ability to attract electrons to an atom.
What is the octet rule?
The concept that atoms and covalent bonds have a tendency to have a full valence shell of a total of eight electrons.
What is a coordinate covalent bond or dative covalent bond?
They coordinate covalent bond is a type of covalent bond in which both electron come from the same atom. Once a coordinate covalent bond has been formed, it is identical to an ordinary covalent bond.
explain the dative covalent bond in NH4+
H+ does not have any electrons with which to form a covalent bond, but NH3 has a long pair of electrons that can be used to form a covalent bond.
What is the bond situation in carbon monoxide?
Both atoms can attain an octet if the oxyen atom donates a pair of electrons to carbon in the formation of a coordinate covalent bond
there is now a triple bond between the two atoms, made up of two ‘ordinary’ covalent bonds and one coordinate covalent bond. both atoms have a lone pair of electrons
What is it called when two molecules are bonded together?
An adduct
What is meant by resonance structure
Individual structures that differ only in the position of the double bond
How to determine central atom in lewis structure?
Usually it is the element with only one atom, or its the least electronegative atom
What is simliar about O3 and NO2-?
they are isoelectronic and has the same lewis stucture
How are NO2+ and CO2 isoelectronic?
same lewis structure
What does the valence shell electron pair repulsion (VSEPR) theory tell us?
It allows us to predict the shapes of molecules
What is the VSEPR theory?
Pairs of electrons in the valence shell of an atom repel each other and will therefore take up positions in space to minimize these repulsion – to be as far apart in space as possible. Can be bonding pairs or non-bonding pairs (lone pairs)
What does the shape of a molecule depend on?
It depends on the number of electron pairs in the outer shell of the central atom.
Why do we use electron domains instead of number of bonds?
Because a double or triple bond behaves as if it were just one electron pair.
Why does a multiple bond count as a single electron pair?
Because the electrons are constrained to occupy the same region of space
What is the shape and bond angle of molecules with 2, 3, 4 electron domains?
2: Linear and 180˚
3: Trigonal planar and 120˚
4: Tetrahedral and 109.5˚
Basic and actual shapes of some specific molecules
What is the angle of bent and trigonal pyramidal?
Bent: 105˚ if two lone pairs 117˚ if one
Trigonal pyramidal: 107˚
Do lone pairs contribute to overal shape?
Yes, it just cannot be seen
What is the order of repulsion strength for pairs of electrons? and Why
lone pairs-lone pairs > lone pairs-bonding pair > bonding pair-bonding pair
Because the lone pairs are held closer to the central nucleus than are bonding pairs. (Long pairs are in the outer shell of the central atom, whereas bonding pair can be imagined as being, on average, halfway between the bonded atoms.
What does the presence of long pairs result in bond angles?
lone pairs repel bonding pairs of electrons more than the bonding pairs repel each other therefore the presence of lone pairs result in a closing up of bond angles.
Why do we call some bonds polar?
Because the electronegativity difference between two atoms covalently bonded together results in the electrons lying more towards one atom than the other.
For a molecule to be polar and must have a positive and a molecule and a negative end.
How can some individual bonds be polar but a molecule is non-polar?
Because of the symmetry of the molecule, the dipole moment of the individual bonds canceled out.
Why is CCl4 for non-polar？
Although each individual bond is polar due to the difference in electronegativity of the atoms, because of the symmetry of the molecule, the dipoles cancel.
What are allotropes of carbon?
Different forms of the same element.
Diamond, graphite, graphene and fullerenes are all allotropes of carbon. They all contain only carbon atoms, but these atoms are joined together differently in each structure.
What is the structure of diamond and graphite and how do they differ? (structure, shape, bonds)
Diamond and graphite has a giant covalent (macromolecular) structure.
Diamond: There are no individual molecules – the whole structure, continuing in 3 dimensions, represents one giant molecule.
Each carbon atom is drawn to for others, in a tetrahedral array, by covalent bonds.
Graphite: it has a layer structure. Each C is covalently bonded to three other layers in a trigonal planner array. There are covalent bonds between the C atoms within a layer but only London forces between the layers. The presence of weak forces between the layers is usually given as the explanation that graphite is a good lubricant.
What are the properties of diamond? (MP/BP, conductivity, solubility)
Diamond has a very high melting and boiling point because a lot of energy must be supplied to break covalent bonds. Has the highest thermal conductivity. Diamond is very hard for the same reason. Diamond is the hardest naturally occurring substance.
Diamond does not conduct electricity because all electrons are held strongly in covalent bonds and are therefore not free to move around in the structure.
Diamond is not soluble in water or organic solvents because the forces between the atoms are too strong. The energy to break these covalent bonds would not be paid back on the C atoms were solvated.
What are the properties of graphite? (MP/BP, bonds, conductivity,
Graphite has a very high melting/boiling point because covalent bonds within the layers must be broken when it is melted/boiled.
Because of the strong covalent bonds between atoms, graphite is not soluble in water or non-polar solvents.
Graphite conduct electricity because each C atom forms only 3 covalent bonds - the extra electrons not used in these bonds are able to move within layers. The free electron is called delocalized electron.
What are substances of giant structures called?
Network solids
What are the properties of graphene? (Structure, strength, MP/BP, bonds, heat and electrical conductivity)
Graphing consists of a single layer of graphite. Only very small pieces happen forms so far. It has a very high tensile strength and would be expected to have a very high melting point because covalent bonds need to be broken to break the sheet.
It is a very good electrical and thermal conductor through to the delocalised electron from C only forming 3 bonds.
What are the properties of C60 fullerene (buckminsterfullerene)? (structure, bonds, solubility, conductivity)
It is considered a molecular rather than a giant structure. It consists of individual C60 molecules, with covalent bonds within the molecule and London forces between the molecules.
C60 is insoluble in water but soluble in some organic solvents such as benzene. The energy to overcome the London forces between the C60 molecules is paid back by the energy released when London forces are formed between the C60 molecules and the solvent.
C60 does not conduct electricity. Although each C forms 3 bonds in the C60 molecule, so that there is delocalisation of electricity over the molecule, the molecular structure means that electrons are not able to move from one molecule to the next.
Properties of silicon dioxide?
SiO2 has a giant covalent structure. Silicone atom is bonded to 4 oxygen atoms in a tetrahedral array. Each oxygen is bonded to two silicon atoms. Due to the two lone pairs on each oxygen atom, the basic shape about each Si-O-Si unit is bent.
The oxide has high melting and boiling points, how is covalent bonds between atoms must be broken in order to melt/boil it and it requires a lot of energy.
Diamond vs graphite vs fullerene boiling and melting point.
Diamond and graphite are giant structures but fullerene is a molecular structure.
Diamond and graphite must break covalent bond to be melted
Fullerene only breaks intermolecular forces (London forces) when melted.
Covalent bonds are must stronger than intermolecular forces, therefore more energy is required for diamond and graphite to melt.
What’s the difference between intramolecular forces and intermolecular forces?
Intramolecular forces are forces within a molecule (covalent, ionic, metallic bonding)
Intermolecular forces are forces between molecules (Van der Waals’ forces)
Which one is stronger: intermolecular forces or covalent bonds
Covalent bonds are much stronger than intermolecular forces
Where can you find London forces? And where does it come from?
London forces are present between all molecules in a solid and liquid states.
It is found between non-polar atoms/molecules.
London forces or temporary instantaneous dipoles–induced dipole interactions because electrons in an atom are in constant motion and at any one time they will not be symmetrically distributed about the nucleus. This results in a temporary instantaneous dipoles in the atom which will induce an opposite dipole in a neighbouring atom. These tables will attract each other so that there’s an attractive force between atoms.
What are the Van der Waals’ forces?
Van der Waals’ forces is the collective name given to the forces between molecules and includes London (dispersion) forces, dipole-dipole interactions and dipole–induced dipole interactions.
How does the strength of London forces increase?
In general, London forces get stronger as a relative molecular mass increases or the number of electrons in molecules increase.
What forces are present in polar molecules?
London dispersion and dipole dipole forces
What do dipole dipole forces due to polar molecules?
They cause melting point and boiling points to be higher
What’s the difference between polar vs nonpolar molecules with similar relative molecular mass?
Polar molecules have higher melting and boiling points and non-polar molecules
Why does Br, a non-polar molecules, have a higher boiling point than some other polar molecules?
The highest boiling point as a result of its highest relative molecular mass and therefore strongest London force
why is H2O then what would be expected from a relative molecular mass?
Just suggest that there must be intermolecular forces other than London forces between H2O molecules and these intermolecular forces are stronger than the London forces.
This intermolecular force is called a hydrogen bond
Which is stronger hydrogen bonding or covalent bonding?
Hydrogen bonding is the strongest of the intermolecular forces but it is still much weaker than covalent bonding.
When does hydrogen bonding occur? How can you tell?
Hydrogen bonding occurs between molecules when a very electronegative atom, N, O, F, is joined to a hydrogen atom in the molecule.
As long as there’s N, O, F atom in a molecule there will be hydrogen bonding.
What happens to the bonds when melted or boiled?
Only intermolecular forces are broken when covalent molecular substances are boiled – covalent bonds are not broken.
What is the weakest to strongest intermolecular forces and why it causes differences in melting and boiling points?
London < permanent dipole – dipole < hydrogen bonding
The stronger the intermolecular force the more energy must be supplied to break them and a higher boiling point.
What type of intermolecular forces are in substances containing H bonded to N, O or F.
Hydrogen bonding, permanent dipole – dipole, London forces
What intermolecular forces are found in polar substances?
Permanent dipole – dipole, London forces
What intermolecular forces are found in non-polar substances?
London forces
How does relative molecular mass influence melting and boiling points?
Generally substances with higher relative molecular masses have higher melting point and boiling points due a stronger London forces.
What does ‘like dissolves like’ means?
Generally a substance will dissolve in a solvent if the intermolecular forces in the solute and solvent are similar
What determines if a substance dissolves or not?
It depends (in part) on how much energy is needed to overcome intermolecular forces in the solvent and solute and then how much energy is released, to pay back the energy, when intermolecular forces are formed between solvent and solute molecules in the solution.
What are substances with hydrogen bonding solubility in water?
Participate in hydrogen bonding will generally be soluble in water, because they’re able to hydrogen bonds with water.
Why can pentane dissolve in hexane but pentane cant dissolve in water?
Because pentane and hexane both have London forces between them but pentane does not dissolve in water because only London forces would before between pentane and water molecules
Why is ethanol soluble in water?
It is very soluble in water, able to hydrogen bond to the water. The hydrogen bonding between water molecules and ethanol molecules in the solution releases energy and pays back the energy to break up hydrogen bonds in pure water and pure ethanol.
Why are longer chain alcohol less soluble in water?
Although there is some hydrogen bonding between the O–H group of the alcohol and water molecules, the long hydrocarbon chain prevents water molecules on either side from hydrogen bonding to each other.
Energy is needed to break the hydrogen bonds between water molecules, but this is not paid back as only London forces form between water molecules and the hydrocarbon part of the molecule
How does ionic substances dissolve in polar solvents?
Aqueous solutions contain aqueous ions. Ion – dipole interactions formed between the water and the ions. Energy is required to overcome the electrostatic forces in the ionic lattice but energy is released when hydrated ions are formed. If the energy released when the hydrated ions are formed is comparable to the energy required to break apart an ionic lattice then the substance is generally soluble.
Are ionic substances usually soluble in non-polar solvents?
They are not usually soluble because the interaction between ions and a non-polar solvents would be weak London forces.
What is meant by a delocalized electrons?
If you look lies electron do not belong to anyone metal atom but rather are able to move throughout the structure.
Comparison of the physcial properties of ionic and covalent molecular substances?
What is metallic bonding?
A metallic bond is the electrostatic attraction between the positive ions in the lattice and the delocalised electrons.
What determines the strength of a metallic bond?
The strength is determined by the number of the look like electrons, the charge on the cation, and the radius of the cation
What is the trend in radius on the periodic table?
Radius get smaller across, due to the number of protons in the nucleus, they become stronger and pulls more electrons towards the nucleus.
The radius increases as it goes down the periodic table since there are more shells
Describe a metallic structure?
Metals contain a regular lattice arrangement of positive ions surrounded by a ‘sea’ of delocalized electrons
Suggest reasons why magnesium has a higher melting point than sodium
1) Mg forms a 2+ ion compared with Na, which forms a 1+ ion. This means that the electrostatic attraction between ions and delocalized electrons is stronger in magnesium
2) magnesium ion is smaller than the sodium ion, because the delocalised electrons are closer to the nucleus of the positive ion in Mg and are more strongly attracted.
3) There are 2 delocalized electrons per atom in magnesium. Therefore there will be a stronger electrostatic attraction between ions and the delocalized electrons.
Define lustrous
Shiny, when freshly scratched/cut
Define ductile
Ability to be drawn out in a thin wire
Define malleable
May be hammered into shape – many uses arise from the ability to easily sheet metal
e.g. making car bodies
How do metals conduct electricity?
Because the delocalized electrons are free to move around
Why are metals malleable/ductile?
Because of the non-– directionality of the bonding. The metal ions in the lattice attract the delocalized electrons in all directions. So when two layers slide over each other, the bonding in the resulting structure it’s exactly the same as in the original.
How do metals do compared to ionic solids in malleability?
Metals are malleable as the resulting structure is the same as the original but ionic solids are brittle and the placement of one layer relative to the other results in like charges repelling each other causing them to fall apart.
Define alloys
Alloys are homogenous mixtures of two or more metals, or of a metal with a nonmetal
Why do alloys tend to be stronger and stiffer than pure metals?
The introduction of a larger atom makes it more difficult for the planes of atom/ions to slide over each other
