An ionic bond is formed when electrons are transferred between one atom (metal) to another atom (non metal)
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Define ionic bonding
the electrostatic attraction between oppositely charged ions
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Why do ionic compounds have high melting points?
Ionic compounds for example a crystal of sodium chloride has strong electrostatic forces of attraction between the oppositely charged ions and a lot of energy must be supplied to overcome such strong forces and break the ionic bonds.
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Why does MgO have a higher melting point than NaCl?
the electrostatic forces of attraction between +2 and -2 ions in the magnesium oxide lattice are much stronger than between +1 and -1 ions in the sodium chloride lattice.
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Can ionic compounds conduct electricity?
in solid form ionic compounds can not conduct electricity as the ions are not free to move, however in molten state in a solution ionic compounds can conduct electricity as the ions can move and carry a current.
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Define covalent bonding
the electrostatic attraction between the shared pair of electrons and the nuclei of the atoms making up the bond, each non metal shares an electron with each other to create a pair and therefore full outer shell.
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How many shared electron pairs are in a single bond, double bond and triple bond?
1, 2, 3
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Write bonds in order of increasing strength
single bond < double bonds < triple bonds
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Write bonds in order of decreasing length
single bond > double bonds > triple bonds
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What has more attraction of the nuclei, two electron pairs in a double bond or one electron pair in a single bond?
the attraction of the two nuclei for two electron pairs is stronger
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Define Coordinate/dative bonding
a type of covalent bond where both electrons come from the same atom
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What are the rules for working out Lewis structures?
1. make sure the outer atoms have 8 electrons (except hydrogen which has two) 2. if the central atom is from period 2 it should have no more than 8 electrons on its outer shell (generally) 3. if the central atom is from period 3 it may have up to 18 electrons in the outer shell
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Define Resonance Structures
Lewis drawings for the same compound but with the double bond in different places and lone pairs accordingly
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What does the shape of a molecule/ion depend on?
it depends on the number of electron domains in the outer shell of the central atom
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What three things count as an electron domain?
1. a lone pair 2. an electron pair 3. the electron pairs that make up a multiple bond i.e. a double or triple bond
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What factor affects the angle between electron domains?
the repulsion between electron domains on the valence shell of the central atom. Bonding pairs of electrons repel as far apart as possible however repulsion between bonding pairs of electrons with lone electron pairs is greater.
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What are the four steps in predicting shapes of molecules/ions?
1. draw a lewis structure 2. count the number of electron domains in the outer shell of the central atom 3. work out the basic shape (Electron domain geometry) 4. work out the actual shape (MDG) by looking at lone pairs and angles.
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Linear
2 electron pairs, 2 bonding pairs, 0 lone pairs, bond angle 180 e.g. CO2
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Trigonal Planar
3 electron pairs, 3 bonding pairs, 0 lone pairs, bond angle 120, e.g. SO3
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Bent
3 electron pairs, 2 bonding pairs, 1 lone pair, bond angle 117, e.g. SO2
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Tetrahedral
4 electron pairs, 4 bonding pairs, 0 lone pairs, bond angle 109.5 e.g. NH4+
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Trigonal Pyramidal
4 electron pairs, 3 bonding pairs, 1 lone pair, bond angle 107 e.g. PCl3
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bent, v-shaped, angular
4 electron pairs, 2 bonding pairs, 2 lone pairs, bond angle 105 e.g H2O
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What is polarity?
polarity is the term used to describe a molecule where two atoms have different electronegativities, causing one atom to have a slightly negative charge and one atom to have a slightly positive charge. This creates a dipole. Additionally, the shape of a molecule has to be asymmetrical for the molecule to be polar.
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The bigger the difference in electronegativity between the atoms…
the more polar the bond
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Why is CO2 a non polar molecule?
even though C -- O is polar the molecule is symmetrical so the dipoles cancel out.
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What is broken when covalent molecular substances are melted or boiled?
intermolecular forces are broken when covalent substances are melted or boiled - not the covalent bonds.
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What are London forces?
intermolecular forces that exist between non-polar molecules. They occur when there is an uneven distribution of electrons in an atom, causing one side to be more negative than the other, attracting oppositely charged areas of other atoms so a temporary dipole is established.
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What factors affect the strength of London forces?
1. the more electrons present in the atom , the stronger the force 2. the greater the relative molecular mass the stronger the forces are.
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What are dipole-dipole forces?
a type of intermolecular force that exists between polar molecules - a positive dipole in one molecule attracts a negative dipole in another molecule.
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True or false intermolecular forces between polar molecules are stronger than between non polar molecules?
true - polar molecules with dipole-dipole IM forces have higher melting and boiling points because the force is stronger.
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What is hydrogen bonding?
hydrogen bonding is a type of intermolecular force that occurs between a hydrogen atom and a very electronegative atom (N, O, F) with a lone pair. The force occurs due to the attraction between the negative dipole of the oxygen, nitrogen or fluorine atom in one molecule and a positive hydrogen atom in another molecule.
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List the intermolecular forces in increasing strength
London Forces < dipole-dipole forces < hydrogen bonding
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Why can substances dissolve in some solvents but not in others?
Normally, a substance will dissolve in a solvent if the intermolecular forces in the solute and solvent and similar e.g. both have dipole-dipole forces. Substances that have hydrogen bonding will dissolve in water as they are able to hydrogen bond to the water.
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What are giant covalent structures?
individual atoms covalently bonded together throughout a lattice structure.
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Name the three allotropes of carbon
diamond, graphite and buckminsterfullerene
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Describe the structure or diamond
giant covalent structure, covalent bonds between all atoms, tetrahedral arrangement of bonds, hexagonal rings of C atoms. Every carbon atom is bonded to four other carbon atoms.
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Describe the chemical and physical features of diamond
high melting and boiling point, extremely hard and durable due to strong covalent bonds, does not conduct electricity as there are no delocalised electrons.
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Describe the structure of graphite
giant covalent structure, layers of atoms joined by covalent bonds, London forces between layers. Each Carbon forms three bonds, planar hexagonal rings of atoms. Individual layers of graphite are called graphene.
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Describe the chemical and physical features of graphite
high melting point because of strong covalent bonds, free electrons as C only forms three bonds, therefore conducts electricity - 1 electron per C atom free to move along the layers. Layers slide over each other so graphite rubs off onto surfaces.
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Describe the structure of Buckminsterfullerenes
C60 molecular structure, hexagons and pentagons of Carbon atoms. Each Carbon forms 3 bonds, London forces between molecules. Cage structure. Not giant covalent structure as only involves 60 atoms and is instead a covalent molecular structure.
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Describe the chemical and physical features of Buckminsterfullerenes
lower melting point than diamond and graphite because only London forces must be broken. Used as catalysts and lubricants.
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Explain, in terms of structure and bonding, why carbon dioxide sublimes at -78 degrees but silicon dioxide melts at about 1600 degrees?
CO2 - covalent molecular and SiO2 is giant covalent. Therefore, when silicon dioxide melts, the strong covalent bonds require more energy to be broken in comparison to CO2, where less energy is needed to overcome weak London forces when it sublimes.
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Describe metallic bonding
metals are made up of a regular lattice arrangement of positive ions surrounded by a ‘sea’ of delocalised electrons. Metals have a giant structure.
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Define Metallic Bonding
the electrostatic attraction between the positive ions in the lattice and the delocalised electrons.
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Explain why magnesium has a higher melting point than sodium?
Magnesium forms a 2+ ion compared to sodium, which forms a +1 ion. This means that the electrostatic attraction between the ions and the delocalised electrons is stronger in magnesium. The Mg2+ ion has a smaller atomic radius than the Na+ ion and therefore the delocalised electrons are closer to the nucleus of the positive ion in magnesium and are more strongly attracted.
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Name some physical properties of metals
1. conductors of electricity (free electrons) 2. malleable (layers of metal ions slide over each other without affecting bonding as the delocalised electrons are attracted by the ions in all directions) 3. can form alloys which are useful in construction
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Define the term ‘alloy’
alloys are homogenous mixtures of two or more metals, or of a metal with a non-metal e.g. steel (Fe and C)
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Why do alloys tend to be stronger then their constituent metals?
a different sized atom will prevent planes of smaller metal atoms sliding over each other, making the overall metal less bendy/soft.
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Why do chemists calculate Formal Charge?
to decide which Lewis structure is most appropriate for the molecule/compound
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Give the Formal Charge Equation
FC = (no. of valence electrons in the uncombined atom) - 1/2(no. of bonding electrons) - (no. of non-bonding electrons)
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What value decides which structure to use?
the value closest to 0.
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How do sigma bonds occur?
from the axial (head-on) overlap of atomic orbitals.
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Where is the electron density concentrated in sigma bonds?
mostly along the axis joining the two nuclei.
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How are pi bonds formed?
the sideways overlap of parallel p orbitals.
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Where is the electron density concentrated in pi bonds?
the electron density lies above and below the internuclear axis.
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trigonal bipyramidal
5 electron domains, 5 bonding pairs, 0 lone pairs, 90 and 120 degrees, e,g, PF5
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see-saw
5 electron domains, 4 bonding pairs, 1 lone pair,
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T - shape
5 electron domains, 3 bonding pairs, 2 lone pairs,
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Linear, TBP
5 electron domains, 2 bonding pairs, 3 lone pairs, 180, e.g. I3-
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octahedral
6 electron domains, 6 bonding pairs, 0 lone pairs, 90, e.g. SF6
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square pyramidal
6 electron domains, 5 bonding pairs, 1 lone pair,
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square planar
6 electron domains, 4 bonding pairs, 2 lone pairs, 90, e.g. XeF4
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Define Hybridisation
the mixing of atomic orbitals (s and p) on a particular atom to produce a new set of orbitals that have both s and p character and are better arranged in space for covalent bonding.
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What hybridisation do molecules with 2 electron domains have?
sp
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What hybridisation do molecules with 3 electron domains have?
sp2
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What hybridisation do molecules with 4 electron domains have?
sp3
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What does sp hybridisation mean in relation to ethyne?
mixing of 1s and 1p orbital on each Carbon
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What does sp2 hybridisation mean in relation to ethene?
mixing of 1s and 2p orbitals on each Carbon
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What does sp3 hybridiation mean in relation to methane?
mixing of 1s and 3p orbitals
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Define the term Delocalisation
the sharing of a pair of electrons between three or more atoms
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Relate declocalisation to O3
The Lewis strucutre of O3 would suggest one short double O bond and one longer O-O bond, however the bond lengths are equal as the two electrons in the pi bond are delocalised over the three O atoms.
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What is the bond order of ozone?
2 sigma bonds and 1 pi bond shared across 2 electron domains = 3/2 = 1.5
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How is the surface of the earth protected by UV rays from the sun?
UV radiation is absorbed by O2 and O3 as they undergo dissociation (where their bonds are broken). O2→ O and O3→ O + O2, O2 absorb higher frequnecy radiation (more damaging) as it has a pi bond which makes the molecule stronger.
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What molecules break down ozone in the atmosphere?
chlorofluorocarbons (CFCs) such as CCl2F2 are broken down by absorbing UV radiation in the upper atmosphere, forming Cl2.
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How does the product of Cl2 affect ozone?
Cl2 has unpaired electrons causing it to take place in a free radical chain reaction which uses up ozone and regenerates the free radical
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