Bonding

Question 1

What is the electron pair repulsion theory?

Answer 1

You can predict the shape of a simple covalent molecule, for example, one consisting of a central atom surrounded by a number of other atoms, using the ideas that:

Each pair of electrons around an atom will repel all other electron pairs

The pairs of electrons will therefore take up positions as far apart as possible to minimise repulsion

Question 2

What does the shape of a simple molecule depend on?

Answer 2

The number of pairs of electrons that surround the central atom

Question 3

What are ions? Why are oppositely charged ions attracted to each other?

Answer 3

Charged particles that result from the transfer of an electron.

Ions are attracted to each other and to other oppositely charged ions by electrostatic forces.

Question 4

What state are ionic compounds at room temperature? Why?

Answer 4

Solids. They have a giant structure and therefore high melting temperatures. This is because in order to melt an ionic compound, energy must be supplied to break up the lattice of ions.

Question 5

Why can ionic compounds only conduct electricity when molten or dissolved in water?

Answer 5

Because the ions that carry the current are free to move in the liquid state but are not free in the solid state.

Question 6

Why are ionic compounds brittle and shatter easily when given a sharp blow?

Answer 6

Because they form a lattice of alternating positive and negative ions. A blow in a certain direction may move the ions and produce contact between ions with like charges.

Question 7

What is a covalent bond?

What is a molecule?

Answer 7

A covalent bond is a shred pair of electrons.

A molecule is a small group of covalently bonded atoms. They’re neutral because no electrons have been transferred from one atom to another.

Question 8

How are atoms with covalent bonds held together?

Answer 8

By the electrostatic attraction between the nuclei and the shared electrons. This takes place within the molecule.

Question 9

Why are molecules poor conductors of electricity?

Answer 9

Because they’re neutral overall meaning there are no charged particles to carry the the current.

Question 10

What is co-ordinate bonding (or dative covalent bonding)?

Answer 10

When one atom provides both electrons for a covalent bond

Question 11

What is metallic bonding?

Answer 11

Metals consist of a lattice of positive ions existing in a ‘sea’ of outer electrons.

Question 12

Why are metals good conductors of electricity and heat?

Answer 12

Electricity - a delocalised electron from the negative terminal of the supply joins the electron sea at one end of a metal wire while at the same time a different electron leaves the wire at the positive terminal.

Heat - the sea of electrons + energy can be spread by increasingly vigorous vibrations of the closely packed ions

Question 13

What 2 things do the strength of a metallic bond depend on?

Answer 13

•The charge on the ion - the greater the charge on the ion, the greater the number of delocalised electrons and the stronger the electrostatic attraction between the positive ions and the electrons.

The size of the ion - the smaller the ion, the closer the electrons are to the positive nucleus and the stronger the bond.

Question 14

Why do metals have high melting and boiling points?

Answer 14

Because they have giant structures. There is a strong attraction between metal ions and the delocalised sea of electrons.

Question 15

Define electronegativity

Answer 15

Electronegativity is the power of an atom to attract the electron density in a covalent bond towards itself.

Question 16

What 3 things does electronegativity depend on?

Answer 16

The nuclear charge, the distance between the nucleus and the outer shell electrons, the shielding of the nuclear charge by electrons in inner shells

Question 17

What is a polar bond?

Answer 17

A covalent bond between two atoms that have a big difference in electronegativity. This difference in electronegativity means that the electrons in the bond will not be shared equally between the atoms.

Question 18

What causes VDWs forces?

Answer 18

Electrons in charge clouds are always moving really quickly. At any moment, the electrons in an atom are likely to be more to one side than the other. At this moment, the atom would have a temporary dipole.

This dipole can cause another temporary dipole in the opposite direction on a neighbouring atom. The two dipoles are then attracted to each other.

The second dipole can cause yet another dipole in a third atom.

Because the electrons are constantly moving, the dipoles are being created and destroyed all the time.

Question 19

Why does the size of the VDWs increase with the number of electrons present?

Answer 19

The dipole is caused by the changing position of the electron cloud, so the more electrons there are, the larger the instantaneous dipole will be.

Question 20

What is the electron repulsion theory?

Answer 20

Each pair of electrons around an atom will repel all other electron pairs.

The pairs of electrons will therefore take up positions as far apart as possible to minimise repulsion.

Question 21

Name and bond angle for a molecule with 2 shared electron pairs (no lone pairs)

Answer 21

Linear

180

Question 22

Name and bond angle for a molecule with 3 shared electron pairs (no lone pair)

Answer 22

Trigonal planar

120

Question 23

Name and bond angle for a molecule with 4 shared electron pairs (no lone pair)

Answer 23

Tetrahedral

109.5

Question 24

Name and bond angle for a molecule with 4 electron pairs (3 shared and 1 lone)

Answer 24

Trigonal pyramidal

107

Question 25

Name and bond angle for a molecule with 4 electron pairs (2 shared and 2 lone)

Answer 25

Bent

104.5

Question 26

Name and bond angle for a molecule with 5 shared electron pairs (no lone pair)

Answer 26

Trigonal bipyramidal

90 and 120

Question 27

Name and bond angle for a molecule with 5 electron pairs (4 shared and 1 lone)

Answer 27

Seesaw

102 and 87

Question 28

Name and bond angle for a molecule with 5 electron pairs (3 shared and 2 lone)

Answer 28

T-shaped

88

Question 29

Name and bond angle for a molecule with 6 electron pairs (no lone pair)

Answer 29

Octahedral

90

Question 30

Name and bond angle for a molecule with 6 electron pairs (2 lone and 4 shared)

Answer 30

Square planar

90

Question 31

What happens when you first heat a solid and supply energy to the particles?

Answer 31

It makes them vibrate more about a fixed position. This slightly increases the average distance between the particles and so the solid expands.

Question 32

What is the enthalpy change of melting?

Answer 32

The energy needed to weaken the forces that act between particles, holding them together in the solid state.

Question 33

What is the difference between enthalpy and temperature?

Answer 33

Enthalpy is the heat energy change measured under constant pressure whilst temperature depends on the average kinetic energy of the particles and is therefore related to their speed - the greater the energy, the faster they go.

Question 34

What is the enthalpy change of vaporisation?

Answer 34

The energy needed to break all the intermolecular forces between particles (turning a liquid into a gas).

Question 35

Describe ionic crystals

Answer 35

Strong electrostatic attractions between oppositely charged ions.

Have a high melting point due to the strong electrostatic attractions which extend throughout the structure. These require a lot of energy to break in order for the ions to move apart from each other.

Question 36

Describe metallic crystals

Answer 36

Metals exist as a lattice of positive ions embedded in a delocalised sea of electrons.

The attraction of positive to negative extends throughout the crystal.

The high melting temperature is a result of these strong metallic bonds.

Question 37

Describe a molecular crystal

Answer 37

Consist of molecules held in a regular array by intermolecular forces.

Covalent bonds within the molecules hold the atoms together but they do not act between the molecules.

Intermolecular forces are weak so molecular crystals have low melting temperatures and low enthalpies of melting.

They do not conduct electricity and are soft/break easily.

Question 38

Name 3 macromolecular crystals

Answer 38

Dimond, graphite, silica