2.2.2

Question 1

What does electron repulsion theory state?

Answer 1

It states that the shape of a molecule is determined by the number of electron pairs in the outer shell of the central atom.

Question 2

Shapes and bond angles of molecules with 2-6 electron pairs:

Answer 2

2 electron pairs - linear - 180°
3 electron pairs - trigonal planer - 120°
4 electron pairs - tetrahedral - 109.5°
5 electron pairs - trigonal bipyramid - 90° and 120° (between the in and out ones)
6 electron pairs - octahedral - 90°

Question 3

How do lone pairs affect bonding angles?

Answer 3

Since lone pair repulsion is greater than bond pair repulsion, each lone pair reduces the bond angle by about 2.5°.

Question 4

What is electronegativity and how does it effect chemical bonds?

Answer 4

This is defined by the ability of an atom to attract the bonding electrons in a covalent bond.
Due to it, there is a spectrum of ionicity and covalency in chemical bonds.
If there is a larger difference in electronegativity it is more likely to be ionic.

Question 5

What is a permanent dipole?

Answer 5

A permanent dipole is a small charge difference across a bond that results from a difference in the electronegativity of the bonded atoms.

Question 6

What are the intermolecular forces?

Answer 6

-van der Waals’ forces: permanent dipole - permanent dipole interactions and induced dipole - dipole interactions.
-hydrogen bonding.

Question 7

What is an intermolecular force?

Answer 7

An attractive force between neighbouring molecules.

Question 8

What is the permanent dipole - permanent dipole force?

Answer 8

A weak attractive force between permanent dipoles in neighbouring polar molecules.

Question 9

What are London forces?

Answer 9

Attractive forces between induced dipoles in neighbouring molecules. These dipoles are transient as they are created in atoms with the electron cloud being in constant motion.

Question 10

What is induced polarity?

Answer 10

When a molecule with a temporary polarity approaches one which is non-polar, giving it a polarity for that precise moment.

Question 11

What affects the strength of London forces?

Answer 11

If atoms have more electrons, more fluctuations in electron cloud possibilities would exist leading to a greater London Force.

Question 12

What is hydrogen bonding?

Answer 12

Hydrogen bonding is the strongest intermolecular force, it is the dipole - dipole interaction between an electron deficient hydrogen atom (O-H, N-H, or F-H) on one molecule and a lone pair of electrons on a highly electronegative atom (H-O, H-N, or H-F) on a different molecule.

Question 13

How does hydrogen bonding explain water’s properties?

Answer 13

Relatively high melting point: hydrogen bonding is stronger than other intermolecular forces so needs more energy to overcome.
Low density of ice: water molecules arrange themselves into an ordinary pattern and hydrogen bonds form between the molecules. This creates an open lattice with longer hydrogen bonds holding the water molecules apart.

Question 14

What is ionic bonding?

Answer 14

The strong electrostatic attraction between positive and negative ions.
Ionic compounds have a giant lattice structure. Using NaCl as the example, each Na+ ion is surrounded by 6 Cl- ions and vice versa.

Question 15

What and why are the properties of ionic compounds?

Answer 15

-High melting/ boiling points: strong electrostatic attractions between oppositely charged ions which need lots of energy to overcome. Compounds with larger charges (e.g. Mg2+O2+) will be more strongly attracted to each other which means more energy will be needed to overcome the higher electrostatic attraction. This means they will have a higher melting/boiling point.
-ionic compounds do not conduct electricity in the solid state because the ions are in fixed positions within the giant ionic lattice and are therefore not free to move. However, when molten or dissolved in solution, the ionic lattice breaks down and the ions become free to move and conduct electricity.
-solubility: ionic compounds are generally soluble in water, which is polar. To be able to do this the giant ionic lattice needs to break down. The ions are stabilised in solution by the slightly polar water molecules. The ions in the giant ionic lattice are attracted by the water molecules and are removed from the lattice. The ions are stabilised by coordination with water.

Question 16

What’s the bonding and properties of simple covalent compounds?

Answer 16

Simple covalent compounds are held together by weak intermolecular forces, when melting or bonding, only the intermolecular forces break.
Simple molecular structures have no free electrons and therefore do not conduct electricity.
Simple molecular structures dissolve in non-polar solvents, when they do so, they form weak electrostatic interactions with the solvent.
Water is a polar substance and it is energetically unfavourable to break bonds between molecules to dissolve the molecular structures.

Question 17

What’s the bonding and properties of different giant covalent structures?

Answer 17

Diamond: each carbon atom is bonded to 4 other carbon atoms. The shape around each carbon is tetrahedral meaning bond angles are 109.5°. Diamond has high melting and boiling points as it needs enough energy to break the strong covalent bonds between the atoms. Diamond does not conduct electricity as it has no free electrons so can’t carry a charge.
Graphite: each carbon in bonded to 3 other carbon atoms. The structure is formed of hexagonal rings that are bonded together in layers. Each carbon has one unbounded electron which forms an electrostatic force of attraction between the layers. The electrons can be delocalised within the layers. The force between the layers is weak so the layers can slide over one another. The melting and boiling points of graphite is high as lots of energy is require to break the strong covalent bonds. Graphite conducts electricity as the delocalised electrons between the layers can move and carry charge.
Graphene: is a two dimensional, single layer of graphite. It is strong, lightweight, and conducts electricity.
Giant covalent structures are not soluble. The bonds that exist in these structures are too strong for a solvent to overcome.

Question 18

What is the structure of metallic bonding?

Answer 18

Metallic bonding has cations occupying fixed positions in a giant lattice. The outer shell electrons are delocalised. The metals are held together by a strong electrostatic attraction between positive ions and delocalised electrons. A sea of delocalised electrons move throughout the whole structure which must remain charge neutral.

Question 19

What are the properties of metallic bonding?

Answer 19

Melting and boiling points: due to the strong attraction between the cations and electrons, a large amount of energy is needed to dislodge the ions.
Electrical conductivity: as the outer shell electrons are free to move throughout the lattice, they can carry charge.
Malleability and ductility: the electrons can move which allows atoms or layers of regularly arranged atoms to slide over each other.
Solubility: metals are insoluble. The strength of the metallic bonds is too great for any solvent to overcome.

Question 20

What is the average bond enthalpy?

Answer 20

A measure of the strength of a covalent bond

Question 21

What is a dative covalent bond?

Answer 21

A dative covalent bond (written as A->B, where A is donating a pair to B) is where one of the atoms supplies both the shared electrons to the covalent bond.

Question 22

What is a polar molecule?

Answer 22

A molecule with polar bonds and dipoles which do not cancel out due to molecular symmetry.

Question 23

What are giant covalent lattices?

Answer 23

Networks of carbon or silicon atoms bonded by strong covalent bonds.