Unit 1-Periodicity

Question 1

What is meant by the covalent radius of an atom?

Answer 1

The covalent radius is a measure of the size of an atom.

Question 1

Explain the trend in covalent radius across a period

Answer 1

Across a period from left to right, the covalent radius decreases.

Increasing/greater/stronger/larger nuclear charge (holds electrons more closely)
OR
Increasing number of protons

Question 1

Explain the trends in covalent radius down a group.

Explain why this arises.

Answer 1

As you move down a group in the periodic table, the covalent radius increases. Atoms increase in size.

This is because of the extra outer energy level and the screening effect of the outer electrons are further away from the nucleus and so are not as attracted to the positive charge.

Question 1

What is meant by the ionisation energy of an atom?

Answer 1

The ionisation energy is the energy involved in removing one mole of electrons from one mole of atoms in the gaseous state.

Question 1

Magnesium has a 1^st ionisation energy of 744 kJmol^-1

2^nd ionisation energy of 1460 kJmol^-1

3^rd ionisation energy of 7750 kJmol^-1

Explain this trend.

Answer 1

The third ionisation energy shows a massive increase because it requires an electron to be removed from magnesium’s second energy level.

Question 2

Explain the trends in ionisation energy across a period.

Answer 2

Across a period from left to right, the ionisation energy increases.

This is due to the increase in atomic charge having a greater pull on the electrons and therefore more energy is required to remove electrons.

Question 3

Explain the trends in ionisation energy down a group.

Answer 3

Going down a group, the ionisation energy decreases.

This is due to the outer electrons being further away from the nucleus and so the attraction is weaker and they are more easily removed.

Question 4

What is meant by an atom’s electronegativity?

Answer 4

Electronegativity is a measure of an atom’s attraction for the electrons in a bond.

Question 5

Explain the trend in electronegativity across a period.

Answer 5

Across a period from left to right the electronegativity of atoms increases.

As you move from left to right across the periodic table, atoms have a greater charge in their nucleus and a smaller covalent radius. This allows the nucleus to attract the bonding electrons more strongly.

Question 6

Explain the trend in electronegativity down a group.

Answer 6

Going down a group, the electronegativity of atoms decreases.

As you move down a group in the periodic table, atoms increase in size, with a greater number of energy levels.

The extra energy levels and increased covalent radius keep the bonding electrons further away from the nucleus.

This screening effect means that atoms further down groups have less attraction for the bonding electrons.

Question 7

The first 20 elements in the Periodic Table are categorised according to which four bonding and structures (hydrogen to calcium)

Answer 7

• metallic (Li, Be, Na, Mg, Al, K, Ca)
• covalent molecular (H₂, N₂, O₂, F₂, Cl₂, P₄, S₈ and fullerenes (eg C₆₀))
• covalent network (B, C (diamond, graphite), Si)
  
  • monatomic (noble gases)

Question 8

Describe bonding and structure in metallic elements.

Answer 8

Metallic bonding occurs between the atoms of metal elements. The outer electrons are delocalised (free to move).

This produces an electrostatic force of attraction between the positive metal ions and the negative delocalised electrons.

Question 9

Describe bonding and structure in covalent molecular elements.

Answer 9

Covalent molecules are small groups of atoms held together by strong covalent bonds inside the molecule and weak intermolecular forces between the molecules.

Examples (H₂, N₂, O₂, F₂, Cl₂, P₄, S₈ and fullerenes (eg C₆₀₎)

Question 10

Describe bonding and structure in covalent network elements.

Answer 10

Covalent networks are large, rigid three-dimensional arrangements of atoms held together by strong covalent bonds.

Question 11

Describe bonding and structure in monatomic elements.

Answer 11

Single, unattached particles. They are stable atoms. Weak London dispersion forces.

Question 12

Compare the densities, melting point and electrical conductivity of elements which exists as:

1. Monatomic
2. covalent molecules
3. covalent networks
4. metallic lattices

Answer 12

1. Monatomic (noble gas) - low density, low melting point, non-conductor
2. Covalent molecular (N₂, P₄, S₈) - low density, low melting point, non-conductor
3. Covalent network - very high density, very high melting point, non-conductor (except graphite)
4. Metallic lattices (all metals) - high density, high melting point, conductor