I1: 8 Periodicity

Question 1

What is the trend in atomic radius as you go across Period 3?

Answer 1

Atomic radius decreases.
This is because the number of protons and electrons increases but the shielding stays similar. The nuclear charge is greater so outer electrons are pulled in close.

Question 2

What is the trend in first ionisation energies as you go across Period 3?

Answer 2

First ionisation energies increases.
This is because the number of protons increases so there’s increased attraction between the nucleus and outer electrons, but shielding stays the same.

Question 3

Why are there deviations from the trend in first ionisation energies as you go across Period 3?

Answer 3

There’s a dip from Group 2 to 3. This is because there’s a change in orbitals and p-electrons are slightly further away from the nucleus so less energy is required to remove an electron.
There’s also a dip from Group 5 to 6. This is because there’s a pair of electrons in Group 6’s highest energy level which repel each other, making it easier to remove an electron.

Question 4

What is the trend in melting points as you go across Period 3?

Answer 4

Melting points increase from Na to Si because Na, Mg, and Al are all metals; their metallic bonding means there’s strong electrostatic attraction between positive ions and the sea of delocalised electrons. The metallic bonding strength increases because the number of electrons increases so the attraction increases.
Si is macromolecular so the melting point is even higher due to the strong covalent bonds.
Melting points decrease from Si to Ar because P, S, and Cl are all molecular.
There’s a slight increase from P to S because, in their natural states, S has more covalent bonds (P4, S8).

Question 5

Why would an element be classified as s, p, d, or f block?

Answer 5

Its highest energy electrons are in that block.