3.2.1 Periodicity

Question 1

Describe the trend in atomic radius of the period 3 elements

Answer 1

Decreases across the group

Question 2

Describe the trend in melting point of the period 3 elements (4)

Answer 2

Increases - Na, Mg, Al
Spikes upwards - Si
Much lower, increasing - P, S
Decreases - Cl, Ar

Question 3

Describe the trend in first ionisation energy of the period 3 elements (2)

Answer 3

Increases in general

Decreases - Al, S

Question 4

Explain the trend in atomic radius of the period 3 elements (3)

Answer 4

Number of protons increases so nuclear charge increases
Electrons are pulled closer to the nucleus
Shielding stays the same

Question 5

Explain the trend in melting point of the period 3 elements (3)

Answer 5

Na, Mg and Al are all metals - m.p increases across the period as the charge on the positive ions increases, there are more delocalised electrons and atomic radius decreases
Si is macromolecular - a lot of strong covalent bonds require a lot of energy to break
P4, S8, Cl2 and Ar are all simple covalent molecules - the bigger the molecule, the stronger the Van der Waals forces

Question 6

Explain the trend in first ionisation energy of the period 3 elements (3)

Answer 6

General increase across period due to increasing nuclear charge
Al - decreases slightly as outer electron is in P sub-shell rather than S (higher energy sub-shell so less energy needed)
S - decreases slightly as there are three orbitals in P sub-shell so the outer electron is paired with another in one of the orbitals and there is extra repulsion