2.1.2 Physical Properties of Period 3 Elements

Question 1

Explain why phosphorus has a lower melting point than sulphur.

Answer 1

• phosphorus exists as P₄ whilst sulphur exists as S₈
• sulphur is bigger than phosphorus (more electrons)
• sulphur has stronger Van Der Waals forces
• so more energy is required to break the intermolecular forces of sulphur

Question 2

State & explain the general trend in first ionisation energy across period 3.

Answer 2

generally increases as across the period:
→ increasing nuclear charge
→ similar shielding
→ stronger attraction between outer e- & nuclues
→ more energy required to overcome

Question 3

Why is the 1st ionisation energy of aluminium less than that of magnesium?

Answer 3

• the Al electron is in a 3p orbital
• the Mg electron is in 3s orbital
• 3p has a higher energy than 3s

Question 4

Why is the 1st ionisation energy of krypton greater than that of bromine?

Answer 4

• Krypton has more protons
• its outer electrons are in the same shell
• stronger attraction between e- & nucleus
• requires more energy

Question 5

Explain why the 1st ionisation energy of sulfur is less than that of phosphurus.

Answer 5

• in sulfur, the electrons start pair up in 3p orbital
• this causes repulsion
• therefore requires less energy to remove the e-

Question 6

State & explain the general trend in atomic radius energy across period 3.

Answer 6

increases across the period:
→ increasing nuclear charge
→ similar shielding
→ stronger attraction between outer e- & nucleus
→ (shells are pulled in more)

Question 7

State & explain the general trend in conductivity across period 3.

Answer 7

decreases across the period:
→ Na-Al are metals
→ Si-Ar are non-metals
→ non-metals cannot conduct electricity as they have no free electrons