9. Periodicity

Question 1

What is the trend in atomic radius across Period 3?

Answer 1

• Decrease in radius from Na to
• Nuclear charge increases, shielding stays relatively constant
• Greater attraction from nucleus to valence electrons

Question 2

What is the trend in ionic radius across Period 3?

Answer 2

• Decrease in radius from Na to Si (cations) due to increase in positive nuclear charge yet same electron configuration
• Jump from Si to P due increase in shielding as switch from cation to anion
• Decrease in radius from Si to Cl (anions) due to increase in nuclear charge

Question 3

What is the trend in melting point across Period 3?

Answer 3

• Na to Al increase due to increase in cation charge, greater ESFA to delocalised e-
• Al anomaly due to electron in 3s2 orbital, partial shielding causes weaker attraction
• Si has highest MP due to strong bonds in giant covalent structure, requires a lot of energy to break
• P4, S8, Cl2 all simple covalent molecules, only experience London forces, MP ∝ number of e-
• Ar has lowest MP as single atoms, only very weak London forces to overcome

Question 4

What is the trend in electrical conductivity across Period 3?

Answer 4

• Na to Al increase due to increase in number of delocalised electrons free to carry charge
• Silicon is a semiconductor
• The rest are insulators due to lack of delocalised electrons

Question 5

What is the trend in ionisation energy across Period 3?

Answer 5

• General increase due to increase in number of protons, constant shielding, stronger ESFA
• Mg to Al drop as ionising from a higher energy orbital
• P to S drop as electron pairing creates repulsion