Periodic Table (JC)

Question 1

How does atomic radii changes across the Period?

Answer 1

Across a Period, atomic radii of the elements decrease gradually. This is because there is an increase in nuclear charge, while shielding effect by inner principal quantum shells of electrons remains relatively constant as the number of inner principal quantum shells remains the same. Hence, effective nuclear charge increases.

Question 2

How does atomic radii of elements change down the group?

Answer 2

Down the group, atomic radii of the elements increase gradually. This is because number of filled principal quantum shells increases. Effective nuclear charge is relatively constant as the increase in nuclear charge is largely cancels out by the increase in shielding effect by the increase in inner principal quantum shells of electrons.

Question 3

How does electrical conductivities vary across Period 3?

Answer 3

Electrical conductivities are high for the metals Na, Mg and Al, and increase from Na to Al. This is because Na, Mg, and Al have giant metallic structures, and number of delocalised valence electrons increases from Na to Al.
Electrical conductivity drops sharply at Si as Si has a giant covalent structure and is a semiconductor.
Electrical conductivity drops to zero to P4 and remains at zero to Ar. P4 to Cl2 have simple covalent structures, and Ar has a monoatomic structure. Hence, there are no mobile charge carriers to conduct electricity.

Question 4

Describe reaction of NaCl with water (with equations)

Answer 4

NaCl(s) dissolves readily in water to form a neutral solution of pH=7. Only hydration of ions takes place and no hydrolysis of Na+ occur as Na+ has a low charge density and hence low polarising power.

Question 5

Describe reaction of MgCl2 with water (with equations)

Answer 5

MgCl2 dissolves readily in water to form a weakly acidic solution of pH=6. Both Hydration and partial hydrolysis occurs. Hydrolysis occurs due to the polarisation of water molecules by the Mg2+ ion.

Question 6

Describe and explain AlCl3 reaction with water (with equations)

Answer 6

AlCl3 dissolved readily in water to form an acidic solution of pH=3. Both hydration of ions and substantial hydrolysis of Al3+ occurs. Due to its high charge density, Al3+ is highly polarising and weakens the O-H bonds in the water molecules of the complex, causing the O-H bonds to break and hence release the hydrogen ions.

Question 7

Describe and explain SiCl4(l) reaction with water (with equation) + Explain why CCl4 does not react with water.

Answer 7

SiCl4 dissolves in water to form a strongly acidic solution of pH=2. SiCl4 undergoes hydrolysis in water as it has energetically accessible vacant 3d orbitals for dative bonding with water molecules.
SiCl4(l) + 2H2O(l) -> SiO2(s) + 4HCl(aq)
CCl4 do not react with water as it has no energetically accessible 3d orbitals for dative bonding, as the vacant 3s,3p and 3d orbitals are too high in energy and not available for bonding.

Question 8

Describe and explain Pcl5(s) reaction with water [Limited and Excess]. (with equations).

Answer 8

PCl5(s) and PCl3(l) undergo hydrolysis in water to give strongly acidic solution of pH=2, as P atom is able to use energetically accessible 3d orbitals for dative bonding with water molecules.
Limited Water: PCl5(s) + H2O -> POCl3(l) + 2HCl (aq)
Excess water: POCl3(l) + 3H2O(l) -> H3PO4 (aq) + 3HCl (aq)