Periodic Table

Question 1

Effective Nuclear Charge Z_eff

Answer 1

Net nuclear charge experienced by an outer electron

Z_eff = Z - S

Z = nuclear charge
S = Shielding effect

Question 2

Ionic radius across the period

Answer 2

Ionic radius decreases across the period from Na⁺ to Si⁴⁺ and P^3- to Cl^-

Same no, of electrons now attracted more strongly by the increasing nuclear charge

Question 3

Size of anions vs cations

Answer 3

Anions are larger than cations due to one more occupied shell of electrons

Question 4

Electronegativity across the period

Answer 4

Eneg increases across the period
(Eneg is a measure of the tendency of an atom to attract the bonding pair of electrons)

• NC increases, shielding effect constant
• ENC increases
• Stronger EFOA between nucleus and bonding electrons

(similar explanation to IE and radius)

Question 5

Melting Point across period 3

Answer 5

1. Increases across Na, Mg and Al:
   - Increasing no of valence electrons to delocalise
   - Decreasing size of cations leading to increasing charge density
2. Si has highest melting point
   - Large amount of energy needed to overcome the strong covalent bonds between Si atoms in giant covalent structure
3. Lower melting point
   - S₈ > P₄ > Cl₂
   - As no of electrons decrease, size of electron cloud decreases
   - Electron cloud becomes less polarisable and less needy require to over the weaker id-id attractions

Question 6

Electrical conductivity across period 3

Answer 6

1. Na, Mg and Al have increasingly higher electrical conductivity
   - Increase in no. of valence electrons contributing to sea of delocalised electrons
2. Si has low (not zero) electrical conductivity
   - Si is a metalloid
3. P₄, S₈, Cl₂ are non-conductors of electricity
   - Absence of mobile ions or sea of delocalised electrons

Question 7

Covalent character

Answer 7

1. Polarising power of cation (charge density, q/r)
   - ability to distort electron cloud of anion
2. Polarisability of anions (radius of anion)
   - how easily anion electron cloud is distorted

Question 8

Trend of bonding/structure of period 3 oxides across the group

Answer 8

1. Na₂O, MgO and Al₂O₃ are giant ionic lattices (LE, q⁺q^- / r⁺ + r^-)
2. SiO₂ is giant covalent (strong covalent bonds)
3. P₄O₆, P₄O₁₀ and SO₂, SO₃ are simple molecular (weak id-id)

• Across the period the bonding of the oxides change from ionic to covalent.
• This is due to the difference in Eneg decreasing across the period, leading to bonding becoming increasingly covalent

Question 9

Melting point of period 3 oxides across the period

Answer 9

1. Na₂O < Al₂O₃ < MgO
   - LE ∝
2. SiO₂ is giant covalent (strong covalent bonds)
3. P₄O₆, P₄O₁₀ and SO₂, SO₃ are simple molecular (weak id-id)

Question 10

Oxidation number of period 3 oxides across the period

Answer 10

1. Oxidation number of metal in oxides increase across the period
   - No. of valence electrons available for bond formation increase
2. From Na to Si, the oxidation number of the oxide is equal to the number of valence electrons in an atom of the element
3. P and S can exhibit multiple oxidation numbers due to vacant and energetically accessible 3d orbitals

Question 11

Why are ionic (metal) oxides basic?

Answer 11

The basic O^2- reacts with H₂O to get OH^-

O^2- + H₂O → 2OH^-

Question 12

Why are covalent (non-metal) oxides acidic?

Answer 12

They produce H₃O⁺ and an oxo-anion (SO₄^2- and PO₄^3-)

Question 13

Why is Al₂O₃ amphoteric

Answer 13

Al₂O₃ has both ionic and covalent properties

Al³⁺ has a high charge density and thus giving it great polarizing power, polarizing the O^2- ion and conferring some covalent character.

Question 14

Na₂ O in water

Answer 14

Na₂O + H₂O → 2NaOH
pH = 13

Question 15

MgO in water

Answer 15

MgO + H₂O → Mg(OH)₂
pH = 9

Question 16

Al₂O₃ in water

Answer 16

Insoluble, pH = 7

Question 17

SiO₂ in water

Answer 17

Insoluble, pH = 7

Question 18

P₄O₁₀ in water

Answer 18

P₄O₁₀ + 6H₂O → 4H₃PO₄
pH = 2

Question 19

SO₃ in water

Answer 19

SO₃ + H₂O → H₂SO₄