Inorganic: Chemical Periodicity (Period 3)

Atomic radius across Period 3

Atomic radius decreases

1) Nuclear charge increases as no. of electrons increases

2) Shielding effect is the same

3) The more electrons, the more the nucleus pulls them closer

Why are anions bigger than cations?

1) Anions have one more shell of electrons than cations, making the outer electrons less strongly attached

2) Less electrons means less shielding of the outer electrons

3) This makes cations smaller

Melting point across Period 3

• Silicon has the highest melting point due to its giant molecular structure with covalent bonds

• P, S, Cl & At all have low melting points due to their simple molecular structure with id-id bonds

Melting points of Period 3 oxides

• MgO has a higher m.p than because of the stronger ionic bonds due to the extra electrostatic attraction by Mg2+

• P4O10 onwards are all simple molecular structures with weak intermolecular forces.

Period 3 oxides with water

Na2O → 2NaOH : pH ≈ 13

MgO → Mg(OH)2 : pH ≈ 9

P4O10 → 4H3PO4 : pH ≈ 2

SO2 and SO3 → H2SO4 and H2SO3 : pH = 1