Chem Chapter 11 (Period 3)

What structures do Period 3 elements have across the period?

Metallic → giant covalent → simple molecular → atomic.

Why do melting points increase from Na to Al?

Increasing ionic charge and number of delocalised electrons strengthens metallic bonding.

Why does silicon have the highest melting point in Period 3?

It has a giant covalent lattice with many strong Si–O (covalent) bonds.

Why do P, S and Cl have low melting points?

They have simple molecular structures with weak van der Waals forces.

Why is sulphur’s melting point higher than phosphorus’?

S₈ molecules have more electrons and greater surface contact than P₄.

What factors affect density?

Atomic mass, atomic size and packing of atoms/molecules.

Why does density increase from Na to Al?

Atomic mass increases and atomic radius decreases.

Why does silicon have a lower density than aluminium?

It has a more open giant covalent lattice.

Why does conductivity increase from Na to Al?

Increasing number of delocalised electrons per atom.

Why is silicon a semiconductor?

It has no delocalised electrons, but some can move at higher temperatures or when doped.

Why do P and S not conduct electricity?

They are covalent molecules with no free electrons.

Why does electronegativity increase across a period?

Increasing nuclear charge attracts bonding electrons more strongly.

Why does atomic radius decrease across Period 3?

Increasing nuclear charge with no extra shielding.

Why are cations smaller than their atoms?

Loss of outer-shell electrons reduces repulsion.

Why are anions larger than their atoms?

Gaining electrons increases electron–electron repulsion.

How does sodium react with water?

Violently, forming NaOH and H₂.

: How does magnesium react with water?

Very slowly with cold water; readily with steam.

Why does aluminium not react with cold water?

A protective oxide layer forms on its surface.

How does chlorine react with water?

Reacts slightly to form a mixture of HCl and HClO.

What type of oxides do Na and Mg form?

Basic oxides.

What type of oxide is SiO₂?

Acidic oxide.

Which Period 3 chlorides are ionic?

NaCl and MgCl₂.

How does oxide acidity change across Period 3?

Basic → amphoteric → acidic.

Why is anhydrous AlCl₃ covalent?

Al³⁺ is small and highly charged, causing ion polarisation.

What happens when SiCl₄ reacts with water?

It hydrolyses to form SiO₂ and HCl.