The reaction of hydrides of period 3 elements with water.

Due to the high polarity of the bonds in the hydrides NaH and MgH2, they react with water readily producing hydrogen and the metal hydroxides.

NaH (s) + H₂O (l) → NaOH (aq) + H₂ (g)
MgH₂ (s) + 2H₂O (l) → Mg(OH)₂ (aq) + 2H₂ (g)

Oxides of period three elements

They are formed by reacting the elements with oxygen. The following table describes the formula and physical properties of period three oxides

• Across the period there is a gradual change in the character of oxides from strongly basic to strongly acidic. Al₂O₃ is an amphoteric oxide, i.e., possesses both basic and acidic properties.
• Na₂O, MgO and Al₂O₃ have high melting points due to the strength of ionic bonding.
• The melting point of Al₂O₃ is lower than that of MgO because Al₂O₃ has bigger covalent character due to high polarizing power of Al³⁺ ions.
• Cl₂O₇ has a low melting point because the molecules are held together by the weak van der Waals forces.

Chlorides of period 3 elements

Physical properties of period 3 chlorides.

NaCl and MgCl₂ have high melting points due to the strength of the ionic bonding. AlCl₃ has a fairly high melting point because, in the solid-state, it consists of Al₂Cl₆ molecules and not simple AlCl₃. These molecules are produced through dative bonding between Al and Cl in the Al2Cl6 molecules.

SiCl₄ and S₂Cl₂ are liquids and consist of respective simple SiCl₄ and S₂Cl₂ molecules and in the solid-state the molecules are held by weak van der Waals force which explains their very low melting points. PCl₅ a pale yellow solid has a fairly high melting point because the solid undergoes partial ionisation.

2PCl₅ ⇋ PCl₄⁺ + PCl₆^–