The structure and physical properties of the elements :

• Sodium Na, magnesium Mg and aluminium Al are silvery solids, with a metal lattice structure, high boiling points and are good conductors of heat/electricity due to the delocalised free electrons moving between the immobile metal ions.
• Si has a non-metallic giant covalent structure based on a tetrahedral arrangement of Si-Si bonds and is a poor conductor of heat/electricity.
• Phosphorus P₄, sulfur S₈ and chlorine Cl₂ are simple-small covalent molecules and Ar consists of single atoms.

The hydrides of period 3 elements.

The hydrides formed by period 3 elements are NaH, MgH₂, AlH₃, SiH₄, PH₃, H₂S, HCl. These hydrides are formed by action of hydrogen on the elements.

NaH and MgH₂ have high melting and boiling points due to the strength of ionic bonds. H₂S and HCl have relatively higher melting and boiling points than SiH₄ and PH₃ because their molecules are polar molecules and are held together by hydrogen bonds whereas the molecules of SiH₄ and PH₃ are non-polar and therefore, held together by the weak van der Waals forces.