3.1.4 Periodicity - Properties of the elements of Period 3


Students should:
  • to illustrate periodic trends
  • be able to describe the trends in atomic radius, first ionisation energy, melting and boiling points of the elements Na to Ar
  • understand the reasons for the trends in these properties

3.2 - Physical Properties

Characteristic Trend (left to right) Reason
Atomic radius decreases in size from left to right increased attractive force (acting on the same energy shell) of the nucleus increases as the number of protons increases
Ionic radius decreases across the period until formation of the negative ions then there is a sudden increase followed by a steady decrease to the end In general as above. The sudden increase on formation of negative ions is due to the new (larger) outer shell
Electronegativity Increases More electron attracting power of the larger nuclear charge as we move to the right
Metallic character Decreases - Na, Mg, Al metals; Si metalloid; P, S, Cl, Ar non-metals Metallic character is a measure of the ease of loss of electrons from the outer shell. This decreases with increasing nuclear charge.
Melting point Na Al steady increase Increasing availability of electrons in the metallic bonding associated with greater charge density of the metal ion
Si massive increase Si giant macromolecular structure
P large decrease P4 molecules
S small increase S8 crown shaped molecules
Cl Ar decrease Cl2 molecules and Ar atoms


quick test