Period 3 is the third row of the Periodic Table: sodium, magnesium, aluminium, silicon, phosphorus, sulfur, chlorine and argon. Across the period each element has one more proton and one more electron, and the outer electrons fill the 3s and then the 3p sub-level (Na and Mg are s-block; Al to Ar are p-block).The structure and bonding of the elements changes across the period:
• Na, Mg and Al are metals with metallic bonding (a lattice of positive ions in a sea of delocalised electrons).
• Si is macromolecular (giant covalent), like diamond.
• Phosphorus (P4), sulfur (S8) and chlorine (Cl2) are simple molecular, with weak van der Waals forces between the molecules.
• Argon exists as single atoms.So the period runs from metals on the left, through silicon, to non-metals on the right. Metallic character decreases because nuclear charge increases while shielding stays about the same, so the outer electrons are held more tightly and are less easily lost to form positive ions.The same change shows up in the oxides. The metals Na, Mg and Al form ionic oxides, while Si, P and S form covalent oxides. This difference in bonding explains how the elements react, the melting points of their oxides and whether each oxide is basic, amphoteric or acidic, which the next lessons cover.Example: sulfur is 1s2 2s2 2p6 3s2 3p4. It is a p-block non-metal, so it shares or gains electrons rather than losing them.
• Na, Mg and Al are metals with metallic bonding (a lattice of positive ions in a sea of delocalised electrons).
• Si is macromolecular (giant covalent), like diamond.
• Phosphorus (P4), sulfur (S8) and chlorine (Cl2) are simple molecular, with weak van der Waals forces between the molecules.
• Argon exists as single atoms.So the period runs from metals on the left, through silicon, to non-metals on the right. Metallic character decreases because nuclear charge increases while shielding stays about the same, so the outer electrons are held more tightly and are less easily lost to form positive ions.The same change shows up in the oxides. The metals Na, Mg and Al form ionic oxides, while Si, P and S form covalent oxides. This difference in bonding explains how the elements react, the melting points of their oxides and whether each oxide is basic, amphoteric or acidic, which the next lessons cover.Example: sulfur is 1s2 2s2 2p6 3s2 3p4. It is a p-block non-metal, so it shares or gains electrons rather than losing them.