Properties and Reactions of Group 2 elements and compounds

Trends in first ionization energy

The first ionization energy decreases down Group 2 because:

•  Atomic radius is increasing as the additional electrons fill higher energy levels, so the distance between the positive nucleus and the negative electrons is increasing and the force of attraction is less.

• Shielding of the nucleus increases. Inner shell electrons shield the two outer electrons from the nucleus, further reducing the force of attraction.

These two factors far outweigh the increase in nuclear charge down the group.

Reactions of Group 2 elements

Reactivity usually decreases down the group, as the ionization energy decreases.

• All the Group 2 metals burn in air or oxygen to form solid metal oxides, often burning with a very bright flame. Reactivity increases down the group. In general:

Mg(s) + 0.5O2(g) > MgO(s)

For Barium: Ba(s) + 0.502(g) > BaO(s)

• All the Group 2 metals burn in Chlorine to form solid metal chlorides. In general, reactivity increases down the group.

Mg(s) + Cl2(g) > MgCl2(s)

Ca(s) + Cl2(g) > CaCl2(s)

• Calcium and the elements of the group below it react with water similarly to form hydrogen and the corresponding hydroxide. Reactivity increases down the group.

Mg(s) + 2H2O(g) > Mg(OH)2(aq) + H2(s)

Sr(s) + 2H2O(l) > Sr(OH)2(aq) + H2(g)

Beryllium does not react with water, due to the formation of an insoluble oxide layer. Magnesium reacts very slowly with cold water, but with steam it reacts vigorously to give Magnesium Oxide, which is almost insoluble.

Reaction of Group 2 oxides

• From calcium down the group, the oxides react with water to form the corresponding hydroxides. For example:

CaO(s) + H2O(l) > Ca(OH)2(aq)

Calcium hydroxide has many uses in water purification, and making whitewash, mortar and plaster.

• All the oxides neutralize hydrochloric acid and nitric acid to form the corresponding chlorides or nitrates:

MgO(s) + 2HCl(aq) > MgCl2(aq) + H2O(l)

CaO(s) + 2HNO3(aq) >Ca(NO3)2(aq) + H2O(l)

The hydroxides of the Group 2 elements react with dilute acids in a similar way.

Sr(OH)2(aq) + 2HCl(aq) >SrCl2(aq) + 2H2O(l)

Solubility of Group 2 hydroxides and sulfates

• The solubility of hydroxides increases down the group. Calcium hydroxide is slightly soluble giving a solution called limewater. This solution reacts with carbon…