Groups IIA, IIIA, and IVA
The elements in Group IIA (Be, Mg, Ca, Sr, Ba and Ra) are all metals, and all but Be and Mg are active metals. These elements are often called the alkaline-earth metals. The term alkaline reflects the fact that many compounds of these metals are basic or alkaline. The term earth was historically used to describe the fact that many of these compounds are insoluble in water. Most of the chemistry of the alkaline-earth metals (Group IIA) can be predicted from the behavior of the alkali metals (Group IA). Three points should be kept in mind, however.
- The alkaline-earth metals tend to lose two electrons to form M 2+ ions (Be2+, Mg2+, Ca2+, and so on).
- These metals are less reactive than the neighboring alkali metal. Magnesium is less active than sodium; calcium is less active than potassium; and so on.
- These metals become more active as we go down the column. Magnesium is more active than beryllium; calcium is more active than magnesium; and so on.
The alkaline-earth metals react with nonmetals to give the products expected from the electron configurations of the elements.
Mg(s) + Cl2(g) MgCl2(s)
3 Mg(s) + N2(g) Mg3N2(s)
Ca(s) + H2(g) CaH2(s)
Because they are not as active as the alkali metals, most of these elements form oxides.
2 Mg(s) + O2(g) 2 MgO(s)
Calcium, strontium, and barium can also form peroxides.
Ba(s) + O2(g) BaO2(s)
The more active members of Group IIA (Ca, Sr, and Ba) react with water at room temperature. The products of these reactions are what we might expect. Calcium, for example, loses two electrons to form Ca2+ ions when it reacts with water.
Ca Ca2+ + 2 e-
These electrons are picked up by the water molecules to form H2 gas and OH- ions.
2 H2O + 2 e- H2 + 2 OH-
Combining the two halves of the reaction so that electrons are conserved gives the following result.
Ca(s) + 2 H2O(l) Ca2+(aq) + 2 OH-(aq) + H2(g)
Although Mg does not react with water at room temperature, it will react with steam. The products of this reaction can't be aqueous Mg2+ and OH- ions because there is no liquid water around to stabilize these ions. The products of this reaction are H2 gas and magnesium oxide, MgO.
Mg(s) + H2O(g) MgO(s) + H2(g)
|Practice Problem 2:
Magnesium reacts with hydrogen to form compound A, which is a white solid at room temperature. It also reacts with hydrochloric acid to form gas B and an aqueous solution of compound C. Identify the products of these reactions and write balanced equations for each reaction.
The elements in Group IIIA (B, Al, Ga, In, and Tl) can be divided into three classes.
- Boron is the only element in this group that is not a metal. It behaves like a semimetal or even a nonmetal.
- Aluminum is the third most abundant element in the earth's crust. It is just slightly less reactive than the active metals.
- The other three elements in this group are active metals, but they are so scarce they are of limited interest. Gallium, indium, and thallium combined total less than 10-10% of the earth's crust.
Discussions of the Group IIIA metals therefore revolve around the chemistry of aluminum, which can be understood by assuming that the metal reacts to form compounds in which it has an oxidation number of +3.
2 Al(s) + 3 Br2(l) Al2Br6(s)
4 Al(s) + 3 O2(g) 2 Al2O3(s)
16 Al(s) + 3 S8(s) 8 Al2S3(s)
Aluminum reacts with concentrated acids to give Al3+ ions and H2 gas.
2 Al(s) + 6 H+(aq) 2 Al3+(aq) + 3 H2(g)
It also reacts with concentrated bases to give H2 gas and the aluminate ion, Al(OH)4-, in which aluminum is in the +3 oxidation state.
2 Al(s) + 2 OH-(aq) + 6 H2O(l) 2 Al(OH)4-(aq) + 3 H2(g)
The elements in Group IVA can be divided into three classes:
- carbon, which is a nonmetal;
- silicon and germanium, which are semimetals; and
- tin and lead, which are metals. Tin and lead are among the oldest metals known.
Tin and lead are much less reactive than any of the groups IA, IIA, or IIIA metals. According to the argument that elements become more metallic and therefore more active as we go down a column of the periodic table, lead should be more reactive than tin.
Lead reacts with air to form a thin coating of PbO and/or PbCO3, which protects the metal from further reaction.
2 Pb(s) + O2(g) 2 PbO(s)
PbO(s) + CO2(g) PbCO3(s)
When finely divided, lead is pyrophoric it bursts into flame in the presence of oxygen.
Tin does not react with either air or water at room temperature. When heated until white hot, tin reacts with air to form SnO2.
Sn(s) + O2(g) SnO2(s)
At high temperatures it also reacts with steam to give SnO2.
Sn(s) + 2 H2O(g) SnO2(s) + 2 H2(g)
Tin and lead are both less active than aluminum. Neither metal reacts with either dilute hydrochloric acid or dilute sulfuric acid at room temperature. Tin, when heated, reacts with either concentrated hydrochloric acid or concentrated sulfuric acid.
Sn(s) + 2 H+(aq) Sn2+(aq) + H2(g)
Lead reacts slowly with hydrochloric acid at room temperature and with concentrated sulfuric acid at temperatures above 200C.
Pb(s) + 2 H+(aq) Pb2+(aq) + H2(g)
Tin and lead are main group metals that form compounds in more than one oxidation state. As a rule, the lower oxidation state becomes more stable as we go down a column of the periodic table. Lead, for example, is more likely to be found in the +2 oxidation state than tin. Thus, while tin reacts with oxygen at high temperatures to form SnO2, lead forms PbO. (PbO2 cannot be formed by the direct reaction between lead and oxygen.)