Group 2 Elements (OCR A Level Chemistry A): Revision Note

Structure & Ions

• All elements in Group 2 (also called alkali earth metals) have two electrons in their outermost shell in the s sub-shell (s² electron configuration)

• Redox reactions are the most common reaction of the Group 2 elements

• Each metal atom is oxidised, losing its two valence electrons to form a 2+ ion with a noble gas configuration

Ca → Ca²⁺ + 2e^-

• The calcium changes from an electron configuration of [Ar]4s² to [Ar]

• Another species will gain the two electrons lost

  • This species will therefore be reduced

• The Group 2 element can be referred to as a reducing agent as it has reduced another species

Relative Reactivity

• The reactivity of Group 2 elements increases down the group.

  • As the atomic radii increase there is more shielding

  • The nuclear attraction decreases and it is easier to remove outer electrons

  • Cations form more easily

Reactions with water and oxygen

• The reaction of group 2 metals with oxygen follows the following general equation:

2M (s) + O₂ (g) → 2MO (s)

          Where M is any metal in group 2

• We can see this is a redox reaction by looking at the changes in oxidation number of the metal, M, and oxygen

  • M = 0 → +2 so the metal has been oxidised

  • O = 0 → -2 so the oxygen has been reduced

• The reaction of all group 2 metals with water follows the following general equation:

M (s) + 2H₂O (l) → M(OH)₂ (s) + H₂ (g)

• We can again see this is a redox reaction by taking a look at changes in the oxidation number of each element in the reaction

  • M = 0 → +2 so the metal has been oxidised

  • H = +1 → 0 so the hydrogen has been reduced

  • No change in oxidation number for oxygen

• The solubility of the hydroxides increases down the group

Reactions of Group 2 metals with acid

• The Group 2 metals will react with dilute acids to form metal salts 

  • For example, they will form metal chlorides if reacted with hydrochloric acid, HCl

• When metals react with an acid, the by-product of this reaction is hydrogen gas

  • The reaction of the metals with dilute HCl follows the following general equation:

M (s) + 2HCl (aq) → MCl₂ (aq) + H₂ (g)

• We can again see this is a redox reaction by taking a look at changes in the oxidation number of each element in the reaction

  • M = 0 → +2 so the metal has been oxidised

  • H = +1 → 0 so the hydrogen has been reduced

  • No change in oxidation number of chlorine

• The reaction of the metals with dilute sulfuric acid, H₂SO₄, follows the following general equation:

M (s) + H₂SO₄ (aq) → MSO₄ (aq) + H₂ (g)

• When some of Group 2 metals react with sulfuric acid rather than hydrochloric, an insoluble sulfate forms

• Going down the group, the Group 2 sulfates become less and less soluble

  • Calcium sulfate is sparingly soluble, but strontium sulfate and barium sulfate are insoluble

Trends in Ionisation Energy

• We can think about the increasing reactivity moving down Group 2 in terms of ionisation energies

• The atoms of Group 2 elements react by losing electrons to form +2 ions

• The formation of +2 ions from gaseous atoms requires two ionisation energies

M(g) → M⁺(g) + e^-

M⁺(g) → M²⁺(g) + e^-

• Both the first and second ionisation energies decrease down the group as

  • The attraction between the nucleus and outer electrons decreases

  • Atomic radius increases, and there is increased shielding

• Despite other energy changes taking place when Group 2 elements react, the first and second ionisation energies make up most of the energy input

• Therefore, as the total energy input from ionisation energies to form 2+ ions decreases down the group, the elements become more reactive and stronger reducing agents

First and second ionisation energies of Group 2 elements