Group 2 Reactivity

Learning Objectives (OCR Spec 3.1.2)

By the end of this lesson you should be able to:

• State the outer electron configurations of Group 2 elements (Be → Ba)
• Describe and explain the trend in reactivity down Group 2
• Write balanced equations for the reactions of Group 2 elements with water, oxygen and dilute acids
• Describe observations when Group 2 metals react with water
• Explain the trend in first ionisation energy and how it links to reactivity

Outer Electron Configurations

All Group 2 (alkaline earth) metals have two electrons in their outer s sub-shell (ns2):

Element	Z	Configuration	Outer
Be	4	1s2 2s2	2s2
Mg	12	[Ne] 3s2	3s2
Ca	20	[Ar] 4s2	4s2
Sr	38	[Kr] 5s2	5s2
Ba	56	[Xe] 6s2	6s2

In all their reactions, Group 2 elements lose their two outer s electrons to form 2+ ions with a noble gas configuration. Hence Group 2 chemistry is dominated by the half-reaction:

M(s) → M2+(aq) + 2e- (oxidation)

Group 2 metals are therefore reducing agents - they lose electrons to reduce other species.

Trend in Reactivity Down the Group

Reactivity increases down Group 2.

Metal	Reactivity with water
Be	No reaction (even at high T)
Mg	Very slow with cold water; rapid with steam
Ca	Moderate - fizzing in cold water
Sr	Faster - vigorous fizzing
Ba	Very rapid - similar to Group 1 alkali metals

Explanation

Down the group:

• Atomic radius increases (extra shells) → outer electrons are further from the nucleus
• Shielding increases (more inner shells) → outer electrons less strongly held
• First and second ionisation energies decrease → easier to form M2+ ions
• Although nuclear charge increases, the distance and shielding effects dominate

Since reactions of Group 2 metals involve losing electrons, and losing electrons becomes easier down the group, reactivity increases down the group. This is the mirror-image pattern of Group 7 (halogens) reactivity, which decreases down the group because gaining electrons becomes harder.

Element	IE1 / kJ mol-1	IE2 / kJ mol-1	Sum IE1+IE2
Be	899	1757	2656
Mg	738	1451	2189
Ca	590	1145	1735
Sr	549	1064	1613
Ba	503	965	1468

The sum IE1 + IE2 is what matters because both electrons must be removed to form M2+.

Reaction with Water

Group 2 metals react with water to form a metal hydroxide and hydrogen gas:

M(s) + 2H2O(l) → M(OH)2(aq) + H2(g)

Example: Calcium

Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)

Observations:

• Metal sinks (briefly) then rises as hydrogen bubbles lift it
• Vigorous effervescence (H2 gas)
• Metal gradually dissolves
• Colourless solution forms
• Solution becomes alkaline (Ca(OH)2 is a base); phenolphthalein turns pink
• A white suspension may form if Ca(OH)2 exceeds its (low) solubility

Magnesium Exception

Mg reacts very slowly with cold water:

Mg(s) + 2H2O(l) → Mg(OH)2(s) + H2(g) [very slow]

A coat of insoluble Mg(OH)2 forms on the surface, passivating the metal. With steam, however, Mg reacts rapidly to form magnesium oxide (not hydroxide) and hydrogen: