Group 2 (Alkaline Earth metals)

Electron arrangement

Two electrons in an outer s-orbital

This s-orbital is gets further and further away from the nucleus going down the group

Size of the atoms trend

Going down group, atomic radius increases

Melting point trend

Going down the group melting point decreases with the exception of Mg which has the lowest melting point in the group

Reason for melting point trend

Going down the group, delocalised electrons are further away from the positive nuclei

Strength of metallic bonds decrease going down the group

Charge of the group 2 metals

+2 (they lose their two outer electrons)

Trend in first ionisation energy going down the group

Going down the group ionisation energy increases as there is greater shielding between the outermost electrons and the positively charged nucleus, and the electrons being removed are further away from the nucleus

Trend in reactivity going down the group

Going down the group reactivity increases as the outermost electrons are lost much more easily and require less energy to be removed (Shielding + Atomic radius)

Reaction of a group 2 metal with water

M_(s) + 2H₂O_(l) → M(OH)_{2 (aq)} + H_{2 (g)}

Formula for milk of Magnesia

Mg(OH)_{2 (aq)}

Purpose of milk of magnesia

Used in indigestion remedies, neutralising excess stomach acid which causes heartburn, indigestion and wind

Reaction of Magnesium with water vs steam

• Magnesium reacts very slowly with cold water, but rapidly with steam

• Reaction with water produces magnesium hydroxide

• Reaction with water produces magnesium oxide

Reaction of group 2 metals with water vs steam

• Going down the group both reactions become more vigorous as the group 2 metal increases in reactivity going down the group

• Reaction with cold water is slower

• Reaction with steam is more vigorous

• Reaction with water produces a metal hydroxide

• Reaction with steam produces a metal oxide

Trend in solubility of hydroxides

Going down the group the metal hydroxides become more soluble

• Mg → almost insoluble, sold as a suspension in water rather than a solution

• Ca → Sparingly soluble, a solution is used in water

• Sr → more soluble

• Ba → dissolves to produce a strongly alkaline solution

Dissolving of Barium hydroxide

Ba(OH)_{2 (s)} + aq → Ba²⁺_(aq) + 2OH^-_(aq)

Trend in solubility of sulfates

Going down the group solubility of sulfates decreases

Barium sulfate is virtually insoluble

What is another name for barium sulfate

A barium meal

What is the purpose of a barium meal

To outline the gut in medical X-rays

Barium is very good at absorbing X-rays

The test is safe because barium sulfate is so insoluble

(Barium compounds are highly toxic)

Test for sulfate ions in solution

• Solution is acidified with nitric or hydrochloric acid

• Barium chloride solution is added to the solution

• If a sulfate is present, a white precipitate of barium sulfate is formed

• Ba²⁺_(aq) + SO₂^2-_(aq) → BaSO_{4 (s)}

What is the purpose of adding the acid when testing for sulfate ions in solution

It removes carbonate ions as carbon dioxide

Barium carbonate is also a white insoluble solid and would be indistinguishable from barium sulfate