Group 1 and 2 elements and their compoundsEdexcel A-Level Chemistry: Revision notes

Group 2 elements have two outer electrons and form M²⁺ ions. Going down the group:

• Atomic radius increases because there are more electron shells
• Shielding by inner electrons increases
• The outer electrons are further from the nucleus and less strongly attracted, even though nuclear charge increases
• So first ionisation energy decreases (Mg 738, Ca 590, Sr 550, Ba 503 kJ mol⁻¹)

Because the outer electrons are lost more easily, reactivity increases down the group. Reactivity is explained by how easily the atoms lose electrons, which depends on the first and second ionisation energies.

With oxygen: 2M + O₂ → 2MO (white solid oxides; Mg burns with a bright white flame).

With chlorine: M + Cl₂ → MCl₂ (white solid chlorides).

With water:

• Magnesium reacts very slowly with cold water but burns in steam: Mg + H₂O(g) → MgO + H₂
• Calcium, strontium and barium react with cold water, more vigorously down the group: M + 2H₂O → M(OH)₂ + H₂

In each reaction the metal is oxidised (0 to +2), consistent with the increase in reactivity down the group.

Group 2 oxides react with water to form hydroxides: CaO + H₂O → Ca(OH)₂. The solution is alkaline because of OH⁻ ions (pH about 12 for calcium hydroxide). MgO is only slightly soluble, so the pH is lower.

Oxides and hydroxides are bases and neutralise dilute acids to form a salt and water:

• MgO + 2HCl → MgCl₂ + H₂O
• Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O

Ionic equations: O²⁻ + 2H⁺ → H₂O and OH⁻ + H⁺ → H₂O. Magnesium hydroxide is used in indigestion remedies and calcium hydroxide (lime water) is used to neutralise acidic soil.

Hydroxides: solubility increases down the group. Mg(OH)₂ is almost insoluble and Ba(OH)₂ is quite soluble, so the pH of the saturated solution increases down the group.

Sulfates: solubility decreases down the group. MgSO₄ is soluble and BaSO₄ is insoluble.

Barium sulfate is the basis of the test for sulfate ions: add dilute hydrochloric acid and barium chloride solution; a white precipitate of BaSO₄ shows sulfate is present. Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). Barium sulfate is used as a 'barium meal' for X-rays as it is insoluble and so is not absorbed.

Group 2: carbonates and nitrates decompose on heating.

• MCO₃ → MO + CO₂
• 2M(NO₃)₂ → 2MO + 4NO₂ + O₂ (brown gas NO₂, and O₂ which relights a glowing splint)

Group 1: carbonates do not decompose at Bunsen temperatures, except lithium: Li₂CO₃ → Li₂O + CO₂. Nitrates give a nitrite and oxygen: 2NaNO₃ → 2NaNO₂ + O₂, except lithium nitrate: 4LiNO₃ → 2Li₂O + 4NO₂ + O₂.

Explanation: a small cation with a high charge has a high charge density, so it is polarising and distorts the electron cloud of the anion, weakening it. Group 2 ions (2+, smaller) are more polarising than Group 1 ions (1+), so Group 2 compounds decompose more easily. Stability increases down the group as the cation gets larger, with lower charge density. Li⁺ is the smallest Group 1 ion so behaves like Group 2.

In a flame test heat promotes electrons in metal ions to higher energy levels. When the electrons fall back they release energy as visible light of specific wavelengths. Each metal has different energy gaps, so each gives a characteristic colour:

• Lithium: red (crimson)
• Sodium: yellow
• Potassium: lilac
• Calcium: brick-red
• Strontium: red (crimson)
• Barium: apple-green

The colours must be learned. Magnesium compounds give no distinct colour.

Flame test: clean a nichrome or platinum wire by dipping it in concentrated hydrochloric acid and heating it until it gives no colour. Dip it into the solid (or its solution) and hold it in the edge of a blue Bunsen flame. Record the colour.

Thermal decomposition: heat a small amount of the nitrate or carbonate in a hard glass test tube. For nitrates, look for brown fumes and test for oxygen with a glowing splint. For carbonates, pass the gas through limewater, which turns cloudy, to show CO₂. Compare the time taken or the strength of heating needed for each compound, keeping the mass, tube and heating conditions the same.

Safety: nitrogen dioxide is toxic, so work in a fume cupboard and wear eye protection.