Part 1: Trends, electron loss and reactions

Connect Group 2 structure to electron loss, then distinguish reactions with cold water, steam, oxygen and acids.

The common pattern: forming 2+ ions

Magnesium, calcium, strontium and barium have two outer s electrons: Mg [Ne]3s², Ca [Ar]4s², Sr [Kr]5s² and Ba [Xe]6s². Their usual ions are M²⁺. They act as reducing agents when the metal donates its two outer electrons.

The third ionisation energy is much greater than the second because the third electron is removed from an inner shell of the 2+ ion. Do not stop at “a full shell”: identify that the electron is closer to the nucleus and less shielded.

Down the group: radius rises and first ionisation energy falls

Each step down adds an occupied principal shell. The outer electron is farther from the nucleus and shielded by more inner electrons. Although proton number also increases, distance and shielding reduce the net attraction to the outer electron. Less energy is needed to remove it.

The metals generally react more vigorously down Mg–Ba because loss of the outer electrons becomes easier. Keep the electron-loss explanation separate from the collision-theory explanation for changing the temperature of one reaction.

Metallic bonding and the non-smooth melting-point pattern

All four are metallic: positive metal ions are attracted to delocalised electrons. Each contributes two electrons in the simple metallic model. Greater ion size tends to reduce attraction between ions and delocalised electrons, so less energy is needed to disrupt the lattice.

However, the actual melting points do not decrease at every step: calcium melts above magnesium (RSC values: Mg 650 °C; Ca 842 °C). Crystal structures also affect the pattern. Describe the data supplied instead of forcing a smooth decline; the simple size argument cannot explain every adjacent pair.

Water versus steam: choose the right product

With liquid water, magnesium reacts very slowly at room temperature; warming makes its reaction easier to observe. A coating of sparingly soluble magnesium hydroxide limits contact. Heated magnesium reacts rapidly with steam to give magnesium oxide and hydrogen, often with a bright white glow.

Calcium, strontium and barium react with cold water, increasingly vigorously. Expect hydrogen bubbles, disappearance of the metal and warming. The dissolved hydroxide makes the mixture alkaline. Calcium hydroxide is only partly soluble, so a white cloudy suspension can form.

Use the state implied by the conditions: dissolved Ca(OH)₂ is (aq), but excess solid in the cloudy suspension is (s). Do not call all reaction mixtures clear solutions. Hydrogen gives a squeaky pop with a lighted splint when tested appropriately in a supervised practical.

Oxygen, surface coatings and acids

Magnesium burns in oxygen with an intense white light and forms white MgO. Its oxide has a giant ionic structure with strong attractions between oppositely charged ions. This gives a high melting point.

Before comparing magnesium reaction rates, clean off the surface coating with emery paper. MgO reacts with hydrochloric acid without producing hydrogen; magnesium metal produces hydrogen. A coating can therefore delay the gas-producing reaction and consume some acid.

With dilute hydrochloric acid, Group 2 metals form chlorides and hydrogen. Sulfuric acid can be a poor choice for comparing the more reactive metals: an insoluble sulfate coating can restrict further reaction. Nitric acid is oxidising, so do not automatically predict hydrogen with every acid.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources