Group 2 Trends: Atomic Properties and Reactions Revision Notes | A Level AQA Chemistry

1. Why Group 2 Trends Matter

Group 2 is one of the most pattern-rich areas of A Level inorganic chemistry. Every property you study — atomic radius, ionisation energy, melting point, reactivity with water — follows a clear, explainable trend from magnesium down to barium.

The real skill the examiners are testing is not whether you can recall the trend, but whether you can explain it using electron shielding, nuclear charge, and metallic bonding. That distinction between describing and explaining is worth several marks on every paper.

2. Atomic Radius: Getting Bigger Down the Group

Picture the electron shells of magnesium: two inner shells and an outer shell containing the two valence electrons. Now move down to calcium — there are three shells. Strontium has four, barium has five. Each new period adds a complete electron shell, pushing the outermost electrons further from the nucleus.

The result is straightforward: atomic radius increases from Mg to Ba. The nuclear charge does increase too (more protons), but the additional inner shells provide substantial shielding — they reduce the effective nuclear charge felt by the outer electrons. The shielding effect outweighs the increase in nuclear charge, so the outer electrons are held less tightly and sit further away.

The chart confirms the trend clearly: each step down the group adds roughly 20–40 pm to the atomic radius. Notice the increase becomes smaller between Sr and Ba — the additional shielding from d-block electrons in Ba slightly moderates the expansion, but the overall trend remains upward.

3. First Ionisation Energy: Getting Easier to Remove an Electron

First ionisation energy (IE₁) measures how strongly an atom holds onto its outermost electron. Down Group 2, IE₁ decreases from Mg to Ba. Two factors drive this, and you need both for full marks in an explain question.

• Increasing atomic radius: the outer electron is further from the nucleus, so the electrostatic attraction is weaker.
• Increasing shielding: more inner electron shells screen the outer electron from the full nuclear charge, reducing the effective nuclear charge experienced by the valence electron.
• Combined effect: less energy is needed to remove the outer electron, so IE₁ falls.

Although nuclear charge increases down the group (Mg has 12 protons, Ba has 56), this is more than compensated for by the shielding and distance effects. The outer electron in Ba is so well shielded and so far from the nucleus that it requires considerably less energy to remove than the outer electron in Mg.

4. Melting Points: Structure and Bonding in Group 2 Metals

Group 2 elements are all metals with a giant metallic lattice structure. In this structure, positive metal ions (cations) sit in a regular arrangement surrounded by a delocalised sea of electrons. The melting point depends on the strength of the electrostatic attraction between the cations and the delocalised electrons.

Each Group 2 atom contributes two delocalised electrons to the sea (they have two valence electrons). This means Group 2 metals have a higher charge density on their cations (2+ ions) compared to Group 1 metals (1+ ions), which is why Group 2 melting points are generally higher than those of Group 1.

The trend in melting points from Mg to Ba is not a simple, smooth decrease. This is one of the most commonly misunderstood aspects of Group 2 at A Level, and examiners know it.

The melting point rises from Mg to Ca, then falls from Ca to Ba. This irregular pattern arises because melting point depends on several competing factors: the charge and size of the metal ion, the number of delocalised electrons, and the crystal structure adopted by each element. As the ionic radius increases down the group, the charge density of the 2+ ion decreases, weakening the metallic bonding and generally lowering the melting point — but structural changes mean the trend is not perfectly smooth.

5. Reactions of Group 2 Elements with Water

Now that you understand the atomic properties, you can use them to explain reactivity. The reactions of Group 2 metals with water follow a clear trend: reactivity increases from Mg to Ba. This is directly linked to the decreasing first ionisation energy and increasing atomic radius you have already studied.

When a Group 2 metal reacts with water, it forms a metal hydroxide and hydrogen gas. The general equation is:

Magnesium is the exception here. It reacts very slowly with cold water — so slowly that you would barely notice a reaction at room temperature. However, magnesium reacts vigorously with steam, producing magnesium oxide rather than magnesium hydroxide:

Calcium reacts steadily with cold water, producing a white precipitate of calcium hydroxide (which is only slightly soluble) and hydrogen gas. Strontium and barium react more vigorously still, with barium reacting rapidly and producing a visible effervescence of hydrogen gas.

The increasing reactivity down the group is explained by the same factors that reduce IE₁: the outer electrons become easier to remove as atomic radius increases and shielding increases. A metal that loses its electrons more readily will react more readily with water, which acts as an oxidising agent in this context.

6. Extraction of Titanium Using Magnesium

Titanium is a highly valuable metal — strong, lightweight, and corrosion-resistant — but it cannot be extracted by carbon reduction because titanium carbide (TiC) would form instead of pure titanium. This is where magnesium plays a critical industrial role.

The Kroll process uses magnesium to reduce titanium(IV) chloride (TiCl₄) to produce pure titanium metal. Magnesium is chosen because it is more reactive than titanium and can therefore displace it from its chloride compound. The reaction takes place at high temperature in an inert atmosphere (argon or helium) to prevent oxidation of the products.

Worked example: Analysing the Kroll Process: Why Magnesium?

Explain why magnesium is used to extract titanium from TiCl4, and why sodium could also be used but is less preferred industrially.

1. $\text{TiCl}_4(g) + 2\text{Mg}(l) \rightarrow \text{Ti}(s) + 2\text{MgCl}_2(l)$ — Magnesium acts as a reducing agent, donating electrons to Ti4+ ions. This is a redox reaction: Mg is oxidised (0 to +2) and Ti is reduced (+4 to 0).
2. Reactivity order: Mg > Ti — A more reactive metal will displace a less reactive metal from its compound. Magnesium is above titanium in the reactivity series, so it can reduce TiCl4.
3. C + TiO₂ → TiC (unwanted) rather than Ti(s) — Carbon reduction of titanium compounds produces titanium carbide, not pure titanium. The product would be contaminated and unsuitable for engineering applications.
4. Na is more reactive than Mg (Na > Mg in reactivity series) — Sodium can also reduce TiCl4 and is used in some processes. However, sodium is more expensive to produce, harder to handle safely (reacts violently with moisture), and the MgCl2 by-product is more easily recycled by electrolysis than NaCl.
5. Temperature: ~800–850°C; atmosphere: argon (inert) — An inert atmosphere prevents Mg and Ti from reacting with oxygen or nitrogen in the air, which would contaminate the titanium product.

Answer: Magnesium is used because it is more reactive than titanium and can reduce TiCl4 to Ti metal. Carbon cannot be used as it would form TiC. The reaction is carried out at high temperature under an inert atmosphere. MgCl2 is a recoverable by-product that can be electrolysed to regenerate Mg.

One detail worth noting: the MgCl₂ by-product is not wasted. It is electrolysed (in the molten state) to recover magnesium metal and chlorine gas, making the overall process more economically viable. This recycling of magnesium is a key reason why the Kroll process remains the dominant industrial method for titanium production.

7. Exam Technique and Mark-Winning Strategies

Group 2 questions on AQA papers tend to follow predictable patterns. The most common errors come not from not knowing the facts, but from using imprecise language that does not match the mark scheme. Here is how to write answers that collect every available mark.

For explain questions about trends, you must give a mechanistic reason — not a restatement of the trend. Writing “atomic radius increases, so ionisation energy decreases” earns one mark at best. Writing “atomic radius increases because additional electron shells are added, placing the outer electron further from the nucleus; combined with greater shielding from inner shells, the effective nuclear charge experienced by the outer electron decreases, so less energy is required to remove it” earns full marks.

For equation questions, always include state symbols. A balanced equation without state symbols will lose the mark for state symbols every time. Check that your equation is balanced by counting atoms on both sides before moving on.

Across this lesson, every trend you have studied — atomic radius, first ionisation energy, melting point, and reactivity with water — connects back to the same underlying principles: electron shell structure, shielding, and effective nuclear charge. Master those three ideas and Group 2 becomes one of the most logical and predictable topics on the entire A Level specification.