27.1 Similarities and Trends in the Properties of the Group 2 Metals, Magnesium to Barium, and Their Compounds

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Section 1: Thermal Stability of Group 2 Nitrates and Carbonates

What is Thermal Stability?

Thermal stability means how much heat a compound can withstand before it breaks apart (decomposes). A compound with high thermal stability needs a lot of heat before it decomposes. A compound with low thermal stability decomposes more easily, at a lower temperature.

The Group 2 metals are: Magnesium (Mg) → Calcium (Ca) → Strontium (Sr) → Barium (Ba), going down the group.

Each of these metals forms compounds with nitrate ions (NO₃⁻) and carbonate ions (CO₃²⁻). The key trend is:

This means magnesium nitrate and magnesium carbonate decompose most easily, while barium nitrate and barium carbonate are the hardest to decompose.

Why Does This Happen? — The Role of Ionic Radius and Polarisation

To understand why this trend exists, you need to understand two key ideas: ionic radius and polarisation.

Ionic radius simply means the size of an ion (an atom that has gained or lost electrons). Going down Group 2, the metal cations (positively charged ions, e.g. Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) get larger in size.

• Mg²⁺ is the smallest cation in the group.
• Ba²⁺ is the largest cation in the group.

Charge density means how concentrated the positive charge is on an ion. Think of it like spreading the same amount of butter on different sizes of toast — a small piece of toast has a lot of butter in a small area (high charge density), while a large piece has the butter spread thinly (low charge density).

• Small ions (like Mg²⁺) have high charge density — the 2+ charge is packed into a small space.
• Large ions (like Ba²⁺) have low charge density — the 2+ charge is spread over a bigger space.