10.1 Similarities and Trends in the Properties of the Group 2 Metals (Magnesium to Barium) and Their Compounds

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1. Introduction to Group 2 Elements

The Group 2 elements are a set of metals found in the second column of the Periodic Table. In this topic, we focus on magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). They are sometimes called the alkaline earth metals.

Here are some important facts about all of them:

• They are all shiny, silvery-white metals.
• Each atom has two electrons in its outermost shell (the shell furthest from the nucleus). This is what makes them so similar to each other.
• When they react, they always lose these two outer electrons to form ions with a 2+ charge (written as M²⁺). For example, magnesium forms Mg²⁺, calcium forms Ca²⁺.
• Because they give away electrons, they act as reducing agents (a reducing agent is a substance that gives electrons to something else).

Their electronic configurations are shown below:

Notice that each element has exactly two electrons in its outermost shell (shown in bold). This is the key similarity across all Group 2 elements.

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2. Physical and Chemical Trends in Group 2 (Syllabus Objective 4)

Understanding why Group 2 elements behave differently as you go down the group is essential. The trend in behaviour is driven by changes in atomic radius and ionisation energy.

2a. Atomic Radius

• Atomic radius means the size of the atom — how big it is from its centre to the edge of its outermost electron shell.
• Going down Group 2 (from Mg to Ba), the atomic radius increases.
• This is because each element has one more electron shell than the one above it. More shells = bigger atom.

2b. Ionisation Energy

• Ionisation energy is the energy needed to remove an electron from an atom. Think of it as "how hard it is to pull an electron away."
• Group 2 elements need to lose two electrons to react, so we care about both the first and second ionisation energies.
• Going down Group 2, both the first and second ionisation energies decrease (get smaller).

Why do ionisation energies decrease going down the group?

There are two reasons that work together:

1. Greater atomic radius: The outermost electrons are further away from the nucleus. The further away an electron is, the less the nucleus can hold onto it, so it is easier to remove.
2. More shielding: As more electron shells are added, the inner shells act like a "shield" between the nucleus and the outermost electrons. This shielding effect reduces how strongly the nucleus pulls on the outer electrons.

Even though the nucleus has more protons going down the group (so more positive charge), the effect of the larger radius and greater shielding outweighs this, making the outer electrons easier to remove.

The values clearly decrease from Mg to Ba.

2c. Chemical Reactivity

• Because it becomes easier to remove the two outer electrons going down the group, the elements become more reactive as you go from Mg to Ba.
• In other words: Ba is the most reactive and Mg is the least reactive of these four elements.
• This is clearly shown in their reactions: for example, when reacted with dilute hydrochloric acid, the bubbles of hydrogen gas are produced much faster for Ba than for Mg.
• Barium is so reactive with oxygen in air that it must be stored in oil to stop it reacting.