Metallic character is linked to ionisation energy

Metallic character is a qualitative property that describes the extent to which an element behaves like a metal.

Elements with high metallic character have the following features:

• They have delocalised electrons in their structure.
• They tend to have lower ionisation energies.
• They are more likely to donate electrons and form cations.

In contrast, elements with low metallic character (high non-metallic character) have higher ionisation energies and tend to accept electrons to form negatively charged ions (anions).

Metallic character of alkali metals increases down the group

The alkali metals (group 1 elements) show a clear trend of increasing metallic character from lithium to francium.

This trend is due to the decreasing ionisation energy down the group, as atomic radius increases and the valence electrons are further from the nucleus, making them easier to remove.

Key points:

• Metallic character increases down group 1.
• Ionisation energy decreases down the group.
• Increased metallic character leads to higher reactivity with water and other substances.

Non-metallic character of halogens decreases down the group

The halogens (group 17 elements) exhibit a decrease in non-metallic character from fluorine to iodine.

This trend is linked to the decreasing electronegativity down the group, as atoms radius increases and the attraction for valence electrons weakens.

Key points:

• Non-metallic character decreases down group 17.
• Electronegativity decreases down the group.
• Decreased non-metallic character results in lower reactivity with other substances.

Reactions of alkali metals with water

The increasing metallic character and reactivity of alkali metals can be demonstrated by their reactions with water.

Alkali metals form aqueous Brønsted–Lowry bases when reacted with water, according to the general equation:

M_(s) + H₂O_(l) ➔ 1⁄2H_2(g) + MOH_(aq)

For example:

Li_(s) + H₂O_(l) ➔ 1⁄2H_2(g) + LiOH_(aq)

Na_(s) + H₂O_(l) ➔ 1⁄2H_2(g) + NaOH_(aq)

K_(s) + H₂O_(l) ➔ 1⁄2H_2(g) + KOH_(aq)

Key points about these reactions:

• They produce hydrogen gas and the corresponding metal hydroxide.
• The reactions occur faster and more vigorously going down the group.
• This is due to the decreasing ionisation energy and increasing metallic character.

Reactions of halide ions with halogens

The reactions between elemental halogens and halide ions demonstrate the decreasing non-metallic character and reactivity of halogens down group 17.

In these reactions, an elemental halogen (X_2_) accepts an electron from a halide ion of a different element (Y^-^), forming a new halide ion (X^-^) and the elemental form of the original halide (Y_2_).

For example, the reaction between chlorine and bromide ions proceeds as follows:

1⁄2Cl₂ + Br^- ➔ Cl^- + 1⁄2Br₂

However, the reverse reaction does not occur because chlorine is more electronegative than bromine and has a greater tendency to gain an electron.

A halogen will only react with halide ions from elements below it in group 17:

The favourability of the reaction between a halogen and a halide ion depends on two factors:

1. The difference in electronegativity between the reacting species - A larger difference in electronegativity leads to a stronger tendency for electron transfer.
2. The position of the halide ion in group 17 - Elements lower in the group have lower electronegativity and are more willing to give up electrons.

Consequently, Cl_2_ reacts faster with I^-^ than with Br^-^ due to the larger difference in electronegativity.

Similarly, Cl_2_ reacts faster with K than with Li, as shown in the equation:

1⁄2Cl_2_ + K ➔ K^+^ + Cl^-^

The greater metallic character and lower ionisation energy of potassium (K) compared to lithium (Li) make it more reactive with chlorine.