Trends in Reactivity (Group 1 and Group 7)

Group 1: Alkali Metals

Group 1 elements are known as the alkali metals. They include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). These metals share similar properties due to their electronic structure, which has one electron in their outermost shell.

Reactivity increases down the group

As you move down Group 1, the reactivity of the alkali metals increases. This happens because the outer electron is further from the nucleus and is shielded by more inner shells of electrons. This makes it easier for the atom to lose its outer electron and react.

Reaction with water

Alkali metals react vigorously with water to form a metal hydroxide and hydrogen gas. The general equation is:

$$\text{Metal} + \text{Water} \rightarrow \text{Metal hydroxide} + \text{Hydrogen}$$

For example, sodium reacts with water:

$$\text{2Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2$$

The metal hydroxide formed is alkaline (basic), which is why these metals are called alkali metals.

Physical properties

Alkali metals are soft and can be cut with a knife. They have relatively low melting points compared to other metals, and these melting points decrease down the group.

Formation of ions

Alkali metals form +1 ions by losing their single outer electron. For example, sodium forms Na⁺ ions. This makes them very reactive, as they tend to lose this electron to form stable ionic compounds.

For instance, potassium reacts with chlorine to form potassium chloride (KCl), where potassium forms K⁺ ions.

Example: Predict which is more reactive, lithium or potassium, and explain why.

Potassium is more reactive because its outer electron is further from the nucleus and more shielded by inner electrons, so it is lost more easily than lithium6s outer electron.

Group 7: Halogens

Group 7 elements are called the halogens. They include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These are non-metals with seven electrons in their outer shell.

Reactivity decreases down the group

Unlike Group 1, the reactivity of halogens decreases as you go down the group. This is because atoms get larger, and the outer shell is further from the nucleus, so it is harder to attract an extra electron.

Exist as diatomic molecules

Halogens exist as molecules made of two atoms (diatomic), such as Cl₂ or Br₂. This is because they share one pair of electrons in a covalent bond to complete their outer shell.

Reaction with metals to form salts

Halogens react with metals to form ionic salts. The metal loses electrons to form positive ions, and the halogen gains electrons to form negative ions (halide ions). For example, chlorine reacts with sodium to form sodium chloride (NaCl):

$$\text{2Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}$$

Displacement reactions

A more reactive halogen can displace a less reactive halogen from a compound. For example, chlorine can displace bromine from potassium bromide solution:

$$\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2$$

This happens because chlorine is more reactive than bromine and can gain electrons more easily.

Example: Explain why iodine is less reactive than chlorine.

Iodine atoms are larger, so the attraction between the nucleus and the extra electron is weaker. This makes iodine less able to gain electrons compared to chlorine, so it is less reactive.

Trends in Reactivity Comparison

Group 1 and Group 7 show opposite trends in reactivity because of their different electronic structures and how they react:

• Group 1: Reactivity increases down the group because atoms lose their one outer electron more easily as it is further from the nucleus and shielded.
• Group 7: Reactivity decreases down the group because atoms gain an electron to complete their outer shell, but this becomes harder as the outer shell is further from the nucleus and shielded.

The reactivity of these groups is linked to their outer electron shell:

• Group 1 elements lose one electron to form +1 ions.
• Group 7 elements gain one electron to form -1 ions (halide ions).

This difference explains why alkali metals are highly reactive metals and halogens are reactive non-metals.

Example: Explain why potassium is more reactive than sodium, but chlorine is more reactive than bromine.

Potassium6s outer electron is further from the nucleus than sodium6s, so it is lost more easily, increasing reactivity down Group 1. In contrast, chlorine6s smaller atom attracts electrons more strongly than bromine, so chlorine is more reactive, meaning reactivity decreases down Group 7.