IGCSE Chemistry 0620: Explaining Group I and Group VII Reactivity Trends

The reactivity of Group I metals increases going down the group, while the reactivity of Group VII non-metals decreases going down the group. Both trends can be explained by looking at atomic structure.

(a) State what happens to atomic radius (atomic size) going down Group I, from lithium to potassium. [1]

(b) A Group I atom reacts by losing its single outer-shell electron to form a $1+$ ion. Using the ideas of atomic radius and the outer-shell electron's distance from the nucleus, explain why this outer electron becomes easier to lose going down Group I, causing reactivity to increase. [3]

(c) A Group VII atom reacts by gaining one electron into its outer shell to form a $1-$ ion. Using the same ideas, explain why an incoming electron becomes harder to gain going down Group VII, causing reactivity to decrease. [3]

(d) State how the reactivity trend down Group I compares with the reactivity trend down Group VII. [1]

Part (a): Atomic radius down Group I

Going from lithium to sodium to potassium, each element down the group has one more occupied electron shell than the element above it (lithium 2,1; sodium 2,8,1; potassium 2,8,8,1). Adding shells makes the atom bigger, so atomic radius increases going down Group I.

Part (b): Why the outer electron is lost more easily down Group I

Going down Group I:

The atom has more occupied electron shells, so its atomic radius is larger.
This means the single outer-shell electron sits further from the nucleus, and is shielded from the nucleus’s pull by more inner shells of electrons.
A more distant, more shielded electron is held by a weaker force of attraction to the nucleus.

Because the outer electron is held less tightly, it is lost more easily going down the group, so reactivity increases going down Group I.

Part (c): Why an extra electron is gained less easily down Group VII

Going down Group VII, the same structural change happens: atoms have more occupied shells, so atomic radius increases.

An incoming electron would have to join the atom’s outer shell, but going down the group, this outer shell is further from the nucleus and shielded by more inner shells, so any extra electron is attracted less strongly.

Because the pull on an incoming electron is weaker, it is gained less readily going down the group, so reactivity decreases going down Group VII.

Part (d): Comparing the two trends

Both trends are explained by the same structural change, atomic radius increasing going down a group, which weakens the force of attraction between the nucleus and the outer shell. However, the outcome is opposite in the two groups:

Group I atoms lose an electron, so a weaker attraction makes this easier $\rightarrow$ reactivity increases down the group.
Group VII atoms gain an electron, so a weaker attraction makes this harder $\rightarrow$ reactivity decreases down the group.

Final answers

(a) Atomic radius increases going down Group I.
(b) The outer electron is further from the nucleus and less strongly attracted, so it is lost more easily. Reactivity increases down Group I.
(c) An incoming electron is further from the nucleus and less strongly attracted, so it is gained less easily. Reactivity decreases down Group VII.
(d) The trends go in opposite directions, even though both come from the same increase in atomic radius down a group.