Taught 10 Nov – 11 Nov · Booklet pp. 32–39 · Homework week 6
Because elements in a group share the same number of outer electrons (1.5), each group reacts in a characteristic way, and reactivity changes smoothly down the group.
Every trend here comes from one idea: how easily an atom loses or gains its outer electron, which depends on how far the outer shell is from the nucleus.
Metals (Groups 1 and 2) react by losing electrons. The further the outer electron is from the nucleus — i.e. the lower down the group — the weaker the pull holding it, so it is lost more easily and the metal is more reactive. That is why potassium beats sodium beats lithium.
Halogens (Group 17) react by gaining an electron. The same distance argument runs the opposite way: lower down the group, the incoming electron would sit further from the nucleus, where the pull is weaker, so it is captured less easily. Reactivity therefore decreases down the group, with fluorine the most reactive at the top.
Why is potassium more reactive than lithium? Both are Group 1, so both react by losing their one outer electron. In potassium that electron is in a shell further from the nucleus than in lithium, so it is held less tightly and lost more easily. Easier electron loss = more reactive.
Fluorine or iodine — which is more reactive? Both are Group 17 and react by gaining one electron. Fluorine's outer shell is closest to the nucleus, so it pulls in an electron most strongly. Fluorine is therefore more reactive than iodine.
The trend you predict on paper plays out dramatically in the lab: drop caesium (low in Group 1) into water and it reacts explosively, far more violently than lithium at the top. The deeper reason — shielding by inner shells and the increasing nuclear charge — is refined in the IB Diploma, but the Year-9 picture (distance from the nucleus) already predicts every trend correctly. These electron gains and losses are also the direct lead-in to ionic bonding: a Group 1 metal hands its electron to a Group 17 non-metal, and the opposite charges hold the compound together.
Diagrams and worked examples are in the Knowledge Book PDF below — they don't reproduce well on a page.
Say it out loud before you tap.
how readily a substance takes part in a chemical reaction
a soft, very reactive metal with one outer electron; reacts with water to give hydrogen
a reactive metal with two outer electrons
a reactive non-metal with seven outer electrons; reacts by gaining one electron
existing as molecules of two atoms, e.g. Cl₂
TRUTH Metals (1 and 2) get more reactive down; halogens (17) get less reactive down. The direction depends on whether the atom loses or gains electrons.
TRUTH They lose their single outer electron. It is the non-metals (like halogens) that gain electrons.
TRUTH It is about how easily the outer electron is lost or gained — i.e. its distance from the nucleus, not the atom's mass on its own.
TRUTH The opposite — for halogens the top is most reactive, because the outer shell is closest to the nucleus.
A
Formulae, symbols & counting atoms
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