Groups 0, 1 and 7

The noble gases (helium, neon, argon, krypton…) are unreactive because their atoms have a full outer shell — 8 electrons, or 2 for helium. This is a very stable arrangement, so they do not need to gain, lose or share electrons. They exist as single atoms.

All are colourless gases at room temperature. Their boiling points increase going down the group, because the atoms get heavier (relative atomic mass increases).

The alkali metals (lithium, sodium, potassium…) have one electron in their outer shell. They are soft, have low densities (Li, Na and K float on water) and low melting points. Going down the group, they get more reactive and their melting points decrease.

They react with:

• water → metal hydroxide + hydrogen: 2Li + 2H₂O → 2LiOH + H₂
• chlorine → metal chloride (a white salt): 2Li + Cl₂ → 2LiCl
• oxygen → metal oxide: 4Li + O₂ → 2Li₂O

With water, lithium fizzes steadily, sodium melts into a ball and whizzes around, and potassium bursts into a lilac flame.

Group 1 gets more reactive down the group. Each atom down the group has an extra shell, so the outer electron is further from the nucleus and more shielded by inner shells. It is less strongly attracted, so it is lost more easily.

Group 7 gets less reactive down the group. Halogens react by gaining an electron. Down the group, the outer shell is further from the nucleus, so an incoming electron is less strongly attracted and is gained less easily.

In both groups, it comes down to how strongly the nucleus attracts the outer electrons.

The halogens are non-metals with seven electrons in their outer shell. They exist as diatomic molecules (F₂, Cl₂, Br₂, I₂). Going down the group they get less reactive, and their melting and boiling points increase.

Halogens react with metals to form ionic salts containing halide ions (Cl⁻, Br⁻, I⁻), and with non-metals to form covalent compounds such as HCl.

In a displacement reaction, a more reactive halogen pushes a less reactive one out of a solution of its salt:

Cl₂ + 2KBr → 2KCl + Br₂