Alkali metals, halogens and noble gases: three groups you should know about

Alkali metals (column 1): highly reactive

Alkali metals (lithium Li, sodium Na, potassium K, etc.) have a single electron in their outermost shell. They seek to lose this electron to achieve a stable configuration, which makes them extremely reactive: metallic sodium reacts violently with water, releasing dihydrogen (2 Na + 2 H₂O → 2 NaOH + H₂). For this reason, they are stored away from air and water, often in oil.

The halogens (Group 17): eager for an electron

In contrast, the halogens (fluorine F, chlorine Cl, bromine Br, iodine I) have 7 electrons in their outermost shell: they are missing just one to complete it. They are therefore also highly reactive, but in the opposite way to the alkali metals: they readily gain an electron. Dichlorine (Cl₂), for example, is a toxic gas widely used in water disinfection.

Noble gases (Group 18): stability

The noble gases (helium He, neon Ne, argon Ar, etc.) have a complete outer shell (2 electrons for helium, 8 for the others). This stability makes them almost chemically inert: they almost never react with other elements, which explains their use in light bulbs or for safely inflating balloons.

The link between the three groups

An alkali metal that loses its outermost electron and a halogen that gains an electron both achieve a configuration similar to that of a noble gas: this is exactly what happens when sodium and chlorine react to form table salt (NaCl).

A common misconception

Don’t assume that ‘reactive’ always means ‘dangerous in the same way’: alkali metals react by giving up an electron, whilst halogens react by gaining one. These are two opposite chemical behaviours, even if the result (often a vigorous reaction) may appear similar.