Predicting Reactivity

Across a period, the elements change broadly from metals on the left to non-metals on the right. Group 1 and Group 2 contain reactive metals. Group 7 contains reactive non-metals called halogens, while Group 0 contains the very unreactive noble gases. A metal reacting with a non-metal commonly forms an ionic compound: the metal loses electrons and the non-metal gains them.

Predicting reactions of metals

Group 1 metals, such as lithium, sodium and potassium, have one outer electron and form ions with a charge of $+1$. They react with water to form a metal hydroxide and hydrogen. Because reactivity increases down Group 1, potassium is expected to react more vigorously with water than lithium. Rubidium is below potassium, so it is predicted to react even more vigorously. These metals also react with oxygen to form oxides and with halogens to form metal halides, such as sodium chloride.

Group 2 metals have two outer electrons and form ions with a charge of $+2$. Their reactivity also generally increases down the group. Barium is below magnesium, so it is predicted to react more readily with dilute acid. A reactive metal and a dilute acid generally produce a salt and hydrogen; the particular salt depends on the acid used. You should use the group trend to compare metals, rather than assume that every metal reacts at the same speed or in exactly the same conditions.

The position of a metal also helps when predicting displacement. A more reactive metal can replace a less reactive metal in a compound. For example, iron displaces copper from copper(II) sulfate solution:

$\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}$

Copper cannot displace iron from iron(II) sulfate because copper is less reactive than iron. This comparison uses the metals’ relative reactivity; simply seeing that both are metals is not enough.

Predicting reactions of halogens

Group 7 halogens have seven outer electrons and tend to gain one electron when forming halide ions. Fluorine, chlorine, bromine and iodine occur as pairs of atoms, written $\mathrm{F_2}$, $\mathrm{Cl_2}$, $\mathrm{Br_2}$ and $\mathrm{I_2}$. Unlike Groups 1 and 2, halogen reactivity decreases down the group: fluorine is more reactive than chlorine, which is more reactive than bromine, which is more reactive than iodine.

This order lets you predict a halogen displacement reaction. Chlorine is above bromine, so chlorine can displace bromine from potassium bromide solution:

$\mathrm{Cl_2 + 2KBr \rightarrow 2KCl + Br_2}$

Bromine is below chlorine, so adding bromine to potassium chloride solution gives no displacement reaction. Halogens can also react with metals to make salts and with hydrogen to make hydrogen halides. Because fluorine is more reactive than chlorine, it is predicted to react more readily with hydrogen than chlorine does.

Knowing the limits of a prediction

Group 0 gases, such as neon, already have full outer electron shells. They have little tendency to gain, lose or share electrons, so a reaction is unlikely under ordinary conditions. For an unfamiliar element, first identify its group, then use known reactions of that group and the direction of its reactivity trend. Finally, check the substances and conditions named in the question. Position provides evidence for a sensible prediction, not a complete description of every possible reaction.