Reactivity trends in Group 1

Group 1 metals become more reactive down the group. Lithium is less reactive than sodium, and sodium is less reactive than potassium.

Each Group 1 atom has one electron in its outer shell. When a Group 1 metal reacts, its atoms lose this outer electron. The more easily an atom loses the electron, the more readily the metal reacts.

Atoms lower down Group 1 are larger, so their outer electron is farther from the nucleus. The attraction between the nucleus and this electron is weaker, so the electron is more easily lost. This makes the metals more reactive down the group.

The trend can be used to make predictions: an alkali metal lower in Group 1 is likely to react more readily than one above it.

Group 1 electron structure and chemical properties

The elements in Group 1 are called the alkali metals. Each of their atoms has one electron in its outer shell. Although their atoms have different numbers of occupied shells, this shared outer-electron structure gives them similar chemical properties.

A Group 1 atom can relatively easily lose its single outer-shell electron during a reaction. The resulting ion has one more proton than electrons, so it has a charge of +1. For example, a sodium atom, $$\ce{Na}$$, forms a sodium ion, $$\ce{Na+}$$, by losing one electron.

Because all Group 1 atoms can lose one outer electron in this way, they form ions with the same charge and take part in similar types of chemical reactions. Their shared chemistry is therefore closely linked to their single outer-shell electron.

Predicting Group 1 properties from trends

A property of a Group 1 element can be predicted by looking at how that property changes down the group. Arrange the known values or descriptions in group order, then look for a trend: does the property increase, decrease or follow another pattern?

Once the pattern is clear, use an element’s position to predict its property or compare it with another element. A prediction may be qualitative, such as “higher” or “lower”, or an approximate numerical value.

Estimating a missing value

Suppose the following melting points are given in group order:

• Lithium: approximately $$181\ ^\circ\mathrm{C}$$
• Sodium: approximately $$98\ ^\circ\mathrm{C}$$
• Potassium: unknown
• Rubidium: approximately $$39\ ^\circ\mathrm{C}$$

The known melting points decrease down the group. Potassium lies between sodium and rubidium, so its melting point is likely to be lower than sodium’s but higher than rubidium’s. A rough estimate halfway between those values is about $$69\ ^\circ\mathrm{C}$$.

This is an estimate, not an observed value. The change between neighbouring elements does not have to be equal, so the actual value need not be exactly halfway between the two known values.

Limits of trend predictions

Establish the direction and size of the trend separately for each property. A decrease in melting point, for example, does not by itself show how another property changes down the group.

A missing value between known data points can be estimated from the surrounding pattern. A prediction beyond the supplied data is less certain: the pattern may not continue in the same way. In either case, an estimated value is less certain than an observed one.