Patterns down a group: alkali metals and halogens
Elements in one group are similar, but they also change in a pattern as we go down. Going down a group, atoms get bigger: the atomic radius grows, because each period adds another energy level. In the alkali metals (Li, Na, K) the activity gets bigger going down, so potassium is more reactive than sodium and sodium more than lithium. The alkaline earth metals are less reactive than the alkali metals. In the halogens the activity gets smaller going down. The melting and boiling points of the alkali metals fall going down, while those of the halogens rise. So at 25 °C lithium, sodium, potassium and iodine are solids, bromine is a liquid and chlorine is a gas. Safety: we never test alkali metals with water and we never handle chlorine, bromine or iodine, because they are dangerous. We learn their behaviour from words and pictures.
| Going down the group | Atomic radius | Reactivity | Melting point |
|---|---|---|---|
| alkali metals | grows | grows | falls |
| halogens | grows | falls | rises |
| Halogen | State at 25 °C |
|---|---|
| chlorine | gas |
| bromine | liquid |
| iodine | solid |
Draw the trend. Make a bar chart with the number of occupied energy levels for Li, Na and K (2, 3 and 4) and write below it: “Bigger atoms down the group.” Then write a prediction for the next alkali metal below K. Remember the safety rule: no experiments with alkali metals or halogens, only words and pictures.