Predicting properties

The halogens show trends in physical properties as you go down the group.

Melting point and boiling point

The halogens have low melting points and low boiling points. This is a typical property of non-metals. Fluorine has the lowest melting and boiling points. The melting and boiling points then increase as you go down the group.

Graph showing melting and boiling points of the halogens, ranging from around minus 200 °C for fluorine to over 300 °C for astatine.Melting and boiling points of Group 7 elements

State at room temperature

Room temperature is usually taken as being 25°C. At this temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. There is therefore a trend in state from gas to liquid to solid as you go down the group.

Colour

The halogens become darker as you go down the group. Fluorine is very pale yellow, chlorine is yellow-green, and bromine is red-brown. Iodine crystals are shiny grey – but easily turn into a dark purple vapour when they are warmed up.

Predictions

When we can see a trend in the properties of some of the elements in a group, it is possible to predict the properties of other elements in that group. Astatine is below iodine in Group 7. The colour of these elements gets darker as you go down the group. Iodine crystals are shiny grey with a purple vapour, and astatine is black.