Group 7 - the halogens
The Group 7 elements are known as the halogens. They are reactive non-metals and are always found in compounds with other elements. Chlorine, bromine and iodine are all halogens.
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.
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 purple - 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 is purple, and astatine is black.