Group 7 properties
The elements in Group 7 of the Periodic Table are called the halogens. They include chlorine, bromine and iodine. The halogens are diatomic - this means they exist as molecules, each with a pair of atoms. Chlorine molecules have the formula Cl2, bromine Br2 and iodine I2.
The halogens show trends in physical properties down the group.
The halogens have low melting points and boiling points. This is a typical property of non-metals. Fluorine has the lowest melting point and boiling point. The melting points and boiling points then increase as you go down the group.
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 down the group.
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.
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, as we would expect, astatine is black.
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