Higher and foundation tier
The group 7 non-metals are called the halogens.
The halogens are fluorine
, chlorine, bromine and
iodine. Astatine at the bottom of group 7 is a very rare and highly radioactive element, its most stable isotope has a half-life of just over 8 hours.
The halogens are all very reactive elements and are not found as elements in nature, instead they are
found combined in compounds in rocks and minerals.
Fluorine, chlorine and bromine are all toxic and corrosive and great care is needed in handling these reactive elements, though
iodine being the least reactive halogen and being a solid is the easiest and safest halogen to handle
in the lab.
Fluorine at the top of the group is a pale green yellow toxic gas, it is perhaps the most reactive elements in the periodic table. Chlorine is a greeny-yellow gas that is also very toxic and reactive. It has a recognisable smell that most people associate with the swimming baths, though the smell at the baths is not chlorine, as even small amounts of chlorine gas are quite toxic. Bromine is a volatile red-brown liquid at room temperarture with an obnxious bleachy odour, if a small amount is placed in a flask it will quickly fill with red-brown bromine vapour. Bromine like fluorine and chlorine is a very toxic and dangerous element to handle. Iodine is a greyish/purple solid with a metallic like sheen at room temperature. With gentle heating it sublimes (turns straight from a solid to a gas) to produce beautiful violet vapours. The image below shows each of the halogens in gas jars along with a sample of solid iodine.
The halogens "go around in pairs"- that is they form molecules made up of two atoms as shown in the image. These diatomic molecules or two atom molecules are quite common for non-metal elements e.g. oxygen, nitrogen and hydrogen also form these diatomic molecules in the elemental state.
The table below lists the melting and boiling points of the halogens. The trend or pattern is fairly obvious, as we go down the group the molecules get larger and the relative mass increases. Larger molecules will result in stronger intermolecular bonding, and this along with the increase in relative mass results in higher melting and boiling points.
|Halogen||Colour||Melting point/0C||Boiling point/0C||state at room temperature|
All the halogens have 7 electrons in their outer shell, so only need to gain one to achieve full last shells. This means that the halogens are used as oxidising agents . That is they accept electrons from other elements, they oxidise them and by accepting electrons they are reduced e.g. All the halogens react with iron wool. The trends are what you might expect:
Iron is a fairly unreactive metal but still displays the trends you would expect. The strongly oxidising fluorine immediately accepts an electron from the iron atoms in the iron wool. The products of the reaction are:
The reaction of iodine and aluminium is much slower. In fact the two substances can be
mixed fairly safely on a clean dry tin lid. A few drops of water are added to catalyse and start the reaction. After about 60 seconds or so the reaction starts.
A bright vivid glow is seen as the aluminium iodide forms, but perhaps the most vivid part of the reaction are
the dense violet coloured fumes of iodine vapour that are produced. The heat produced by the reaction
causes some of the iodine to sublime into
In fact any reactive metal can be used to form salts called metal halides. The salts formed from these reactions are ionic. The metal atoms, lose electrons and are oxidised to form positive metal ions. The halogens always gain electrons, that is they are reduced to form negatively charged halide ions.
Perhaps one of the best reactions to show the reactivity trends in the halogens is the reaction with hydrogen gas. All the halogens react with hydrogen to form hydrogen halide vapours:
Fluorine being the smallest halogen atom will be able to attract a negatively charged electron from a metal atom more strongly towards its positively charged nucleus and so is the most reactive halogen. Iodine being in period 5 of the periodic table has 5 shells of electrons between its nucleus and any electron it tries to attract, these shells shield the positive nucleus from electrons that it is trying to attract. The iodine nucleus may have a much larger positive charge than the small fluorine nucleus, but the effect of shielding and the fact that the nucleus is a long way from any electrons it may try and attract means that the ability to attract electrons decreases as you descend group 7.