Physical properties of the Group 17 elements

The elements in Group 17 of the Periodic Table are known as the halogens. This family includes fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). The defining characteristic of a halogen atom is that it has seven electrons in its outermost shell. To achieve a stable electron arrangement like a noble gas (an octet), they are highly reactive and tend to gain one electron. At room temperature and pressure, they exist not as single atoms but as diatomic molecules, written as F₂, Cl₂, Br₂, and I₂. Within each molecule, the two atoms are joined by a single covalent bond, where they share a pair of electrons.

Volatility refers to how easily a substance turns into a gas. Down Group 17, volatility decreases: fluorine is the most volatile and iodine is the least. This trend is explained by their boiling points, which increase as you go down the group. Halogens have a simple molecular structure. The forces between their diatomic molecules are weak intermolecular forces called van der Waals' forces. These forces arise from temporary fluctuations in electron density, creating instantaneous dipoles that induce dipoles in neighbouring molecules. As you go down the group from F₂ to I₂, the number of electrons in each molecule increases significantly. A larger number of electrons leads to stronger and more frequent instantaneous dipoles, and thus stronger van der Waals' forces. More energy is needed to overcome these stronger intermolecular forces, leading to higher melting and boiling points.

The physical properties of halogens show clear trends down the group. At room temperature (approx. 20°C), their physical states change: fluorine and chlorine are gases, bromine is a volatile liquid, and iodine is a solid. The colour of the halogens also deepens down the group: fluorine is a pale-yellow gas, chlorine is a greenish-yellow gas, bromine is a red-brown liquid, and iodine is a grey-black solid which produces a violet vapour when heated (sublimation). Another important trend is atomic radius, which increases down the group. This is because each element down the group has one more principal electron shell than the one above it. Although the nuclear charge (number of protons) increases, the outer electrons are further from the nucleus and experience greater shielding from the attraction of the nucleus by the inner shells of electrons. This effect of adding a new shell is the dominant factor, causing the atom to be larger.

A key skill in chemistry is using periodic trends to predict the properties of unknown or less-common elements. Astatine (At) sits below iodine in Group 17. By extrapolating the trends we have observed for the other halogens, we can make educated predictions about its physical properties. Since melting/boiling points increase down the group, astatine will have a higher melting and boiling point than iodine, and will therefore be a solid at room temperature. Since the colours darken down the group (yellow -> green-yellow -> red-brown -> grey-black), astatine is predicted to be a black solid. Its volatility will be the lowest in the group, and its atomic radius will be the largest.

The chemical reactivity of the halogens is dominated by their tendency to gain an electron. This makes them powerful oxidising agents (they oxidise other substances by taking their electrons). The reactivity and oxidising power of the halogens decrease down the group. Fluorine is the most powerful oxidising agent, while iodine is the weakest. This trend is explained by changes in atomic structure. As you go down the group: 1. Atomic radius increases: The outer shell is further from the nucleus. 2. Electron shielding increases: There are more inner shells of electrons to shield the outer shell from the positive pull of the nucleus. These two factors mean that the nucleus has a weaker electrostatic attraction for a new electron. Although the nuclear charge increases down the group, the effects of increased distance and shielding are more significant. Therefore, it becomes progressively harder for atoms to attract and capture an electron as you descend the group, making them weaker oxidising agents.