Physical properties and trends

The halogens make up group 7 of the periodic table. This family consists of 5 elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).

As highly reactive non-metals, they tend to form diatomic molecules with one covalent bond between two atoms (e.g. F_2_). Halogens have some notable trends down the group.

The appearances of Cl₂, Br₂ and I₂ at room temperature are shown below:

Melting and boiling points increase down group 7

The melting and boiling points increase down the group because:

• The molecules become larger in size and relative mass.
• This allows for stronger induced dipole-dipole forces to develop between molecules.
• These stronger intermolecular forces require increasing amounts of energy to overcome.
• So the halogens melt and boil at higher temperatures down the group.

Electronegativity decreases down group 7

Related to atomic size, the electronegativity of halogens shows an overall decrease going down the group. Electronegativity refers to the tendency of an atom to attract a bonding pair of electrons.

The electronegativity decreases down the halogens because:

• Nuclear charge - Increases down the group as more protons are added, increasing attraction for electrons.
• Atomic radius - Increases down the group as more electron shells are added, moving electrons away from nucleus.
• Electron shielding - Inner electron shells reduce the atom’s ability to attract electrons. Although the halogens have high electronegativities compared to other elements, the atomic radius and shielding effects down the group are greater than the nuclear charge effect. This leads to an overall decrease in electronegativity on descending group 7.

Halogens form 1- ions by gaining electrons

Halogens react by gaining an electron to form negative ions with a 1- charge (known as halide ions). For example:

X + e⁻ ➔ X⁻

Where X represents a halogen atom.

This electron gain results in the halogen being reduced, as its oxidation number decreases from 0 to -1.

As the halogen gains an electron, it causes another substance to be oxidised - so halogens act as oxidising agents

Reactivity decreases down group 7

The reactivity of the halogens decreases going down the group because:

• Atomic radius increases down the group as more electron shells are added.
• The increasing size leads to the outer electrons being farther from the positive nucleus.
• The outer electrons also experience more shielding from inner electron shells.
• The electrostatic attraction between the outer electrons and nucleus gets progressively weaker.
• The increase in atomic radius and shielding outweigh the increase in nuclear charge so it becomes harder for larger halogens to attract the electron needed to form a negative ion.