Physical Properties of Group 7 (AQA A-Level Chemistry): Revision Notes

2.3.1 Physical Properties of Group 7

The halogens (Group 7 elements) exhibit distinct trends in their physical properties as you move down the group. These trends can be explained by understanding their atomic structure, bonding, and the forces between their molecules. Two key properties you must focus on are electronegativity and boiling points.

Electronegativity Trend in Group 7

Electronegativity is defined as the ability of an atom to attract a bonding pair of electrons in a covalent bond. For halogens, as you move down the group from fluorine (F) to iodine (I), electronegativity decreases.

Explanation

• As you go down Group 7, the number of electron shells increases. This causes the atomic radius to increase.
• The bonding electrons are further away from the positively charged nucleus, and there is increased shielding by inner electrons.
• The reduced attraction between the nucleus and the bonding electrons results in lower electronegativity. For example, fluorine, with only two electron shells, has the highest electronegativity in the group, while iodine, with five electron shells, has much lower electronegativity.

Order of Electronegativity

Fluorine > Chlorine > Bromine > Iodine

Boiling Point Trend in Group 7

Boiling points of halogens increase as you move down the group. This can be explained in terms of molecular size and the intermolecular forces acting between diatomic halogen molecules.

Explanation

• The size of the halogen molecules (e.g., $F_2$, $Cl_2$, $Br_2$, $I_2$) increases down the group due to the addition of more electron shells.
• Larger molecules have more electrons, which leads to stronger van der Waals forces (induced dipole-dipole interactions).
• Stronger van der Waals forces require more energy to overcome, hence the boiling points increase.

Order of Boiling Points

Fluorine < Chlorine < Bromine < Iodine

Oxidising Ability

The halogens are oxidising agents because they gain electrons in reactions, forming halide ions. The oxidising ability of the halogens decreases down Group 7. This means fluorine is the strongest oxidising agent, while iodine is the weakest.

Explanation

• As you go down the group, the atomic radius increases because additional electron shells are added.
• This increased atomic size means that the outermost electrons are further from the nucleus, and there is greater shielding from the inner electron shells.
• Consequently, the ability of the nucleus to attract additional electrons weakens, making it harder for the atom to gain electrons.

Order of Oxidising Power

Fluorine > Chlorine > Bromine > Iodine

Reducing Ability

While halogens are strong oxidising agents, their halide ions (e.g., Cl^-, Br^-, I^-) act as reducing agents. The reducing power of halide ions increases down the group. In other words, iodide ions are the strongest reducing agents, and chloride ions are the weakest.

Explanation

• As you move down the group, the halide ions become larger with more electron shells.
• This makes the outermost electrons further from the nucleus and more shielded, thus reducing the nucleus's attraction to these electrons.
• Therefore, it becomes easier for the ion to lose electrons and be oxidised, making it a stronger reducing agent.

Order of Reducing Power

Iodide > Bromide > Chloride