Physical Properties and Halide Formation

Meeting the halogens

Group 7 contains the halogens. The familiar members are fluorine, chlorine, bromine and iodine. They are non-metals, and each of their atoms has seven electrons in its outer shell. This shared electronic structure gives them similar chemical reactions.

An individual halogen atom is represented by a symbol such as $Cl$. However, an elemental halogen normally exists as molecules containing pairs of atoms:

• fluorine: $F_2$
• chlorine: $Cl_2$
• bromine: $Br_2$
• iodine: $I_2$

They are therefore described as diatomic. The small 2 is part of the formula: it says that each molecule contains two atoms joined by a covalent bond. This matters when writing equations—elemental chlorine is $Cl_2$, not $Cl$.

Physical appearance at room temperature

The halogens change noticeably down the group. Fluorine is a pale yellow gas and chlorine is a pale green gas. Bromine is a dark orange liquid, while iodine is a grey solid. All four are toxic.

These different states can be understood using melting and boiling points. At a given temperature:

• a substance is solid below its melting point;
• it is liquid between its melting and boiling points;
• it is gas above its boiling point.

For example, bromine melts at about $-7\,^{\circ}\mathrm{C}$ and boils at about $59\,^{\circ}\mathrm{C}$. Room temperature lies between these values, so bromine is a liquid. Iodine melts at about $114\,^{\circ}\mathrm{C}$, well above room temperature, so it is solid.

Trends down Group 7

Going down Group 7, the molecules contain atoms with greater relative atomic masses. Consequently, their relative molecular masses increase: approximately 38 for $F_2$, 71 for $Cl_2$, 160 for and 254 for .

Their melting and boiling points also increase down the group. The data show the overall pattern from gases near the top, through liquid bromine, to solid iodine at room temperature.

Data for Melting and boiling points of the halogens

A useful prediction follows from these trends: an element below iodine would be expected to have a greater relative molecular mass and generally higher melting and boiling points. A prediction should follow the trend; an exact value cannot be obtained unless sufficient numerical data are supplied.

What happens when a halogen reacts with a metal?

When chlorine, bromine or iodine reacts with a metal, electrons are transferred. A halogen atom gains one electron to complete its outer shell and forms a negatively charged halide ion:

$Cl + e^- \rightarrow Cl^-$

Similarly, bromine forms $Br^-$ ions and iodine forms $I^-$ ions. The metal loses electrons and forms positive ions. Electrostatic attraction between the oppositely charged ions produces an ionic compound with a giant ionic lattice.

For example, potassium reacts with chlorine to form potassium chloride:

$2K + Cl_2 \rightarrow 2KCl$

The coefficient 2 is needed because chlorine is diatomic. Each of the two chlorine atoms becomes a chloride ion, so two potassium atoms are required to supply two electrons.

The products are named by changing the halogen ending to -ide: chlorine forms chlorides, bromine forms bromides and iodine forms iodides. These metal halides are salts. Alkali-metal halides such as potassium chloride are ionic solids.

What happens when a halogen reacts with a non-metal?

Two non-metals do not normally form ions by transferring electrons from one to the other. Instead, a halogen and another non-metal share electrons, forming covalent bonds. The product consists of molecules rather than a giant ionic lattice.

Therefore, the central distinction is:

Do not assume that every compound containing chlorine, bromine or iodine is ionic. Its nature depends on the other element: a metal usually gives an ionic halide, whereas a non-metal gives a covalent molecular compound.