Bromine vs Iodine
Both are diatomic non-metals, so they share the same broad chemistry.
The differences in plain English
- They sit in the same column, group 17, so they have the same number of outer electrons and react in similar ways; the heavier one, Iodine, is usually the more reactive of the two if they are metals, and the less reactive if they are non-metals.
- At room temperature Bromine is a liquid and Iodine is a solid.
- Iodine melts at 114 °C, Bromine at -7 °C.
- An atom of Iodine is about 59 per cent heavier than an atom of Bromine (126.90 u against 79.90 u).
- Bromine pulls electrons more strongly in a bond (electronegativity 2.96 against 2.66).
Side by side
| Bromine | Iodine | |
|---|---|---|
| Atomic number | 35 | 53 |
| Symbol | Br | I |
| Family | Diatomic non-metal | Diatomic non-metal |
| Group / period | 17 / 4 | 17 / 5 |
| State at room temperature | Liquid | Solid |
| Atomic mass | 79.904 u | 126.904 u |
| Density | 3.1028 g/cm³ | 4.933 g/cm³ |
| Melting point | -7 °C | 114 °C |
| Boiling point | 59 °C | 184 °C |
| Electronegativity | 2.96 | 2.66 |
| First ionisation energy | 1139.9 kJ/mol | 1008.4 kJ/mol |
| Electron configuration | [Ar] 3d10 4s2 4p5 | [Kr] 4d10 5s2 5p5 |
| Discovered by | Antoine Jérôme Balard | Bernard Courtois |
Bromine
Open a bottle of bromine and a heavy red-brown vapour rolls out over the rim. The liquid beneath is dark, dense and almost black in bulk, and the smell is sharp enough that the element was named from the Greek word for stench. Only two elements are liquid at ordinary room temperature, and bromine is the one that is not a metal.
- Bromine and mercury are the only two elements that are liquid at normal room temperature and pressure.
- Balard was 23 years old when he identified bromine in salt marsh brine.
Iodine
Iodine is the heaviest element that the human body needs in significant amounts. Without it, the thyroid gland cannot make the hormones that set the pace of metabolism, and a shortage in childhood stunts both growth and brain development. A pinch of iodised salt is one of the cheapest public health measures ever devised.
- Iodine was discovered by accident when a gunpowder manufacturer added too much acid to seaweed ash and saw violet vapour rise.
- The thyroid hormone thyroxine contains four iodine atoms per molecule, which is why it is abbreviated T4.
Colour the bubbles by density or melting point and the difference jumps out.
More comparisons
Data from Periodic-Table-JSON (CC BY-SA); temperatures rounded to the nearest degree. Written by Anthony.