Chlorine vs Bromine
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, Bromine, is usually the more reactive of the two if they are metals, and the less reactive if they are non-metals.
- At room temperature Chlorine is a gas and Bromine is a liquid.
- Bromine melts at -7 °C, Chlorine at -102 °C.
- An atom of Bromine is about 2.3 times heavier than an atom of Chlorine (79.90 u against 35.45 u).
- Chlorine pulls electrons more strongly in a bond (electronegativity 3.16 against 2.96).
Side by side
| Chlorine | Bromine | |
|---|---|---|
| Atomic number | 17 | 35 |
| Symbol | Cl | Br |
| Family | Diatomic non-metal | Diatomic non-metal |
| Group / period | 17 / 3 | 17 / 4 |
| State at room temperature | Gas | Liquid |
| Atomic mass | 35.450 u | 79.904 u |
| Density | 3.2 g/L | 3.1028 g/cm³ |
| Melting point | -102 °C | -7 °C |
| Boiling point | -34 °C | 59 °C |
| Electronegativity | 3.16 | 2.96 |
| First ionisation energy | 1251.2 kJ/mol | 1139.9 kJ/mol |
| Electron configuration | [Ne] 3s2 3p5 | [Ar] 3d10 4s2 4p5 |
| Discovered by | Carl Wilhelm Scheele | Antoine Jérôme Balard |
Chlorine
Chlorine has two faces. As a free gas it is greenish-yellow, heavier than air and dangerous to breathe, and it was used as a weapon in the First World War. Combined with sodium it is the harmless white crystal you sprinkle on chips. The difference between the two is a single electron, and that electron is the whole story of chlorine.
- Chlorine was the first chemical agent used on a large scale in war, released from cylinders at Ypres in April 1915.
- The swimming pool smell most people call chlorine is mostly chloramines, formed when chlorine reacts with sweat and urine; a well-kept pool smells of very little.
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.
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.