Bromine (35)

Bromine is the only liquid nonmetallic element. It is a member of the halogen group. It is a heavy, volatile, mobile, dangerous reddish-brown liquid. The red vapor has a strong unpleasant odor and the vapor irritates the eyes and throat. It is a bleaching. When spilled on the skin it produces painful sores. It is a serious health hazard, and maximum safety precautions should be taken when handling it.

•Name: Bromine
•Symbol: Br
•Atomic number: 35
•Atomic weight: 79.904
•Standard state: liquid at 298 K
•CAS Registry ID: 7726-95-6
•Group in periodic table: 17
•Group name: Halogen
•Period in periodic table: 4
•Block in periodic table: p-block
•Color: red-brown, metallic luster when solid
•Classification: Non-metallic

Historical information

Bromine was discovered by Antoine-J. Balard in France, 1826. Origin of nameis from the Greek word "bromos" meaning "stench." Bromine was not prepared in quantity until 1860 but compounds of bromine were of some considerable importance well before it was recognized as an element. Long ago an excretion from a particular kind of mussel was used to make a purple dye called "Tyrian purple". It is now known that a key compound in this process is an organobromine compound.

It seems that an undergraduate chemist called Carl Löwig studying at Heidelberg presented one of his lecturers, Leopold Gmelin, with a sample of bromine that he had made over the summer holidays. Löwig's exams interrupted his studies long enough to allow a report from Antoine-Jérôme Balard to take precedence in 1826.

Physical properties

•Melting point: 265.8 [or -7.3 °C (19 °F)] K
•Boiling point: 332 [or 59 °C (138 °F)] K
•Density of solid: 4050 kg m-3

Orbital properties

•Ground state electron configuration: [Ar].3d10.4s2.4p5
•Shell structure: 2.8.18.7
•Term symbol: 2P3/2

Isolation

Bromine is available commercially so it is not normally necessary to make it in the laboratory. Bromine also occurs in seawater as the sodium salt but in much smaller quantities than chloride. It is recovered commercially through the treatment of seawater with chlorine gas and flushing through with air. In this treatment, bromide is oxidized to bromine by the chlorine gas. The principle of oxidation of bromide to bromine is shown by the addition of a little chlorine water to aqueous solutions of bromide. These become brown as elemental bromine forms.

2Br- + Cl2 → 2Cl- + Br2

Small amounts of bromine can also be made through the reaction of solid sodium bromide, NaBr, with concentrated sulphuric acid, H2SO4. The first stage is formation of HBr, which is a gas, but under the reaction conditions some of the HBr is oxidized by further H2SO4 to form bromine and sulphur dioxide. This reaction does not work with the corresponding chlorides and fluorides.

NaBr (s) + H2SO4 (l) → HBr (g) + NaHSO4 (s)

2HBr (g) + H2SO4 (l) → Br2 (g) + SO2 (g) + 2H2O (l)