Introduction to Bromine
Bromine (Br) is a chemical element classified as a halogen, belonging to Group 17 of the periodic table. It is the only non-metallic element that is liquid at room temperature and standard pressure, appearing as a reddish-brown volatile liquid. The name “bromine” originates from the Greek word “bromos,” meaning “stench,” referring to its strong, unpleasant odour.
Occurrence and Uses
Bromine is found naturally in bromide salts dissolved in seawater, salt lakes, and subterranean brine deposits. In India, bromine is primarily extracted from sea bitterns, which are concentrated solutions left after the crystallisation of common salt (sodium chloride) from seawater. Major salt-producing regions, such as the coastal areas of Gujarat, contribute to this extraction process.
Bromine compounds find various applications. They are used in the production of flame retardants, essential for enhancing the fire resistance of plastics, textiles, and electronic components commonly found in homes and industries across India. Bromine compounds are also utilised in disinfectants, agricultural chemicals, and certain pharmaceutical preparations. For instance, some bromide salts historically found use as sedatives.
Atomic Structure of Bromine
The atomic structure of an element describes the arrangement of its subatomic particles: protons, neutrons, and electrons. For bromine, these particles are arranged as follows:
Fundamental Particles
The atomic number (Z) of bromine is 35. This number defines the element and represents the count of protons in the nucleus of every bromine atom.
- Protons: 35 For a neutral atom, the number of electrons is equal to the number of protons.
- Electrons: 35
The number of neutrons can vary, leading to isotopes of the element. The average atomic mass of bromine is approximately 79.904 atomic mass units (amu).
Isotopes
Bromine naturally occurs as two stable isotopes: Bromine-79 ($^{79}\text{Br}$) and Bromine-81 ($^{81}\text{Br}$). An isotope refers to atoms of the same element that have the same number of protons but different numbers of neutrons.
- For Bromine-79 ($^{79}\text{Br}$):
- Mass Number (A) = 79
- Number of Neutrons = Mass Number - Atomic Number = 79 - 35 = 44
- For Bromine-81 ($^{81}\text{Br}$):
- Mass Number (A) = 81
- Number of Neutrons = Mass Number - Atomic Number = 81 - 35 = 46
Electron Configuration
Electron configuration describes the distribution of electrons of an atom in atomic orbitals. For a neutral bromine atom with 35 electrons, its electron configuration follows the Aufbau principle, Hund’s rule, and Pauli’s exclusion principle.
The complete electron configuration for bromine is: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^5$
This can also be written in a condensed form using the noble gas preceding bromine in the periodic table, which is Argon (Ar): $[Ar] 3d^{10} 4s^2 4p^5$
This configuration indicates that the first 18 electrons fill the orbitals up to the 3p subshell, mimicking Argon’s configuration. Following this, the 4s subshell is filled, then the 3d subshell, and finally, the 4p subshell accommodates the remaining electrons.
Valence Electrons
Valence electrons are the electrons located in the outermost shell of an atom. These are the electrons that participate in chemical bonding and largely determine an element’s chemical properties.
For bromine, the highest principal quantum number (n) in its electron configuration is 4 (corresponding to the 4th shell). The electrons in this shell are found in the $4s^2$ and $4p^5$ subshells.
Therefore, the number of valence electrons for bromine is the sum of electrons in the $4s$ and $4p$ subshells: $2 + 5 = 7$.
This count of 7 valence electrons is characteristic of elements in Group 17 (halogens), which typically gain one electron to achieve a stable octet configuration like a noble gas.