Bromine (Br)

Atomic Structure of Bromine (Br)

Atomic Number and Mass Number

Atomic Number (Z): 35 (indicates the number of protons in the nucleus).
Atomic Mass (A): 79.904 u (average atomic mass of naturally occurring isotopes).

Subatomic Particles

Based on an atomic number of 35 and an average atomic mass of approximately 80 u:

Protons: 35 (equal to the atomic number).
Electrons: 35 (in a neutral bromine atom, equal to the number of protons).
Neutrons: Approximately 45 (Mass Number - Atomic Number, e.g., for the most common isotope Br-79, it is 79-35 = 44 neutrons; for Br-81, it is 81-35 = 46 neutrons. The average atomic mass reflects the natural abundance of these isotopes).

Atomic Size

Covalent Radius: 120 pm
Van der Waals Radius: 185 pm

Electronic Configuration

Ground State Electronic Configuration

Full Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁵
Condensed Configuration: [Ar] 3d¹⁰ 4s² 4p⁵

Shell Diagram Representation

The electrons in a bromine atom are distributed in four main energy shells:

K-shell (n=1): 2 electrons (1s²)
L-shell (n=2): 8 electrons (2s² 2p⁶)
M-shell (n=3): 18 electrons (3s² 3p⁶ 3d¹⁰)
N-shell (n=4): 7 electrons (4s² 4p⁵) – This is the valence shell, containing 7 valence electrons.

Periodic Trends for Bromine

Bromine is a Group 17 element (Halogen) in Period 4, and its properties align with these classifications.

Ionization Enthalpy

First Ionization Enthalpy: 1140 kJ/mol.
- This is the energy required to remove the outermost electron from a gaseous bromine atom. It is high due to its small size and high effective nuclear charge, but lower than chlorine due to increased atomic size and shielding.

Electronegativity

Pauling Scale Electronegativity: 2.96.
- Bromine is highly electronegative, though less so than fluorine and chlorine, due to its position in the group. It strongly attracts electrons in a chemical bond.

Electron Gain Enthalpy

Electron Gain Enthalpy: -325 kJ/mol.
- This is the energy released when an electron is added to a gaseous bromine atom to form a uninegative ion (Br⁻). The negative value indicates an exothermic process, showing bromine’s strong tendency to gain an electron to achieve a stable noble gas configuration (isoelectronic with Krypton). It is less negative than chlorine due to larger atomic size.

Atomic Radius

Covalent Radius: 120 pm
Van der Waals Radius: 185 pm
- Atomic radius increases down a group and decreases across a period. Bromine’s radius is larger than chlorine but smaller than iodine.

Key Physical Properties of Bromine

Density

Density (liquid at 20°C): 3.1028 g/cm³

State at Room Temperature

State: Liquid (Bromine is one of only two elements that are liquid at room temperature and standard pressure, the other being mercury).

Color

Color: Reddish-brown. Its vapor is also reddish-brown and highly irritating.

Melting Point

Melting Point: -7.2 °C (265.95 K)

Boiling Point

Boiling Point: 58.8 °C (331.95 K)

Bromine (Br) - Atomic Structure & Chemical Properties