Barium (Ba) - Atomic Structure and Properties

Atomic Model Details

Barium (Ba) is an alkaline earth metal found in Group 2, Period 6 of the periodic table.

Subatomic Particles

• Atomic Number (Z): 56
  • Number of Protons: 56
  • Number of Electrons: 56 (in a neutral atom)
• Mass Number (A): The most abundant isotope of Barium is $^{138}$Ba.
  • Number of Neutrons: A - Z = 138 - 56 = 82

Atomic Size

• Atomic Radius (metallic): 215 pm (picometers)
• Ionic Radius (Ba²⁺): 135 pm

Electronic Configuration & Shell Diagram Representation

Electronic Configuration

• Full Electronic Configuration: $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6 4d^{10} 5s^2 5p^6 6s^2$
• Noble Gas Configuration: $[Xe] 6s^2$

Shell Diagram Representation

A shell diagram for Barium illustrates the distribution of electrons in different energy levels (shells) around the nucleus.

• Nucleus: Contains 56 protons and 82 neutrons.
• Shell 1 (K-shell): 2 electrons
• Shell 2 (L-shell): 8 electrons
• Shell 3 (M-shell): 18 electrons
• Shell 4 (N-shell): 18 electrons
• Shell 5 (O-shell): 8 electrons
• Shell 6 (P-shell): 2 electrons (valence electrons)

This configuration indicates that Barium has 2 valence electrons in its outermost shell ($6s^2$), which it readily loses to form a Ba²⁺ ion, achieving a stable noble gas configuration (that of Xenon, [Xe]).

Periodic Trends

Ionization Enthalpy

Ionization enthalpy is the energy required to remove an electron from a gaseous atom in its ground state.

• First Ionization Enthalpy ($IE_1$): 502.9 kJ/mol
• Second Ionization Enthalpy ($IE_2$): 965.2 kJ/mol Barium has a relatively low first ionization enthalpy due to its large atomic size and the shielding effect of inner electrons, making it easy to remove the outermost electron. The second ionization enthalpy is higher but still relatively low, as the removal of the second electron leads to a stable noble gas configuration.

Electronegativity

Electronegativity is a measure of an atom’s ability to attract shared electrons in a chemical bond.

• Pauling Scale Electronegativity: 0.89 Barium is a highly electropositive element, indicating its strong tendency to lose electrons and form positive ions, rather than attract electrons. This is characteristic of alkaline earth metals.

Electron Gain Enthalpy

Electron gain enthalpy is the energy change when an electron is added to a neutral gaseous atom to form a negative ion.

• Electron Gain Enthalpy: ~+14 kJ/mol (Estimated, typically positive for metals) For Barium, like other alkaline earth metals, the electron gain enthalpy is positive. This means energy must be supplied to force Barium to accept an additional electron, forming a highly unstable Ba⁻ ion. Barium prefers to lose electrons rather than gain them.

Atomic Radius

• Atomic Radius (metallic): 215 pm Barium has a large atomic radius compared to elements in earlier periods and groups due to the presence of multiple electron shells and effective shielding of the nucleus by inner electrons. Atomic radius increases down a group.

Key Physical Properties

• Density: 3.62 g/cm³
• State at room temperature (25°C): Solid
• Color: Silvery-white, tarnishes to dull grey upon exposure to air
• Melting Point: 727 °C (999 K)
• Boiling Point: 1897 °C (2170 K)