Lanthanum (La)

Atomic Model Details of Lanthanum (La)

Lanthanum (La) is the first element of the lanthanide series, a f-block element, though its ground state electronic configuration usually places a d-electron.

Atomic Number (Z): 57
Atomic Mass: 138.9055 u
Number of Protons: 57 (equal to atomic number)
Number of Electrons: 57 (for a neutral atom)
Number of Neutrons: 82 (Calculated from mass number 139 - atomic number 57 for the most common isotope, ¹³⁹La)
Atomic Radius (Metallic): 207 pm

Electronic Configuration and Shell Diagram

Electronic Configuration

The ground state electronic configuration of Lanthanum is:

[Xe] 5d¹ 6s²

This configuration signifies that it has the electron configuration of Xenon (Xe) followed by one electron in the 5d subshell and two electrons in the 6s subshell. This d-electron is characteristic of a transition metal, even though La is considered the progenitor of the f-block series.

Shell Diagram Representation (Textual)

The distribution of electrons in different shells (K, L, M, N, O, P) for Lanthanum (Z=57) is as follows:

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): 18 electrons (4s² 4p⁶ 4d¹⁰)
O-shell (n=5): 9 electrons (5s² 5p⁶ 5d¹)
P-shell (n=6): 2 electrons (6s²)

This distribution leads to the configuration 2, 8, 18, 18, 9, 2.

Periodic Trends of Lanthanum

Lanthanum exhibits trends typical of early d-block elements and precedes the contraction observed across the f-block.

Ionization Enthalpy

Ionization enthalpy is the energy required to remove an electron from a gaseous atom.

First Ionization Enthalpy (IE₁): 538.1 kJ/mol (Relatively low, indicating ease of losing the outermost electron)
Second Ionization Enthalpy (IE₂): 1067 kJ/mol
Third Ionization Enthalpy (IE₃): 1850 kJ/mol (Typical for forming a stable +3 ion)

Electronegativity

Pauling Electronegativity: 1.10 (Relatively low, indicative of its metallic character and tendency to lose electrons)

Electron Gain Enthalpy

Electron Gain Enthalpy: Typically positive or very low negative for metals. For Lanthanum, it is generally considered positive or negligible, as metals have a low affinity for gaining electrons.

Atomic Radius

Atomic Radius (Metallic): 207 pm. This is relatively large compared to elements in earlier periods due to the increasing number of electron shells.

Key Physical Properties of Lanthanum

Lanthanum is a soft, silvery-white metal.

Density: 6.16 g/cm³ (at 20 °C)
State at STP: Solid
Color: Silvery-white (tarnishes rapidly in air)
Melting Point: 920 °C
Boiling Point: 3464 °C

Lanthanum (La) - Atomic Structure & Properties