Boron (B) Revision Guide

Atomic Model Details

Boron is the first element of Group 13 in the p-block of the periodic table. It is classified as a metalloid.

• Symbol: B
• Atomic Number (Z): 5 (Indicates 5 protons in the nucleus).
• Mass Number (A): Approximately 10.81 atomic mass units (amu). This is the weighted average of its naturally occurring isotopes.
• Protons: 5
• Electrons: 5 (in a neutral Boron atom)
• Neutrons:
  • Boron-11 ($^{11}\text{B}$), the most common isotope (approx. 80%), has $11 - 5 = 6$ neutrons.
  • Boron-10 ($^{10}\text{B}$), the less common isotope (approx. 20%), has $10 - 5 = 5$ neutrons.
• Atomic Size:
  • Covalent Radius: 85 pm
  • Van der Waals Radius: 192 pm

Electronic Configuration & Shell Diagram

Electronic Configuration

The ground state electronic configuration of Boron is: $1s^2 2s^2 2p^1$

Shell Diagram Representation

The electron distribution in different shells can be represented as:

• K-shell (n=1): Contains 2 electrons ($1s^2$)
• L-shell (n=2): Contains 3 electrons ($2s^2 2p^1$)
  • Specifically, 2 electrons in the 2s orbital and 1 electron in one of the 2p orbitals.

Therefore, Boron has 3 valence electrons in its outermost L-shell, which participate in chemical bonding.

Periodic Trends

Ionization Enthalpy (IE)

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

• First Ionization Enthalpy ($IE_1$): 800.6 kJ/mol. This value is higher than that of Beryllium (Be) due to Boron’s smaller size and greater effective nuclear charge, but lower than Carbon (C). There is an anomaly in the trend from Be to B due to the removal of an electron from a 2p orbital in B (which is higher in energy and less penetrating than a 2s orbital in Be).
• Second Ionization Enthalpy ($IE_2$): 2427.1 kJ/mol
• Third Ionization Enthalpy ($IE_3$): 3659.7 kJ/mol

Electronegativity

• Pauling Scale Electronegativity: 2.04. Boron is more electronegative than most metals but less electronegative than non-metals like Carbon, Nitrogen, and Oxygen.

Electron Gain Enthalpy

• Electron Gain Enthalpy: +27 kJ/mol. Boron has a positive electron gain enthalpy, meaning energy is required to add an electron to its outermost shell. This indicates a low tendency to accept electrons, which is typical for elements that are more inclined to lose electrons or form covalent bonds.

Atomic Radius

• Covalent Radius: 85 pm.
• Boron has a relatively small atomic radius due to its small number of electron shells and a relatively high effective nuclear charge for its period.

Key Physical Properties

• Density:
  • Amorphous Boron: 2.34 g/cm³
  • Crystalline Boron: 2.37 g/cm³
• State at STP (Standard Temperature and Pressure): Solid
• Color:
  • Amorphous Boron: Black
  • Crystalline Boron: Black, with a metallic luster (silvery-gray)
• Melting Point: 2075 °C (2348 K). Boron has an exceptionally high melting point.
• Boiling Point: 3927 °C (4200 K). Boron also exhibits a very high boiling point.
• Hardness: Crystalline boron is extremely hard, ranking second only to diamond and cubic boron nitride among known materials.
• Nature: Metalloid. It exhibits properties intermediate between metals and non-metals. It is a semiconductor.