Introduction

Electronic configuration describes the arrangement of electrons in the shells and subshells of an atom. It explains how electrons are distributed around the nucleus.

Electronic configuration is important because it determines chemical behavior, bonding, electrical conductivity, and other material properties.

In engineering materials, electronic configuration helps explain why metals conduct electricity and why different elements form different types of bonds.

Definition

Electronic configuration is the distribution of electrons in different energy levels, shells, and orbitals of an atom.

Energy Levels (Shells)

Electrons move around the nucleus in specific energy levels called shells.

These shells are represented by:

K, L, M, N shells

Each shell can hold a maximum number of electrons.

Maximum electrons in a shell are given by:

Maximum electrons = 2n²

Where
n = shell number

Examples:

K shell (n = 1) → 2 electrons
L shell (n = 2) → 8 electrons
M shell (n = 3) → 18 electrons

Subshells and Orbitals

Each shell is divided into subshells.

The main subshells are:

s, p, d, f

Maximum electrons in each subshell:

s → 2 electrons
p → 6 electrons
d → 10 electrons
f → 14 electrons

Rules for Electronic Configuration

Electrons fill orbitals according to certain rules.

Aufbau Principle

Electrons fill lower energy orbitals first, then higher energy orbitals.

Example order:

1s → 2s → 2p → 3s → 3p → 4s → 3d

Pauli Exclusion Principle

An orbital can contain maximum two electrons, and they must have opposite spins.

Hund's Rule

Electrons occupy empty orbitals first before pairing occurs.

Examples of Electronic Configuration

Hydrogen (Atomic number = 1)

1s¹

Helium (Atomic number = 2)

1s²

Carbon (Atomic number = 6)

1s² 2s² 2p²

Sodium (Atomic number = 11)

1s² 2s² 2p⁶ 3s¹

Iron (Atomic number = 26)

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

Valence Electrons

Valence electrons are the electrons present in the outermost shell of an atom.

They determine:

Chemical bonding
Reactivity
Electrical conductivity

Example:

Sodium → 1 valence electron
Oxygen → 6 valence electrons

Importance in Engineering Materials

Electronic configuration explains:

Electrical conductivity of metals
Chemical bonding between atoms
Magnetic behavior of materials
Material properties such as strength and hardness

For example:

Metals have free valence electrons, which allows them to conduct electricity and heat.

Exam-Focused Points

Electronic configuration describes distribution of electrons in shells and orbitals.

Maximum electrons in a shell = 2n².

Subshell capacities:

s = 2
p = 6
d = 10
f = 14

Important rules:

Aufbau principle
Pauli exclusion principle
Hund's rule

Valence electrons determine bonding and chemical behavior.

Example Competitive Exam Questions

What is electronic configuration?
Answer — Arrangement of electrons in shells and orbitals of an atom.

Maximum number of electrons in a shell is given by which formula?
Answer — 2n².

Maximum electrons in p subshell?
Answer — 6.

Which rule states that two electrons in an orbital must have opposite spins?
Answer — Pauli exclusion principle.

Which electrons determine chemical bonding?
Answer — Valence electrons.

Quick Revision

Electronic configuration = arrangement of electrons in an atom.

Maximum electrons in shell = 2n².