Electronic Configuration

Electronic Configuration is fundamental in chemistry. It explains the arrangement of electrons in atoms and molecules, influencing their chemical properties and behavior. The rules and principles governing electronic configuration provide a structured approach to predicting and explaining these arrangements.

1.0What is Electronic Configuration?

Electronic Configuration refers to the distribution of electrons of an atom or molecule in atomic or molecular orbitals. It provides a way of understanding the placement of electrons in different energy levels around the nucleus of an atom.

The electronic configuration of an element describes how electrons are distributed in its atomic orbitals. This configuration follows a specific notation that reflects the order in which electrons occupy the available subshells, with the number of electrons in each subshell indicated by a superscript. 

2.0Distribution of Electrons in Shells and Subshells

The maximum number of electrons that can be accommodated in a shell is based on the principal quantum number (n). This capacity is represented by the formula 2n², where ‘n’ is the shell number. The following table summarizes the shells, their corresponding values of n, and the total number of electrons each shell can accommodate:

Here is an example of filling electrons:

Each shell is divided into subshells (s, p, d, f), and these subshells have a specific capacity for electrons:

1. s-subshell:

• Holds a maximum of 2 electrons.
• Appears in all shells.

2. p-subshell:

• Holds a maximum of 6 electrons.
• Appears from the second shell (n=2) onward.

3. d-subshell:

• Holds a maximum of 10 electrons.
• Appears from the third shell (n=3) onward.

4. f-subshell:

• Holds a maximum of 14 electrons.
• Appears from the fourth shell (n=4) onward.

3.0Principles Governing electronic configuration

1. Pauli Exclusion Principle: 

• The Pauli Exclusion Principle is a fundamental concept in quantum mechanics, which states that: A maximum of two electrons, each having opposite spins, can occupy a single orbital.
• This principle can also be expressed as: No two electrons in the same atom can have the same set of all four quantum numbers.

2. Aufbau Principle: 

The Aufbau Principle, from the German word ‘Aufbauen’ meaning ‘build up,’ dictates the order in which electrons occupy orbitals in an atom. According to this principle, electrons fill orbitals starting with the lowest available energy levels before filling higher energy levels.

Energy Order:

• Electrons will occupy the orbitals with lower energies first.
• This is based on minimizing the energy of the atom, leading to the most stable electronic configuration.

Calculating Orbital Energy:

• The energy of an orbital is determined by the sum of the principal quantum number (n) and the azimuthal quantum number (l).
• Lower values of (n + l) correspond to lower energy orbitals.
• If two orbitals have the same (n + l) value, the orbital with the lower n value is filled first.

3. Hund's Rule: 

Hund's Rule describes the order in which electrons are filled in the orbitals of a given subshell. It states that:

Every orbital in a given subshell is singly occupied by electrons before any orbital is doubly occupied.

To maximize the total spin, the electrons in the singly occupied orbitals must have the same spin, meaning they all have the same value of the spin quantum number (m_s).

4.0Filling Order of Electrons

Electrons fill orbitals in the order of increasing energy. The general order is as follows: 

1s→2s→2p→3s→3p→4s→3d→4p→5s→4d→5p→6s→4f→5d→6p→7s→5f→6d→7p

Standard Notation of electronic configuration

In this notation, all electron-containing atomic subshells are placed in a sequence. For example, the electronic configuration of sodium is written as 1s²2s²2p⁶3s¹.

• 1s²:
• • 1: Principal energy level (n=1).
  • s: Type of orbital.
  • 2: Number of electrons in the 1s orbital.

5.0Electronic Configuration Table:

Here is electronic configuration for Elements 1 to 30

6.0Electronic Configuration of Ions

For ions, electrons are added or removed according to the same rules, starting from the outermost shell:

1. Cations (positive ions): Electrons are removed starting from the highest energy level.

• Example: Na⁺ has the configuration 1s²2s²2p⁶ (one electron removed from 3s).

2. Anions (negative ions): Electrons are added to the lowest available energy level.

• Example: Cl⁻ has the configuration 1s²2s²2p⁶3s²3p⁶ (one electron added to 3p).