What are shells and subshells?

Short Answer

Shells are the main energy levels around the nucleus of an atom where electrons are present. They are represented by numbers (n = 1, 2, 3…) or letters (K, L, M, N). Each shell can hold a fixed maximum number of electrons depending on its energy.

Subshells are smaller divisions within each shell. These are named as s, p, d, and f. Each subshell has a different shape and a specific number of orbitals. Subshells help explain how electrons are arranged inside shells and how atoms form chemical bonds.

Detailed Explanation :

Shells and Subshells

In atomic structure, electrons are not placed randomly around the nucleus. Instead, they occupy specific energy levels called shells, and each shell is further divided into subshells. This arrangement helps explain the behaviour of electrons, the formation of bonds, the properties of elements, and the structure of the periodic table. Understanding shells and subshells is essential for studying electronic configuration, chemical bonding, and modern atomic theory.

Shells represent the major energy levels of an atom, while subshells provide more detailed information about how electrons populate these energy levels. Together, they give a clear and systematic picture of the electron distribution in an atom.

Shells

Shells are the main energy levels surrounding the nucleus. They are also known as principal energy levels. Each shell has a specific amount of energy, and electrons in shells farther from the nucleus have higher energy than those closer to it.

Shells are represented in two ways:

  • By numbers: n = 1, 2, 3, 4…
  • By letters: K, L, M, N…

For example:
n = 1 → K-shell
n = 2 → L-shell
n = 3 → M-shell

Each shell can hold a maximum number of electrons, calculated using the formula:

Maximum electrons = 2n²

So,

  • K-shell (n = 1): 2 electrons
  • L-shell (n = 2): 8 electrons
  • M-shell (n = 3): 18 electrons
  • N-shell (n = 4): 32 electrons

These limits ensure that electrons fill shells in a logical and predictable way.

Importance of Shells

Shells help explain:

  • The structure of atoms
  • Electron arrangement
  • Periodic table periods
  • Energy levels of electrons
  • Valency and chemical reactivity

The outermost shell is called the valence shell, and the electrons in it are valence electrons, which play a major role in chemical bonding.

Subshells

Within each shell, electrons are further grouped into subshells. Subshells explain the complex behaviour of electrons and help understand atomic spectra, bonding, and periodicity.

There are four types of subshells:

  • s-subshell
  • p-subshell
  • d-subshell
  • f-subshell

Each subshell has a specific shape and contains a set number of orbitals.

  1. s-subshell
  • Shape: spherical
  • Contains 1 orbital
  • Holds 2 electrons
  1. p-subshell
  • Shape: dumbbell-shaped
  • Contains 3 orbitals
  • Holds 6 electrons
  1. d-subshell
  • Shape: complex (cloverleaf)
  • Contains 5 orbitals
  • Holds 10 electrons
  1. f-subshell
  • Shape: highly complex
  • Contains 7 orbitals
  • Holds 14 electrons

As the shell number increases, the number of possible subshells also increases.

For example:

  • Shell 1 (K) → only s
  • Shell 2 (L) → s, p
  • Shell 3 (M) → s, p, d
  • Shell 4 (N) → s, p, d, f

This pattern helps in writing electronic configurations of atoms.

Relationship Between Shells and Subshells

Shells show the broad energy levels, while subshells show finer divisions within those levels. Subshells refine the electronic structure by dividing shells into regions of slightly different energy.

  • Shells determine the capacity of electrons.
  • Subshells determine the arrangement and behaviour of electrons.
  • Electrons fill subshells in increasing energy order.

This relationship is key to understanding how electrons are arranged.

How Shells and Subshells Affect Chemical Properties

The structure of shells and subshells determines:

  • How many electrons an atom has
  • How easily an atom gains or loses electrons
  • How atoms bond with each other
  • The position of elements in the periodic table
  • Trends like atomic size, ionization energy, and reactivity

For example:

  • Elements with full shells (like noble gases) are stable and unreactive.
  • Elements with one or two electrons in the outer shell (like metals) are highly reactive.

Subshells explain why transition metals show variable oxidation states and why f-block elements have special properties.

Examples of Shells and Subshells in Electronic Configuration

Hydrogen (Z = 1)

Electrons: 1
Shell: K
Subshell: s
Configuration: 1s¹

Carbon (Z = 6)

Electrons: 6
Shells: K, L
Subshells: s and p
Configuration: 1s² 2s² 2p²

Calcium (Z = 20)

Electrons: 20
Shells: K, L, M, N
Subshells: s, p, d
Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

These examples show how shells and subshells guide electron arrangement.

Conclusion

Shells are the main energy levels around an atom’s nucleus, while subshells are divisions within each shell that determine how electrons are arranged. Shells describe the overall structure, and subshells provide detailed information about electron distribution, shapes of orbitals, and chemical activity. Understanding shells and subshells is essential for studying electronic configuration, bonding, periodic trends, and the properties of elements in chemistry.