Shells 

An atom is the smallest particle of matter, consisting of a nucleus surrounded by electrons. The electrons fill specific energy levels called atomic or electron shells, which determine the chemical behaviour and stability of an atom.

1.0Introduction to Shells 

An electron shell is a space outside the nucleus of the atom where electrons are probably to be found. These spaces are labelled with the capital letters K, L, M, N, O, & P, and so on, or with numbers 1, 2, 3, 4, 5, & 6 and so on. Every shell has a unique level of energy, where the closest shell to the nucleus has the lowest.

2.0Shells and Subshells

Each electron shell is further divided into subshells, which are denoted with letters s, p, d, and f. Unlike shells, each subshell holds a different shape and orientation within an atom, and they also hold different numbers of electrons. For instance: 

• s subshell holds 2 electrons,
• p subshell holds 6 electrons,
• d subshell holds 10 electrons,
• f subshell holds 14 electrons.

Shells, too, hold different numbers of subshells according to their sequence. For example: 

• The first shell (K) only has an s subshell and hence holds 2 electrons. 
• The second shell (L) has both s and p subshells and holds 8 electrons
• The third shell (M) contains s, p, and d subshells and holds 18 electrons. And so on.

Note: Orbitals are the regions where there is the most probability of finding electrons in the shell. 

3.0Key Features of Shells

Valence Electrons and Chemical Properties

Valence electrons are the electrons that exist in the outermost shell, and they define the chemical properties of an element and its reactivity. Elements with the same numbers of valence electrons behave similarly, which explains the similarity in properties of elements located in the same column or group of the periodic table.

Shell Capacity

Shell capacity refers to the number of electrons a shell can hold in its orbit. It is also known as the principal quantum number (or shell number) and is denoted with the letter ‘n’. To determine the number of electrons in a shell, the following formula is derived: 

Maximum electrons in shell = 2n2 ,where 'n' is the principle quantum number. 

For example, k shell (n =1) can hold up to 2 electrons; similarly, L shell (n =2) can hold 8, and so on. 

4.0Bohr’s Model of Shells

Niels Bohr proposed the model of electrons orbiting the nucleus in fixed energy levels or shells. Such a model helped explain the quantization of energy in atoms and determine the shell characteristics. The Bohr model is outdated and replaced by quantum mechanics for larger atoms, but it is useful in explaining the behaviour of simpler atoms like hydrogen.

Excited States 

According to Bohr, an electron in an atom will absorb energy and jump to a higher energy level, or we can say excited state. As soon as these electrons fall back to their ground state, they release energy in the form of light, which forms the basis for atomic spectra.

Shell Configuration 

Shell configuration is the arrangement of electrons in the electron shells and subshells. The electrons fill the shells in order of increasing energy, starting from the innermost. Each shell has a maximum capacity, and within the shell, as mentioned earlier, electrons occupy specific subshells - (s, p, d, f). Let’s take an example of the element Sodium (Na atomic no. = 11) to understand it better: Fill shells with increasing energy levels.

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