In 1913, scientist Niels Bohr suggested that electrons rotate around the nucleus in special regions called shells or energy levels. These shells are at fixed distances from the nucleus, and each shell can hold only a specific number of electrons.

The first four shells are represented by letters:

• K — First shell (closest to nucleus)
• L — Second shell
• M — Third shell
• N — Fourth shell

The maximum number of electrons that can fit in any shell is given by the formula:

$\text{Maximum electrons} = 2n^2$

where n is the shell number from the nucleus.

Important note: Even though the third shell can hold up to 18 electrons, it is more stable with only 8 electrons. This is why elements like potassium (19 electrons) and calcium (20 electrons) have only 8 electrons in their third shell, with the remaining electrons in the fourth shell.

Electrons always fill the lowest available shell nearest to the nucleus first. A shell that contains its maximum number of electrons is called a fully-filled shell. Atoms with fully-filled outermost shells are stable.

• Duplet: When the outermost shell has 2 electrons (e.g., helium)
• Octet: When the outermost shell has 8 electrons (e.g., neon, argon)

The electronic configuration shows the number of electrons in each shell, written from the innermost shell to the outermost. The shells are separated by commas or colons.

Worked Example 1

Write the electronic configuration of sodium (Na), which has 11 electrons.

Solution

Step 1: Fill the first shell: 2 electrons → K = 2

Step 2: Fill the second shell: 8 electrons → L = 8

Step 3: Remaining electrons go to the third shell: 11 - 2 - 8 = 1

Therefore, the electronic configuration of sodium is 2:8:1

Worked Example 2

Write the electronic configuration of chlorine (Cl), which has 17 electrons.

Solution

• First shell (K): 2 electrons
• Second shell (L): 8 electrons
• Third shell (M): 17 - 2 - 8 = 7 electrons

Electronic configuration: 2:8:7

• Electrons are arranged in shells around the nucleus
• Each shell can hold a maximum of 2n² electrons
• Electrons fill the lowest shells first
• Electronic configuration shows how electrons are distributed in shells
• Stable atoms have duplet (2) or octet (8) electrons in their outermost shell

Understanding electronic arrangement helps explain why certain elements are chemically reactive while others are stable. For example, neon has a full outer shell (2:8), making it unreactive — this is why neon gas is used in illuminated signs in Dar es Salaam and other Tanzanian towns. In contrast, sodium with configuration 2:8:1 easily loses its one outer electron, which is why sodium metal reacts vigorously with water. This knowledge is applied in designing batteries, understanding how medicines work in the body, and developing new materials for industry.