medium difficulty7 min

Electron Distribution in Shells

Electron distribution in shells means arranging electrons in fixed shells around the nucleus according to their maximum capacity and filling order.

In one line: Fill from home outward: K before L before M.

Practice This Concept

Understand it

In the Bohr-Bury scheme, shells are filled from inner to outer levels. The maximum number of electrons in a shell is given by 2n², where n is the shell number. K shell can hold 2, L shell can hold 8, and M shell can hold 18, but for the first 20 elements, students usually fill as 2, 8, 8 before moving further.

Example

Magnesium has 12 electrons, so its distribution is 2, 8, 2.

Rule to remember

Maximum electrons in a shell = 2n²; K = 2, L = 8, M = 18 by capacity.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

For chlorine, Z = 17, so electrons = 17.

1Fill K with 2, L with 8, and remaining 7 in M.
2Distribution is 2, 8, 7.

Method you can reuse

Find electrons from atomic number, fill K with maximum 2, fill L with maximum 8, then place remaining electrons in the next shell.

Sketch it in your answer

Draw concentric shells around the nucleus and place electrons as dots: K first, then L, then M.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What is the electron distribution of sodium with atomic number 11?

Think first, then show answer

Sodium has 11 electrons in a neutral atom, so its electron distribution is 2, 8, 1.

Common wrong answer

Writing chlorine as 2, 7, 8.

Why it loses marks: Inner shells must be filled before outer shells.

Don't mix this up

Students often put more than 2 electrons in the K shell or fill outer shells before inner shells.

Exam tip

For the first 20 elements, fill K first, then L, then M using 2, 8, 8 as the practical pattern.