How the Shells Are Filled

Electrons in atoms and ions always occupy the lowest available energy level. In other words, they always go into the lowest energy shell which has not yet reached its capacity.

We can use this to work out how many electrons are in each shell in a particular atom or ion.

For example, a nitrogen atom has 7 electrons. Since the first shell has a capacity of 2, we know that the first 2 electrons will be in the first shell. This leaves 5 electrons. The second shell has a capacity of 8, meaning that it can hold all of these 5 electrons. Therefore, a nitrogen atom must have 2 electrons in its first shell and 5 electrons in its second shell. None of the other shells contain any electrons.

In diagrams of atoms and ions, we usually do not show the empty shells. Therefore, in a diagram of a nitrogen atom, only the first two shells are shown.

Diagram of a nitrogen atom. Two electron shells are shown. The first shell (the innermost shell) contains two electrons. The second shell contains five electrons.

A nitrogen atom. There are two electrons in the first shell and five in the second.

The outer shell is the outermost shell that is occupied

The term outer shell refers to the outermost shell that contains electrons. For example, an oxygen atom has electrons in its first shell and its second shell. Therefore, the outer shell of an oxygen atom is the second shell.

Diagram of an oxygen atom. Two shells are shown, both containing electrons. The second shell is labelled as the outer shell.

In an oxygen atom, the outer shell is the second shell, since this is the outermost shell that has electrons in it.


Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.


What is the rule for how the electron shells are filled?


What is the outer shell?


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