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Number of electrons in different shells

The maximum number ($$N$$) each electron shell can hold depends on its shell number ($$n$$). The relationship is $$N=2n^{2}.$$

Shell number Maximum number of electrons
1 2
2 8
3 18

Shell 1 must be completely filled before electrons can occupy the next shell. Similarly, shells 1 and 2 must be filled before shell 3 is filled. Filling of electrons is more complex from shell 3 onwards.

Shells are often drawn as circular "orbits" around a nucleus for simplicity. In reality, an electron shell is a complex region where electrons can be found.

The electrons are each drawn as dots on the circles representing their shells.

A carbon atom has a total of 6 electrons. It has 4 electrons in its outer shell.
A carbon atom has a total of 6 electrons. It has 4 electrons in its outer shell.

The electron configuration of an atom or ion can be also be written down. The number of electrons in each shell, starting from the smallest, is written separated by commas or full stops (periods).

A sodium atom has electron structure 2.8.1