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# Arrangement of electrons around an atom

## Electron shells

The electrons of an atom are arranged around the nucleus in shells. A shell can be thought of as a region around the nucleus.

Shells can also be envisioned as energy levels as they indicate the energy of the electrons within them.

An electron with a higher energy occupies a shell farther away from the nucleus.

It has a greater ability to resist the attractive force of the nucleus and is more easily removed than a lower energy electron.

The shell closest to the nucleus is assigned the number 1. Larger shells are assigned higher numbers according to their size rankings (e.g. the second smallest shell is assigned the number 2).

Representation of a carbon atom showing the arrangement of electrons in circles.

Even though electrons are sometimes drawn as dots and crosses to indicate that they come from different atoms, they are indistinguishable in a real molecule.

Each circle represents a shell (e.g. the innermost orbit represents shell number 1).

## 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.

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