Electron arrangement
An The order electrons are arranged into between different energy levels. is the way in which Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. are arranged in an The smallest part of an element that can exist..
Electrons in shells
Electrons occupy different energy levels, or shells. Each shell can hold a maximum number of electrons. Moving through the A substance made of one type of atom only. in the periodic table, each atom has one more electron than the last because the number of electrons is the same as the The number of protons in the nucleus of an atom. Also called the proton number.. Electrons occupy the shells in order, starting with the shell that is nearest the The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei.. They begin to occupy the next shell only when this shell becomes full.
For elements with atomic number 1 to 20:
| Electron shell | Maximum number of electrons |
| First | 2 |
| Second | 8 |
| Third | 8 |
| Electron shell | First |
|---|---|
| Maximum number of electrons | 2 |
| Electron shell | Second |
|---|---|
| Maximum number of electrons | 8 |
| Electron shell | Third |
|---|---|
| Maximum number of electrons | 8 |
Predicting an electron arrangement
The electron arrangement of an atom can be worked out from its atomic number. For example, the atomic number of sodium is 11. Sodium atoms have 11 Subatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1. and so 11 electrons.
- 2 electrons occupy the first shell
- 8 electrons occupy the second shell
- 1 electron occupies the third shell
This electron arrangement can be written as 2.8.1 (each dot separates one shell from the next). It can also be shown as a diagram. In these diagrams:
- each shell is modelled as a circle
- each electron is modelled as a dot or a cross
