Different shells can hold different maximum numbers of electrons. Electrons occupy shells starting with the innermost one. They begin to occupy the next shell when a shell becomes full.
For elements with atomic number 1 to 20:
|Electron shell||Maximum number of electrons|
The electron arrangement of an atom can be predicted from its atomic number. For example, the atomic number of sodium is 11. Sodium atoms have 11 protons and so 11 electrons:
This electron arrangement can be written as 2.8.1 (each dot separates one shell from the next). This electron arrangement can also be shown as a diagram. In these diagrams:
The electron arrangement of an element is related to its position on the periodic table.
|Electron arrangement feature||Link to the periodic table|
|Number or numbers of circles||Period number|
|Number of electrons in outermost shell||Old group number|
|Total number of electrons in all shells||Atomic number|
The electron arrangement of sodium is 2.8.1. This shows that sodium:
The electron arrangement of nitrogen is 2.5. Explain what this shows about the position of nitrogen in the periodic table.
2.5 shows that nitrogen: