Electronic structure
An A set of numbers to show how the electrons in an atom are arranged in shells, or 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 atom.
Electrons in shells
Electrons in atoms occupy energy levels, also called electron shells, outside the nucleus. Different shells can hold different maximum numbers of electrons. The electrons in an atom occupy the lowest available energy level first. This is the shell nearest the nucleus. When this shell is full the electrons begin to occupy the next energy level:
| Energy level shell | Maximum number of electrons |
| First | 2 |
| Second | 8 |
| Third | 8 |
| Energy level shell | First |
|---|---|
| Maximum number of electrons | 2 |
| Energy level shell | Second |
|---|---|
| Maximum number of electrons | 8 |
| Energy level shell | Third |
|---|---|
| Maximum number of electrons | 8 |
- the first shell, which is the one closest to the nucleus of an atom, can hold a maximum of two electrons
- the second shell, which is a little further from the nucleus, can hold a maximum of eight electrons
- the third shell, which is further from the nucleus, can hold a maximum of eight electrons
Any other electrons will be found in the fourth shell. This model will work for any atom in the periodic table up to calcium.
Predicting an electronic structure
The electronic structure of an atom can be predicted from its The number of protons in the nucleus of an atom. Also called the proton number.. For example, the atomic number of sodium is 11. Sodium atoms have 11 protons and so 11 electrons:
- two electrons occupy the first shell
- eight electrons occupy the second shell
- one electron occupies the third shell
This electronic structure is written as 2,8,1 (each comma separates one shell from the next). This electronic structure can also be shown as a diagram. In these diagrams:
- each shell is shown as a circle
- each electron is shown as a dot or a cross
Electronic structure and the periodic table
The electronic structure of an element is related to its position in the A tabular representation of all known elements in order based on atomic number, eg all the noble gases are found on the right of the periodic table..
| Electronic structure feature | Link to the periodic table |
| Number or numbers of shells | Period number |
| Number of electrons in outermost shell | Group number |
| Total number of electrons in all shells | Atomic number |
| Electronic structure feature | Number or numbers of shells |
|---|---|
| Link to the periodic table | Period number |
| Electronic structure feature | Number of electrons in outermost shell |
|---|---|
| Link to the periodic table | Group number |
| Electronic structure feature | Total number of electrons in all shells |
|---|---|
| Link to the periodic table | Atomic number |
The electronic structure of sodium (2,8,1) shows that sodium, Na:
- is in period 3
- is in group 1
- has an atomic number of (2 + 8 + 1) = 11