# Electronic structure

An is the way in which 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 shellMaximum number of electrons
First2
Second8
Third8
• 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 . 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
The electronic structure of sodium as a diagram

### Electronic structure and the periodic table

The electronic structure of an element is related to its position in the .

Electronic structure featureLink to the periodic table
Number or numbers of shellsPeriod number
Number of electrons in outermost shellGroup number
Total number of electronsAtomic 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