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|
Any other electrons will be found in the fourth shell. This model will work for any atom in the periodic table up to calcium.
The electronic structure 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 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:
The electronic structure of an element is related to its position in 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||Atomic number|
The electronic structure of sodium (2,8,1) shows that sodium, Na: