Energy LevelsOxford AQA IGCSE Chemistry: Revision notes

Electrons in an atom do not orbit randomly — they occupy specific energy levels, also called shells, around the nucleus.

• Electrons always fill the lowest available energy level first, before starting the next one
• Each energy level can only hold a certain maximum number of electrons
• For the first twenty elements: the first shell holds up to 2 electrons, the second shell up to 8, and the third shell up to 8 (before a new shell begins)

The electronic structure of an element can be shown using number notation, listing the number of electrons in each shell starting from the one closest to the nucleus, separated by commas.

For the first twenty elements, the number of electrons in each shell can be worked out from the atomic number (which equals the number of electrons in a neutral atom).

Applying the filling rule (2, then 8, then 8) to the first twenty elements:

Notice each shell fills completely before the next one starts to be occupied.

The number of electrons in the outer (highest) energy level determines many of an element's chemical properties, including:

• Which group of the periodic table it belongs to
• How reactive the element is
• What kind of bonds it is likely to form

Elements in the same group have the same number of outer-shell electrons, which is why they react in similar ways.