In 1913 Niels Bohr developed the nuclear model by proposing that electrons orbit the nucleus in fixed shells, orbitals, or energy levels located at set distances from the nucleus.

Each orbital has a different energy associated with it, with the higher energy orbitals being located further away from the nucleus.

This model solved the question of why the atom doesn’t collapse inwards due to the attraction between the positive nucleus and negative electrons circling the nucleus.

The schell closest to the nucleous has the lowest energy

First schell holds 2 electrons, the second and next, can hold 8 electrons to give a stable arrangement. (2, 8, 8, 18.)

Number of outer electrons = number of the group in the periodic table

Electrons are arranged in electron shells and want to have full outer shells (full set of valency electrons), this is why they react.

The elements like noble gases that have full outer shells don't need to react. More reactive elements have a greater desire to have a full outer shell, so also form more stable compounds.

The outermost shell of an atom is called the valence shell and an atom is much more stable if it can manage to completely fill this shell with electrons.

In most atoms this shell is not full and they react in order to gain electrons to fill this shell.

In some cases, atoms lose electrons to entirely empty this shell so that the next shell below becomes the now full outer shell.