Question

why shells have electron not proton and neutron

Answer 1

The maximum number of electrons that can occupy a specific energy level can be found using the following formula:

Electron Capacity = (2n)2

The variable n represents the Principal Quantum Number, the number of the energy level in question.

Answer 2

Shells have negatively charged particles which revolve around the nucleus. Electron is just a name given to these negatively charged particles. As neutrons have no charge (positive or negative) they cannot revolve around the nucleus because of lack of strong attractive force. Also, protons are much heavier (around 1800 times electron) than electrons so they are included in the nucleus while the electrons go round the nucleus.
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