| Particle | Symbol | Relative Charge | Mass Unit |
| Electron | e- | -1 | 5.4858 x 10-4 |
| Proton | p+ | +1 | 1.0073 |
| Neutron | n | 0 | 1.0073 |
Protons = Electrons ⇒ Atom. Neutral.
Protons ≠ Electrons ⇒ Ion. Charged.
Electrons are surrounding the nucleus, consisting of protons and neutrons.
Mass number are the number of protons and neutrons in the nucleus.
Atomic number are the number of protons in nucleus.
For example, 42He, where mass number is written on top left, and atomic number on bottom left.
The elements are arranged in the order of increasing atomic number, from left to right, from up to down.
Each element has a unique atomic number (or the number of proton).
The maximum number of electron that a shell can hold is 2(n)2
| n | max |
| 1 | 2 |
| 2 | 8 |
| 3 | 18 |
| 4 | 32 |