The universe is made up of matter. All matter is made up of molecules, which are made up of combinations of atoms.
There are many different types of atom (elements) and these are categorised by their structure, the atomic no. - (the no. of protons and electrons).
The atom contains 3 smaller particles. Electrons which have a negative charge, and a mass about 1/1836 of that of a neutron & proton. Protons which have a positive charge and a mass of 1. And Neutrons which have a neutral charge and also a mass of 1.
Atoms are neutral Particles they have no charge.
The total no. of Protons and neutrons in an atom is called the mass no.
The no. of protons or electrons in an atom = atomic no.
Isotopes = elements which contain the same no. of protons but a different no. of neutrons.
238 - mass no.
92 - atomic no. = 92 protons, 146 neutrons