How is an atom represented? Why?

(N/A) An atom is represented by the notation $^A_Z X$,where $X$ is the chemical symbol of the element,$Z$ is the atomic number (number of protons),and $A$ is the mass number (total number of protons and neutrons).
The positive charge in the nucleus is due to the protons. $A$ proton carries one unit of fundamental charge and is stable.
All the electrons of an atom are outside the nucleus and revolve around it in circular orbits due to the electrostatic Coulomb force.
The number of electrons in a neutral atom is equal to the atomic number $Z$. Hence,the total charge of the electrons is $(-Ze)$.
Since the atom is electrically neutral,the total positive charge of the nucleus must be $(+Ze)$. Therefore,the number of protons in the nucleus is equal to the atomic number $Z$.