What is an atom made up of ?

All atoms have 3 constituent particles: protons, neutrons
and electrons.  (The exception here is the isotope of hydrogen with mass number 1 which
does not have neutrons.)

The protons and neutrons make up
the nucleus of the atom which is at the center of the atom and around which the
electrons revolve.

Protons and electrons are electrically
charged with protons having a single positive charge and electrons having a single
negative charge. Neutrons are electrically neutral. The mass of protons and neutrons are
approximately the same and their sum gives the mass of the atom. Electrons have an
extremely low mass and which can be ignored.

The atoms of
different elements have differing numbers of constituent particles. The number of
protons gives the atomic number of elements. Atoms of the same element can have
different numbers of neutrons which changes their atomic mass but not their atomic
number. These are called isotopes of the element.