All atoms of a particular element have the same number of protons in the nucleus .This number is a basic property of an element,called it atomic number (Z) .
The atomic number (z) is the number of protons in the nucleus of an atom.
Z = number of protons
For example : the hydrogen atom have one proton in the nucleus; and uranium have 92 protons in the nucleus.So,
H atom Z = 1
U atom Z = 92
Mass number : We characterize a nucleus by its atomic number (Z) and its mass number (A).The mass number (A) is the total number of protons and neutrons in a nucleus.
For example : sodium atom has number of protons 11 and number of neutrons 12 .So,
Mass number of Na atom = number of protons + number of neutrons = 11 + 12 = 23
A nuclide is a particular nucleus characterized by a definite atomic number and mass number. The shorthand notation for any nuclide consists of the symbol of the element with the atomic written as a subscript on the left and the mass number as a superscript on the left.