Atomic Number (Z) and Mass Number (A)
The number of protons present in an atom is known as its atomic number. It is generally denoted by the letter Z and written on the bottom of the symbol of the atom.
Since electrons have negligible mass, the entire mass of the atom lies in its protons and neutrons which are present in the nucleus. Further since protons and neutrons have approximately 1 unit mass, the sum of number of protons and neutrons is called the mass number of the atom. Mass number is generally denoted by the letter A and written on the top of the symbol of the atom.
It is important to note that the mass number, being the sum of the number of protons and neutrons in the nucleus, is always a whole number. On the other hand, since mass of a proton or a neutron is not a whole number (on atomic weight scale), atomic weight is not necessarily a whole number. For example, the isotopes of oxygen having mass numbers 17 and 18, have atomic weights equal to 17.00045 and 18.0037, respectively.
Following is a general symbol for an atom of an element (E) indicating its mass number (A) and atomic number (Z).
ZEA or ZEA
Thus knowing the mass number (A) and atomic number (Z) of an element we can find out the number of electrons, protons and neutrons present in the atom.
Atomic number (Z) = Number of protons
= Number of electrons
Mass number (A) = Number of protons + No. of neutrons
A = Z + No. of neutrons
or A - Z = No. of neutrons
In short, No. of electrons = Z
No. of protons = Z
No. of neutorns = A - Z
