Question

give reason for mass no. of atom exclude the mass of an electron

Answer 1

Generally element is represented by  A
                                                             X
                                                           Z
   where A= mass number ,Z= atomic number and X= element.
Mass number (A) of an atom is the sum of protons and neutrons and mass of electrons  are not considered in mass number because of the following reasons:
1.An atom consists of protons , neutrons and electrons.
2. The mass of electron is about 1/1840 of the mass of hydrogen atom ( which is the lowest mass) . Since mass of hydrogen atom is 1u, we can say that the relative mass of electron is 1/1840 u. the absolute mass of an electron is however, 9 x 10⁻²⁸ g . The mass of electron is so small that is considered to be negligible.
Hence mass of electrons is ignored while calculating  mass number  of an element.

Answer 2

an atom consists of and protons, neutrons and electrons second. the mass of electron is about 1/1840of the mass of hydrogen atom (which is the lowest cost)