Question

Tell me the mass number of first forty elements

Answer 1

In the modern periodic table, the elements are listed in order of increasing atomic number.  The atomic number is the number of protons in the nucleus of an atom.  The number of protons define the identity of an element (i.e., an element with 6 protons is a carbon atom, no matter how many neutrons may be present).  The number of protons determines how many electrons surround the nucleus, and it is the arrangement of these electrons that determines most of the chemical behavior of an element.