The mass of a given heavy nucleus is slightly less than the sum of the masses of the protons and neutrons that combine to form the nucleus,. Why?

Statement 1: Mass of O^16 nucleus is less than the sum of masses of 9 protons and 8 neutrons. Statement 2: Some internal energy is needed to keept the protons and neutrons bound in the nucleus. (A) Statement 1 is true, Statement 2 is true. Statement 2 is not a correct explanation for statement 1. (B) Statement 1 is true, Statement 2 is true. Statement 2 is a correct explanation for statement 1. (C) Statement 1 is true, Statement 2 is false. (D) Statement 1 is false, Statement 2 is true.

Mention the symbol and the mass number of an element which contains two neutrons in the nucleus.

What is mass defect of a nucleus?

Statement I:Some energy is released when a heavy nucleus disintegrate into two nuclei of moderate size. Statement II: The more the mass number of the nucleus , more is the binding energy for each proton or neutron

The atomic number of an element is 17 and its mass number is 37. The number of protons, electrons and neutrons present in the neutral atom are :

Why was it necessary to consider the existence of neutrons in the nucleus of an atom?