Newton define the mass as the amount of matter in an object. i.e., number of particles in an object. Actually this definition is not true!!!
Newton gives the equation with his second law F=ma.
This equation gives us an idea that the mass is amount of restriction given by the object to accelerate. Though this equation is true, he considers the mass of an object is only determined by no of particles inside that object which is really not an good idea!
We know that atom is composed of nucleus and one or more electrons bound to the nucleus and evolving around it.For example we take a simple Hydrogen atom which has only one proton in its nucleus and only one electron evolving around that. Consider that I have given you the mass of proton and mass of electron to you and I want you to calculate the mass of Hydrogen atom. So everyone will try to sum up both masses of electron and proton together with the help of calculator and claim the result as mass of Hydrogen atom! Isn’t it?
In reality the mass of Hydrogen atom is lesser than the value that you have calculated! We come back to Newton’s view on Mass. He thought that Mass is determined by amount of matter in an object. If this statement is true, the value you have calculated should match the mass of Hydrogen atom. You wonder how this discrepancy happens!!!