Question

why the mass of an atom is concentrated in the nuclues

Answer 1

The atom consists of mainly three important subatomic particles - electrons , protons and neutrons.

While one may argue there are several other subatomic particles like gluons ,quarks, etc. but all of these are present inside nucleus except electron which is in the huge space around the nucleus( I say huge because the volume of atom is around 10^15 times that of the nucleus)

Now, the mass of the nucleons ( neutrons and protons) is around 1.67 * 10^-27 kg each. Compare it with the mass of an electron which is 9.1*10^-31 kg (almost 1840 times less)

Its like comparing the weight of a cricket ball. and a thermocol ball ( the tiny well) and that too when the cricket balls are in the centre of a cricket field and thermocol ball spread throughout the field.We can certainly neglect the mass of the electrons with a very good approximation .

(The example is not to scale and is given based on the assumption that the readers would not go through the pain of comparing the actual masses of cricket ball and thermocol ball to see if the ratio comes the same.)