Do all atoms of a particular element have always the same mass?

(a) Which of the nuclear particles is present in the same fixed number of the atoms of any particular element ? (b) What do we call this number which is characteristic of a particular element ?

Atoms of different elements have the same mass .

What do we call the atoms of different elements with same mass number

Do all the photons of differents colour have the same mass? If not, why?

Do all individuals of a particular species have constant chromosome number ?

The atomic particle whose number in the atoms of an element always remains the same and which forms the real basis for the modern classification of elements is: