Question

1.) why isotopes have same atomic number but different mass numbers?

2.) why is the whole mass of an atom considered to be concentrated at the centre?

Answer 1

the isotopes have same atomic number but different mass number because of the difference in the number of neutrons present in the nucleus

the whole mass of an atom is considered to be concentrated at the centre that is in the nucleus because neutrons and protons together constitut mass of an atom and electrons have negligible mass when compared to neutrons and protons