An atom has mass number 209 and atomic number 82. The number of electrons in it is

The correct option is A

82

The correct option is (A).

Explanation of correct option:

1. The number of protons present in the nucleus of an atom of an element is known as its atomic number.
2. The number of protons in an atom is equal to the number of electrons, since an atom, on the whole, is electrically neutral.
3. $\mathrm{Atomic}\mathrm{number}=\mathrm{Number}\mathrm{of}\mathrm{protons}=\mathrm{Number}\mathrm{of}\mathrm{electrons}$
4. Atom has mass number 209 and atomic number 82. Therefore, there are 82 electrons.

Explanation of incorrect option:

Option (B): Atomic mass number is a sum of neutron and proton in an atom. And here the atomic number is 209. So this option is incorrect.

Option (C): The number of neutrons of an atom is 127.

Mass number = No. of proton + No. of neutron

No. of neutron = Mass number - No. of proton

= 209 - 82

= 127

Option (D): This is also an incorrect option. As, no. of proton=82, no. of electron=82, and no. of neutron=127 of an atom.

Thus, An atom has a mass number 209 and an atomic number is 82. The number of electrons in it is 82.