By total number of nucleons, we mean :

1. the number of protons + number of electrons
2. the number of electrons + number of neutrons
3. the number of electrons + number of protons + number of neutrons
4. the number of neutron + number of protons

If Z is the atomic number and A is the mass number of an atom, then which of the following relations is incorrect ?

1. Number of electrons = Z
2. Number of protons = A - Z
3. Number of neutrons = A - Z
4. Number of neutrons + number of protons = A

A certain nucleus P has a mass number 15 and atomic number 7. The number of neutrons is :

1. 7
2. 15
3. 8
4. none of the above

The atoms of the same element, having the same atomic number Z but different mass number A are called :

1. Isotopes
2. Isobars
3. Isotones
4. None of the above