Isotopes are defined as atoms of same element with different mass number and same atomic number.
Here, atomic number is equal to number of protons and number of electrons, it is denoted by symbol Z thus,
and, mass number is equal to sum of number of protons and neutrons, it is denoted by symbol A thus,
The three isotopes of tin are given, Sn-116, Sn-120 and Sn-126, mass number for these three isotopes is 116, 120 and 126 respectively. Since, they are isotopes thus, atomic number remains the same that is 50.
Since, thus, number of protons and electrons will be 50.
For Sn-116,
Or,
Or,
For Sn-120,
For Sn-126,
Therefore, mass number, number of protons, electrons and neutrons in all three isotopes are:
Sn-116:
Mass number= 116
Protons=50
neutrons=66
electrons=50
Sn-120:
Protons=50
neutrons=70
electrons=50
Sn-126:
Protons=50
neutrons=76
electrons=50