Different isotopes of the same element differ from each other ...

[futuristic]

Different isotopes of the same element differ from each other because they have different
  ___________________ _. Fill in the blank(s) with the appropriate word(s).

Question 2

The atomic mass of an atom is defined as the sum of the number of its ___________________ _ plus
  ___________________ _. Fill in the blank(s) with the appropriate word(s).

[Kjones0604]

Answer to Question 1

atomic masses

Answer to Question 2

protons, neutrons

[futuristic]

Great answer, keep it coming

[mochi09]

Thanks for the timely response, appreciate it