The mass number of an atom is the sum of the number of protons and neutrons in its nucleus. The number of charged particles in the center of an atom refers to the number of protons, which determines the atom's atomic number.
Given that there are 7 charged particles (protons) in the center of the atom, we can infer that the atomic number is 7.
To find the mass number, we need to consider both the charged particles (protons) and the uncharged particles (neutrons) in the nucleus. The difference between the mass number and the atomic number represents the number of neutrons.
In this case, the number of uncharged particles (neutrons) is given as 9. Therefore, the mass number is:
Mass number = Atomic number (protons) + Number of neutrons
Mass number = 7 (protons) + 9 (neutrons)
Mass number = 16
So, the mass number of the atom is 16.