To build a neutral atom, there must be an equal number of protons and electrons.

Refresh the simulation, and then add six protons and six electrons to make a neutral carbon atom. Click the plus sign on

the Mass Number window. What's the mass number of this carbon atom?

Answer: The mass number of this carbon atom is 12.

Explanation:

Atomic number is defined as the number of protons or number of electrons that are present in an atom.

Atomic number = Number of electrons = Number of protons = 6

Mass number is defined as the sum of number of protons and neutrons that are present in an atom.

Mass number = Number of protons + Number of neutrons = 6 + 6 = 12

Thus the mass number of this carbon atom is 12.