All the atoms of a particle element have an equal number of protons and neutrons (Z=N). Which one is true? A) The atomic weight must be a whole number. B) The mass number must be exactly twice the atomic number for each atom.
The number of protons is the atomic number of an element. The mass number is the sum of the protons and neutrons in an atom. So if the number of protons (atomic number) and neutrons are equal, the mass number will be twice the atomic number (number of protons) for each atom.
An atom has 10 neutrons and 10 protons. The mass number is the the sum of the neutrons and the protons, which is 20. Since the number of protons is the atomic number, the mass number is 2 x 10 = 20.