An atom with 17 protons, 18 neutrons, and 17 electrons is a chlorine atom. The mass of this atom is primarily determined by the sum of protons and neutrons, as electrons have negligible mass. Mass Calculation:

• Number of protons = 17
• Number of neutrons = 18
• Mass number (A) = protons + neutrons = 17 + 18 = 35

Therefore, the mass number of this chlorine atom is 35 atomic mass units (amu)

. Electrons do not significantly contribute to the mass, so the atomic mass is effectively the mass number, which is 35 amu.