The atomic number equals the number of protons; atomic mass equals protons plus neutrons.

The atomic number equals the number of protons; atomic mass equals protons plus neutrons.

• A. Atomic number equals electrons; Atomic mass equals protons
• B. Atomic number equals the number of neutrons; Atomic mass equals protons plus neutrons
• C. Atomic number equals the number of protons; Atomic mass equals protons minus neutrons
• D. Atomic number equals the number of protons; Atomic mass equals protons plus neutrons

Answer: (D)

The main idea is how the numbers that identify an atom relate to its subatomic particles. The atomic number tells you how many protons are in the nucleus, and that count also defines the element’s identity. The atomic mass is the total number of nucleons in the nucleus, so it equals protons plus neutrons because those two particles make up nearly all the atom’s mass. Electrons orbit the nucleus and contribute negligible mass, so they don’t determine the atomic mass. That’s why the statement about the atomic number matching protons and the atomic mass matching protons plus neutrons is correct. The other options mix up which particle sets define the atomic number or misstate the mass formula (electrons instead of protons, or mass as protons alone or protons minus neutrons).

The main idea is how the numbers that identify an atom relate to its subatomic particles. The atomic number tells you how many protons are in the nucleus, and that count also defines the element’s identity. The atomic mass is the total number of nucleons in the nucleus, so it equals protons plus neutrons because those two particles make up nearly all the atom’s mass. Electrons orbit the nucleus and contribute negligible mass, so they don’t determine the atomic mass.

That’s why the statement about the atomic number matching protons and the atomic mass matching protons plus neutrons is correct. The other options mix up which particle sets define the atomic number or misstate the mass formula (electrons instead of protons, or mass as protons alone or protons minus neutrons).