The atomic number of an atom would be equal to its mass number if it

Analyze the question: We're asked when an atom's atomic number (Z) equals its mass number (A).

Concept:

• Atomic number (Z) = number of protons
• Mass number (A) = number of protons + number of neutrons
• So A = Z + N, where N is the number of neutrons

Solution: For A to equal Z:

A = Z + N Z = Z + N N = 0

So the atom must have no neutrons.

Checking the options:

• A. A filled valence shell describes a noble gas, such as helium (Z = 2, A = 4). It has neutrons, so this is wrong.
• B. A high charge to mass ratio is a property of particles in a deflection experiment. It has nothing to do with equating Z and A.
• C. An atom with no neutrons has A = Z. The only common example is protium (hydrogen-1), with 1 proton and 0 neutrons. Correct.
• D. Isotopy means atoms of the same element have different numbers of neutrons. It doesn't make A equal to Z.

Exam tip: Remember A = Z + N. Whenever a question asks when A equals Z, set N = 0. The classic example is hydrogen-1 (protium), the only stable atom with no neutrons.