The atomic number of an isotope of hydrogen is equal to its mass number because it

Analyze the question: The question asks why one isotope of hydrogen has an atomic number equal to its mass number.

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, the number of neutrons (N) must be zero:

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

Hydrogen has three isotopes:

• Protium (¹₁H): 1 proton, 0 neutrons, so A = 1 and Z = 1
• Deuterium (²₁H): 1 proton, 1 neutron, so A = 2 and Z = 1
• Tritium (³₁H): 1 proton, 2 neutrons, so A = 3 and Z = 1

Only protium, the most common isotope, has A = Z, and this is because it has no neutrons.

Checking the other options:

• A. Hydrogen's valence shell (1s) holds only 2 electrons, and protium has just 1, so it is not filled. Wrong.
• B. Charge-to-mass ratio does not determine the relationship between mass number and atomic number. Wrong.
• D. Isotopy is simply the existence of atoms with the same Z but different A. It does not explain why A equals Z. Wrong.

Answer: C. does not contain neutrons

Tip: Whenever a question says "mass number equals atomic number", think "no neutrons". Only protium (¹H) does this among all elements.