The atomic number of an atom would be equal to its mass number if it
Answer Details
The atomic number of an atom is defined as the number of protons in the nucleus of the atom. The mass number of an atom, on the other hand, is the sum of the number of protons and neutrons in the nucleus of the atom.
Therefore, the atomic number and mass number of an atom would only be equal if the atom does not contain any neutrons. This is because the mass number of an atom is determined by the number of protons and neutrons in its nucleus, while the atomic number is determined only by the number of protons.
If an atom does not contain any neutrons, then its mass number would be equal to the number of protons (i.e., the atomic number). However, most naturally occurring elements have isotopes, which means that they have varying numbers of neutrons in their nuclei. This results in different mass numbers for the same element, while the atomic number remains constant.
Therefore, the correct answer is: "the atomic number of an atom would be equal to its mass number if it does not contain neutrons".