Which of the following is most likely true of an atom with a mass of 15 amu?

• A. It contains a total of 15 protons and neutrons.
• B. It must have 15 electrons.
• C. Its atomic number is 15.
• D. It is an isotope of carbon.

Answer: (A)

Verified step by step guidance

1. Understand that the mass of an atom in atomic mass units (amu) is approximately the sum of the number of protons and neutrons in its nucleus. This sum is called the mass number (A), where \(A = Z + N\), with \(Z\) being the number of protons and \(N\) the number of neutrons.
2. Recognize that the atomic number (\(Z\)) is the number of protons in the atom, which defines the element. The number of electrons in a neutral atom equals the number of protons, but the problem only gives the mass, not the charge or element identity.
3. Note that the mass of 15 amu suggests the total number of protons and neutrons is 15, but this does not necessarily mean the atom has 15 protons or 15 electrons.
4. Recall that isotopes are atoms of the same element (same number of protons) but different numbers of neutrons. Since carbon has an atomic number of 6, an isotope of carbon with mass 15 would have 6 protons and 9 neutrons, totaling 15 amu.
5. Conclude that the statement 'It contains a total of 15 protons and neutrons' is the most generally true for an atom with a mass of 15 amu, because the mass number counts both protons and neutrons together.