Which of the following statements correctly describes the number of protons in an atom?

• A. It varies depending on the isotope of the element.
• B. It is equal to the atomic number of the element.
• C. It is equal to the mass number of the atom.
• D. It is always equal to the number of neutrons.

Answer: (B)

Verified step by step guidance

1. Recall that the number of protons in an atom is a fundamental property that defines the element itself. This number is called the atomic number (Z).
2. Understand that isotopes of an element differ in the number of neutrons, not protons. Therefore, the number of protons remains constant for all isotopes of a given element.
3. Recognize that the mass number (A) is the sum of protons and neutrons in the nucleus, so it is not equal to the number of protons alone.
4. Note that the number of neutrons can vary independently of the number of protons, so the number of protons is not always equal to the number of neutrons.
5. Conclude that the correct statement is: the number of protons in an atom is equal to the atomic number of the element.