Which of the following best describes the composition of an isotope of calcium (atomic number 20)?

• A. 20 neutrons, 20 electrons, and 20 protons always
• B. 20 protons, 20 neutrons, and a variable number of electrons
• C. 20 protons, 20 electrons, and a variable number of neutrons
• D. A variable number of protons, 20 neutrons, and 20 electrons

Answer: (C)

Verified step by step guidance

1. Recall that the atomic number of an element represents the number of protons in its nucleus. For calcium, the atomic number is 20, so every calcium atom has 20 protons.
2. Understand that isotopes of an element have the same number of protons but differ in the number of neutrons. This means the number of neutrons can vary while the number of protons remains constant.
3. Recognize that the number of electrons in a neutral atom equals the number of protons, so a neutral calcium atom has 20 electrons. However, the number of electrons can vary if the atom is an ion.
4. Analyze each option by checking if the number of protons is fixed at 20 and if the number of neutrons varies, which is the defining characteristic of isotopes.
5. Conclude that the best description of a calcium isotope is one with 20 protons, 20 electrons (if neutral), and a variable number of neutrons, since isotopes differ in neutron count but not in proton number.