The most common form of calcium has 20 protons, 20 neutrons, and 20 electrons. Which of the following elements would be an isotope of calcium?

• A. An atom with 21 protons, 20 neutrons, and 21 electrons
• B. An atom with 20 protons, 20 neutrons, and 18 electrons
• C. An atom with 21 protons, 21 neutrons, and 21 electrons
• D. An atom with 20 protons, 21 neutrons, and 20 electrons

Answer: (D)

Verified step by step guidance

1. Understand that isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons.
2. Identify the atomic number of calcium, which is determined by the number of protons. Calcium has 20 protons.
3. Recognize that for an atom to be an isotope of calcium, it must also have 20 protons, as isotopes of an element have the same number of protons.
4. Examine the given options to find the atom with 20 protons. This is the key characteristic that defines the element as calcium.
5. Determine that the correct isotope of calcium is the atom with 20 protons, 21 neutrons, and 20 electrons, as it maintains the same number of protons but has a different number of neutrons compared to the most common form of calcium.