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?

1. an atom with 20 protons, 21 neutrons, and 20 electrons

2. an atom with 21 protons, 21 neutrons, and 21 electrons

3. an atom with 21 protons, 20 neutrons, and 21 electrons

4. an atom with 20 protons, 20 neutrons, and 18 electrons

5. All of the listed responses are correct.

Answer : The correct option is, (1) an atom with 20 protons, 21 neutrons, and 20 electrons.

Explanation:

Isotope : It is defined as the element that have the same number of protons but have the different number of neutrons of each of the atom.

Atomic number is defined as the number of protons or number of electrons.

Atomic number of = number of protons = number of electrons

Mass number is defined as the sum of number of protons and number of neutrons.

Number of neutrons = Mass number - Number of protons

As we are given,

Element = Calcium

Number of protons = 20

Number of electrons = 20

Number of neutrons = 20

For the isotope of calcium, the element should have the same number of protons but the different number of neutrons of each of the atom.

From this we conclude that, the option (1) would be an isotope of calcium because in this the protons and electrons are same but neutrons are different.

Hence, the correct option is, (1) an atom with 20 protons, 21 neutrons, and 20 electrons.