The atomic number of calcium (Ca) is 20, and it's mass number is 40. What is true about the subatomic particles that make up a neutral atom of calcium?

A) It has 20 protons, 20 neutrons and 20 electrons.

B) the total number of subatomic particles in Ca is 40.

C) The number of electrons cannot be determined from the information given.

Answer: A. It has 20 protons, 20 neutrons and 20 electrons

Explanation: The atomic number of an atom is equal to the number of protons inside the nucleus. In a neutral atom the number of electrons and protons are equal.

The number of neutrons can be determined by this formula

Atomic Mass - Atomic Number = n

40 - 20 = 20 neutrons

Protons = 20

Electrons = 20

Neutrons = 20

Answer: A

• Mass number = tell you the protons and neutrons

• Atomic number = protons and electrons

• To find the number of neutrons:

Mass number - atomic number

Neutrons: 40 - 20 = 20