Question

The atomic number of an atom is 19 and its mass number is 39. Which of the following are correct?

• A. Number of neutrons = 19
• B. Number of electrons = 20
• C. Number of protons = 19
• D. Number of neutrons = 20

Answer 1

Answer: (C), (D)

Number of neutrons = 20

The atomic number gives the number of protons present in an atom. For a neutral atom, it also represents the number of electrons.
Hence, number of protons = 19

Mass number is the sum of the number of protons and neutrons present in the nucleus of an atom.
Hence, no. of neutrons = mass number - number of protons $=39-19=20$.