The atomic number of an atom is 19 and its mass number is 39. Which of the following are correct?
Number of neutrons = 20
Number of protons = 19
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.