An atom has an atomic number of 19 and a mass number of 39. How many electron ,protons and neutrons does it have?

The atomic number is the number of protons . In a neutral atom, the number of electrons equals the number of protons. Mass number is the sum of the protons and neutrons of a particular isotope. If you have mass number and atomic number, subtract the atomic number from the mass number to get the number of neutrons.

(Mass number)−(atomic number)=number of neutrons

Here,
Number of protons= atomic number=19

since it is neutral
Number of protons=number of electrons=19

Number of neutrons= mass number - atomic number=39-19=20

Electrons = 19
Protons = 19
Neutrons = 20