Determine the number of protons and neutrons in each isotope.
The isotopes of the same element have similar chemical behavior.
(a) In 1535P the atomic number is 15, so there are 15 protons and 15 electrons. The mass number is 35, so there are 10 (=35-15) neutrons.
(b) In 1940K the atomic number is 19, so there are 19 protons and 19 electrons. The mass number is 40, so there are 21 (=40-19) neutrons.