First diagram shows 3 electrons, 3 protons and 3 neutrons.
Second diagram shows 3 electrons, 3 protons and 4 neutrons.
Isotopes are atoms of the same element with different number of neutrons.
Two elements are the same if they have the same number of protons and electrons and are isotopes if they differ only in the number of neutrons, which is the case.
2) mass number = number of protons + number of neutrons.
