The atomic number of an atom represents the number of protons in its nucleus, while the mass number represents the total number of protons and neutrons in the nucleus. If an atom has an atomic number of 19, it means it has 19 protons in its nucleus.
However, the mass number is not necessarily the same as the atomic number. The mass number depends on the specific isotope of the atom. Isotopes are atoms of the same element with different numbers of neutrons.
Given that an atom has an atomic number of 19, it could refer to the element potassium (K). Potassium has multiple isotopes, with the most abundant one being potassium-39 (39K) which has a mass number of 39 (19 protons + 20 neutrons).
Therefore, an atom with an atomic number of 19 and a mass number of 39 would be potassium-39 (39K).