To identify the atom or ion with 18 electrons, 15 protons, and 16 neutrons, we start by looking at the number of protons. The atomic number of an element is defined by the number of protons it has. In this case, the atom has 15 protons, which corresponds to the element phosphorus (P) on the periodic table, as phosphorus has an atomic number of 15.
Next, let’s consider the electrons. The atom has 18 electrons, which is more than the 15 protons it has. In neutral atoms, the number of electrons equals the number of protons. However, since this atom has 3 more electrons than protons, it implies that it is an anion (a negatively charged ion). Specifically, since it has gained three extra electrons, it would be a phosphorus ion with a charge of -3, commonly written as P3-.
Lastly, we can verify the mass number of this ion. The mass number is calculated by adding the number of protons and neutrons. For this atom, we have:
Mass Number = Protons + Neutrons = 15 + 16 = 31
In conclusion, the atom or ion with 18 electrons, 15 protons, and 16 neutrons is a phosphorus ion (P3-) with a mass number of 31.