The atomic number of an atom represents the number of protons in its nucleus. In this case, the atomic number is given as 15, which means the atom has 15 protons.
The atomic mass of an atom represents the sum of the number of protons and neutrons in its nucleus. Here, the atomic mass is given as 31.
To determine the number of neutrons, we subtract the atomic number from the atomic mass: Number of neutrons = Atomic mass - Atomic number = 31 - 15 = 16.
Since the atom has a valency of -3, it means the atom gains 3 electrons to achieve a stable electron configuration. Valency represents the combining capacity of an atom.
To find the total number of electrons in the atom, we need to consider that the atom is electrically neutral. In an electrically neutral atom, the number of electrons is equal to the number of protons.
Therefore, the atom in question would have 15 electrons, as the number of protons (given by the atomic number) is equal to the number of electrons in an electrically neutral atom.