## Chemistry: The Science in Context (4th Edition)

An atom of argon (Ar) has an atomic number of 18, which means it has 18 protons. In a neutral argon atom, there would also be 18 electrons to balance the positive charges of the protons. (a) To figure out how many electrons a neutral atom of sulfur (S) contains, we look to the periodic table. Sulfur has an atomic number of 16, so there are 16 protons. In a neutral atom, the number of protons equals the number of electrons, which means that the neutral sulfur atom also contains 16 electrons. However, in this case, we have a sulfur ion with a 2- charge, meaning there are two extra negative charges or two extra electrons. This sulfur ion has 18 electrons. (b) An phosphorus (P) atom has an atomic number of 15, which means it has 15 protons. In a neutral atom, phosphorus should have 15 electrons. However, in this situation, phosphorus has a 3- charge, meaning it has three extra negative charges or three extra electrons. Therefore, this P$^{3-}$ ion has 18 electrons instead of the regular 15. (c) A beryllium (Be) has an atomic number of 4, which means it has 4 protons. In a neutral atom, beryllium should have 4 electrons. However, in this situation, beryllium has a 2+ charge, meaning it has two fewer negative charges or two fewer electrons. Therefore, this Be$^{2+}$ ion has 2 electrons instead of the regular 4. (d) A calcium (Ca) atom has an atomic number of 20, which means it has 20 protons. In a neutral atom, calcium should have 20 electrons. However, in this situation, calcium has a 2+ charge, meaning it has two fewer negative charges or two fewer electrons. Therefore, this Ca$^{2+}$ ion has 18 electrons instead of the regular 20. Options (a), (b), and (d) are correct. A neutral argon atom has 18 electrons, and the ions of options (a), (b), and (d) have 18 electrons each.