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

An atom of Krypton (Kr) has an atomic number of 36, which means it has 36 protons. In a neutral krypton atom, there would also be 36 electrons to balance the positive charges of the protons. (a) To figure out how many electrons a neutral atom of selenium (Se) contains, we look to the periodic table. Selenium has an atomic number of 34, so there are 34 protons. In a neutral atom, the number of protons equals the number of electrons, which means that the neutral selenium atom also contains 34 electrons. However, in this case, we have a selenium ion with a 2- charge, meaning there are two extra negative charges or two extra electrons. This Se$^{2-}$ ion has 36 electrons instead of the regular 34. (b) An arsenic (As) atom has an atomic number of 33, which means it has 33 protons. In a neutral atom, arsenic should have 33 electrons. However, in this situation, arsenic has a 3- charge, meaning it has three extra negative charges or three extra electrons. Therefore, this As$^{3-}$ has 36 electrons instead of the regular 33. (c) A calcium (Ca) 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. (d) A potassium (K) atom has an atomic number of 19, which means it has 19 protons. In a neutral atom, potassium should have 19 electrons. However, in this situation, potassium has a 1+ charge, meaning it has one fewer negative charge or one fewer electron. Therefore, this K$^{+}$ ion has 18 electrons instead of the regular 19. Options (a) and (b) are correct. A neutral krypton atom has 36 electrons, and the ions of options (a) and (b) have 36 electrons each.