## Chemistry: Principles and Practice (3rd Edition)

(a) $^{35}_{17}$Cl$^{-}$ (b) $^{39}_{19}$K$^{+}$
To find the full chemical symbol of an element/ion, we need to find the following pieces of information: elemental symbol, mass number, atomic number, and charge. (a) $^{35}_{17}$Cl$^{-}$ Looking at the periodic table, the halogen (group 17) element that has mass number closest to 35 is chlorine (Cl). To find the full chemical symbol for this element, we also need its atomic number, which is 17, according to the periodic table. We will also include the charge of the element, which is 1-, in the chemical symbol. (b) $^{39}_{19}$K$^{+}$ The alkali metals are the elements of group 1 on the periodic table. If the ion has a 1+ charge, that means that the number of protons is one greater than the number of electrons; if the number of electrons is 18, then the number of protons for this element is 19. According to the periodic table, the alkali metal with an atomic number of 19 is potassium (K). To find the mass number, we add the number of protons (19) to the number of neutrons (20) to get 39.