Chemistry: Principles and Practice (3rd Edition)

(a) $^{39}_{18}$Ar (b) $^{40}_{20}$Ca$^{2+}$
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) $^{39}_{18}$Ar We want to find a noble gas with 21 neutrons. Scanning the noble gases group (group 18) of the periodic table, we start subtracting the atomic numbers of each of the noble gases from their respective mass numbers to see which one comes up with the difference of 21. The noble gas that most closely fits this description is argon (Ar). The atomic number of argon is 18, according to the periodic table. The mass number can be found by adding the 21 neutrons to the atomic number of 18 to get a mass number of 39. We write the elemental symbol with the mass number as a superscript to the left of the symbol and the atomic number as a subscript also on the left of the elemental symbol. Since the atom is neutral, there is no charge to be reflected in the chemical symbol. (b) $^{40}_{20}$Ca$^{2+}$ The alkaline earth metals are the elements of group 2 on the periodic table. We scan the periodic table to find which alkaline earth metal has a mass number close to 40, and we find that this element is calcium (Ca). Calcium has an atomic number of 20. We write the elemental symbol with the mass number as a superscript to the left of the symbol and the atomic number as a subscript also on the left of the elemental symbol. We then add the charge of 2+ as a superscript to the right of the elemental symbol.