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

(a) $_{17}^{37}$Cl (b) $_{20}^{40}$Ca
(a) To write the complete symbol, we first locate the element chlorine on the periodic table. We see that its chemical symbol is Cl. We also know from the periodic table that its atomic number is 17. We write the atomic number to the left of the chemical symbol as a subscript. The mass number is the number of protons and the number of neutrons. The number of protons is the same number as the element's atomic number. The number of neutrons is already given in the problem. So we have the number of protons as 17 and the number of neutrons as 20. When we add 17 to 20, we get 37; this is the mass number. We write the mass number as a superscript to the left of the chemical symbol as well. The complete symbol for this particular atom of chlorine is $_{17}^{37}$Cl. (b) To write the complete symbol, we first locate the element calcium on the periodic table. We see that its chemical symbol is Ca. We also know from the periodic table that its atomic number is 20. We write the atomic number to the left of the chemical symbol as a subscript. The mass number is the number of protons and the number of neutrons. The number of protons is the same number as the element's atomic number. The number of neutrons is already given in the problem. So we have the number of protons as 20 and the number of neutrons as 20. When we add 20 to 20, we get 40; this is the mass number. We write the mass number as a superscript to the left of the chemical symbol as well. The complete symbol for this particular atom of chlorine is $_{20}^{40}$Ca.