Chapter 2 - Atoms, Molecules, and Ions - Questions and Exercises - Exercises - Page 89: 2.131

(a) $^{23}_{11}$Na$^{+}$ (b) $^{121}_{51}$Sb$^{3+}$ (c) $^{84}_{36}$Kr

(a) $^{23}_{11}$Na$^{+}$ The cation with an atomic number of 11 is sodium (Na). We write the mass number as a superscript to the left of the element symbol, the atomic number as a subscript also to the left of the element symbol, and the charge as a superscript to the right of the element symbol. (b) $^{121}_{51}$Sb$^{3+}$ We are looking at group 5A on the periodic table; this group begins with the element nitrogen. If it has a 3+ charge, that means that it has three more protons than electrons. Given that this particular atom has 48 electrons, we can deduce that the number of protons, and, thus, the atomic number, is 51. Looking at the periodic table, the element in group 5A with an atomic number of 51 is antimony (Sb). We are also given the piece of information that the number of neutrons is 70. If we add 70 to the number of protons, which is 51, then we get 121, which will be the mass number. We write the mass number as a superscript to the left of the element symbol, the atomic number as a subscript also to the left of the element symbol, and the charge as a superscript to the right of the element symbol. (c) $^{84}_{36}$Kr We are looking for the element on the periodic table in the noble gases group (group 8A) where the number of neutrons (48) added to the atomic number would be close to the atomic mass given on the periodic table. We see that the closest match would be Krypton (Kr) with an atomic number of 36. We write the mass number as a superscript to the left of the element symbol and the atomic number as a subscript also to the left of the element symbol. The atom is neutral, so there is no charge.