Answer
(a) $^{23}_{11}$Na
(b) $^{64}_{28}$Ni
(c) $^{115}_{50}$Sn
(d) $^{42}_{20}$Ca
Work Step by Step
"$Z$" stands for the atomic number whereas "$A$" stands for the mass number. The atomic number is written as a subscript for the element symbol whereas the atomic mass is written as a superscript also to the left of the chemical symbol.
(a) The atomic number given is for that of sodium, Na. The symbol for this isotope of sodium is $^{23}_{11}$Na.
(b) The atomic number given is for that of nickel, Ni. The symbol for this isotope of nickel is $^{64}_{28}Ni$.
(c) The atomic number given is for that of tin, Sn. The symbol for this isotope of tin is $^{115}_{50}$Sn.
(d) The atomic number given is for that of calcium, Ca. The symbol for this isotope of calcium is $^{42}_{20}$Ca.
