Answer
(a) $^{24}_{12}$Mg$^{2+}$
(b) $^{121}_{51}$Sb$^{3+}$
(c) $^{84}_{36}$Kr
Work Step by Step
For these problems, we will need to refer to the periodic table on the inside front cover of our book.
(a) We look at the element that has atomic number 12, which is magnesium (Mg). When we write the chemical symbol for this particular atom, we write the mass number as a superscript to the left of the elemental symbol, the atomic number as a subscript to the left of the elemental symbol below the mass number, and the charge as a superscript to the right of the elemental symbol.
For this ion of magnesium, the chemical symbol is $^{24}_{12}$Mg$^{2+}$.
(b) In this problem, we are not given the atomic number. We are, however, given the number of electrons (48) and the charge (3+). In a neutral atom, the number of electrons will equal the number of protons (which is also the atomic number). This atom has a charge of 3+, meaning there are three more positive charges than negative charges (three more protons than electrons; therefore, we know that we have 51 protons, and the atomic number of this atom is 51. On the periodic table, we see that element 51 is antimony (Sb).
The mass number is the number of neutrons plus the number of protons. If the number of protons is 51 and the number of neutrons is 70, then the mass number of this atom is $51 + 70 = 121$.
We can now write the chemical symbol. We write the mass number as a superscript to the left of the elemental symbol, the atomic number as a subscript to the left of the elemental symbol below the mass number, and the charge as a superscript to the right of the elemental symbol.
For this ion of antimony, the chemical symbol is $^{121}_{51}$Sb$^{3+}$.
(c) We are given very minimal information for this atom. We know that we have a noble gas, which is group 18 on the periodic table. We know that there are 48 neutrons in this atom. Noble gases are normally not reactive, so we are dealing with a neutral atom with the same number of protons and electrons.
Let's look at the periodic table and check to see which element in the noble gases could possibly have 48 neutrons by looking at the average atomic masses listed. We see that the average atomic mass of krypton (Kr) is 83.798 and the atomic number of krypton is 36. If we subtract the atomic number (also the number of protons in the atom) from the average atomic mass, we get a number between 47 and 48; therefore, we know that krypton is the element we are looking for.
Now, we add the number of protons (36) to the number of neutrons (48) to get the mass number. So we have $36 + 48 = 84$ as the mass number.
We can now write the chemical symbol. We write the mass number as a superscript to the left of the elemental symbol, the atomic number as a subscript to the left of the elemental symbol below the mass number, and the charge as a superscript to the right of the elemental symbol.
For this atom of krypton, the chemical symbol is $^{84}_{36}$Kr.
