#### Answer

(a) $_{7}^{15}$N
(b) $_{31}^{70}$Ga
(c) $_{18}^{40}$Ar

#### Work Step by Step

(a) If an element contains 7 protons, that means that it has an atomic number of 7 as well because the atomic number is equal to the number of protons. To write the complete symbol, we first locate the element with an atomic number of 7 on the periodic table. We see that this element is nitrogen, and its chemical symbol is N. 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 7 and the number of neutrons as 8. When we add 7 to 8, we get 15; 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 is $_{7}^{15}$N.
(b) If an element contains 31 protons, that means that it has an atomic number of 31 as well because the atomic number is equal to the number of protons. To write the complete symbol, we first locate the element with an atomic number of 31 on the periodic table. We see that this element is gallium, and its chemical symbol is Ga. 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 31 and the number of neutrons as 39. When we add 31 to 39, we get 70; 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 is $_{31}^{70}$Ga.
(c) If an element contains 18 protons, that means that it has an atomic number of 18 as well because the atomic number is equal to the number of protons. To write the complete symbol, we first locate the element with an atomic number of 18 on the periodic table. We see that this element is argon, and its chemical symbol is Ar. 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 18 and the number of neutrons as 22. When we add 18 to 22, 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 is $_{18}^{40}$Ar.