Answer :
To determine the number of neutrons in an atom, you first need to understand the structure of an atom. An atom is made up of protons, neutrons, and electrons. The number of protons is known as the atomic number, and it defines the element. For gold (Au), the atomic number is 79, which means every gold atom has 79 protons.
The number of neutrons can vary, giving rise to different isotopes of an element. Isotopes have the same number of protons but different numbers of neutrons. The most common isotope of gold is 197Au, which refers to the mass number. The mass number is the sum of protons and neutrons in the nucleus.
To find the number of neutrons in gold, you subtract the atomic number from the mass number of the most common isotope:
Number of neutrons = Mass number - Atomic number
Number of neutrons = 197 - 79
Number of neutrons = 118
Therefore, the number of neutrons in the most common isotope of gold is 118. The correct answer to the question is D. 118.