The mass number of an atom of ¹¹⁸Xe is _ .

Question

Quizplus · Accepted Answer

The mass number of an atom is the sum of the number of protons and neutrons in its nucleus. Counting the number of each in the given atom: 1 sulfur (S) (atomic number 16) has 16 protons, 1 uranium (U) (atomic number 92) has 92 protons, 1118 phosphorus (P) (atomic number 15) have a total of 16,770 protons, 1 xenon (Xe) (atomic number 54) has 54 protons, and 1 oxygen (O) (atomic number 8) has 8 protons. Putting this all together, we get:

16 protons from sulfur 
92 protons from uranium 
16,770 protons from phosphorus 
54 protons from xenon 
8 protons from oxygen

Adding these all up gives a total of 17,940 protons. To find the number of neutrons, we can subtract the atomic number (number of protons) from the mass number.

The atomic number of S is 16, the atomic number of U is 92, the atomic number of P is 15, and the atomic number of Xe is 54. Assuming that the oxygen has a negligible mass (compared to the others) gives us:

1 sulfur has 16 protons and (rounded to the nearest whole number) 16 neutrons, for a mass number of 32. 
1 uranium has 92 protons and roughly 26 neutrons (based on its most common isotope), for a mass number of 118. 
1118 phosphorus have 16,770 protons and roughly 25,767 neutrons (based on the most common isotope of phosphorus), for a total mass of approximately 42,537. 
1 xenon has 54 protons and roughly 77 neutrons (based on its most common isotope), for a mass number of 131. 
1 oxygen has 8 protons and (rounded to the nearest whole number) 8 neutrons, for a mass number of 16.

Adding up all of the protons gives a total of 17,940, and adding up all of the neutrons gives a total of approximately 25,856. Adding these together gives a mass number of approximately 43,796, which is closest to answer choice B, 118.

The Mass Number of an Atom of 118Xe Is _

The Mass Number of an Atom of ¹¹⁸Xe Is _