Solutions to Exercises

Exercise 1. How many protons, neutrons, and electrons are in an atom of \( ^{15}_{7}X \)? What is the identity of the element X?

Element X has 7 protons, 7 electrons, and 15 – 7 = 8 neutrons. The atomic number is 7, and this corresponds to nitrogen.

Exercise 2. How many protons, neutrons, and electrons are in ¹⁶O, ¹⁷O, and ¹⁸O?

Oxygen has an atomic number of 8. O-16 has 8 protons, 8 electrons, and 16 – 8 = 8 neutrons.

Oxygen-17 has 8 protons, 8 electrons, and 17 – 8 = 9 neutrons.

Oxygen 18 has 8 protons, 8 electrons, and 18 – 8 = 10 neutrons.

Exercise 3. Element X is composed of 4 naturally occurring isotopes: X-32, X-33, X-34, and X-36. Calculate the average weighted atomic mass of element X. Use the data in the table below for the calculation. Give the possible identity of element X.

Here we multiply the isotopic masses by the fractional abundance and sum them together.

31.972071 x 0.9502 + 32.971459 x 0.0075 + 33.967867 x 0.0421 + 35.967081 x 0.0002 = 32.06 amu

Exercise 4. Magnesium is composed of 3 naturally occurring isotopes: Mg-24 with an isotopic mass of 23.985 amu and 78.99% abundance; Mg-25 with an isotopic mass of 24.986 amu and 10.00% abundance; and a third isotope with 11.01% abundance. The atomic mass of magnesium is 24.3059 amu. What is the mass of the third isotope?

Let a be the mass of the third isotope. The atomic mass of Mg is 24.3050 amu. We can set up our equation and then solve for a.

24.3050 = 23.985 amu x 0.7899 + 24.986 amu x 0.1000 + a x 0.1101

Solving for a

a = (24.3050 – 23.985 amu x 0.7899 – 24.986 amu x 0.1000)/0.1101 = 25.982 amu

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