Video: Applying Knowledge of Atomic Composition and Atomic Mass

For statements I and II, state for each if they are true or false. I) The isotope sulfur-35 has 16 protons, 16 neutrons, and 16 electrons. II) The atomic mass of sulfur is 32.06 amu. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements I and II, state for each if they are true or false. I) The isotope sulfur-35 has 16 protons, 16 neutrons, and 16 electrons. II) The atomic mass of sulfur is 32.06 amu. If both are true, state if II is a correct explanation for I.

An isotope of an element is a variant of that element where the number of neutrons per nucleus is fixed. Statement I suggests a number of protons, neutrons, and electrons per atom of sulfur-35. In the name sulfur-35, sulfur indicates the element, and 35 indicates the mass number. The mass number is the number of protons and neutrons per nucleus for that particular isotope. However, statement I says that per nucleus of sulfur-35, we have 16 protons and 16 neutrons. That’s 32 protons and neutrons in total. 32 isn’t equal to 35, so statement I is false.

The element sulfur has an atomic number of 16. The atomic number of an element or isotope is the number of protons per nucleus. So, in actual fact, for a sulfur-35 atom, we’d expect it to contain 16 protons, 19 neutrons — that’s the mass number minus the atomic number — and 16 electrons. That’s as many electrons as there are protons, giving us an overall neutral atom.

Now, onto statement II. Statement II says that the atomic mass of sulfur is 32.06 atomic mass units. The atomic mass of an element like sulfur is the average mass of an atom of that element, based on the natural abundance of the isotopes. For instance, on Earth, about 75 percent of all chlorine atoms have a mass number of 35, while 25 percent have a mass number of 37. This means that the average mass of a chlorine atom on Earth is about 35.5 atomic mass units.

You can find the numerical value for the atomic mass of any element at the bottom of its square on the periodic table. If you look it up on the periodic table, you will see indeed that the atomic mass of the element sulfur is 32.06 atomic mass units. So, statement II is true. Only one of the statements is true, so we don’t need to address the last part of the question.

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