Video Transcript
Which of the following is an isotope of carbon-12? (A) Boron-11, (B) carbon-14, (C) nitrogen-15, (D) oxygen-12, (E) oxygen-16.
Isotopes are atoms that have the same number of protons but a different number of neutrons. In hyphen notation, the number which appears after the hyphen is the mass number. The mass number is the sum of the number of protons and the number of neutrons in a nucleus. As isotopes are atoms that have the same number of protons but a different number of neutrons and the mass number is the sum of the protons and neutrons, we can also say that isotopes are atoms that have the same number of protons, but a different mass number.
We can use the periodic table to determine the number of protons in an atom, isotope, or ion of a particular element. When looking at a cell of the periodic table, the number frequently written above the chemical symbol is the atomic number. This number represents the number of protons in an atom, isotope, or ion of this element. Each element has a different atomic number. This means that atoms will only have the same number of protons if they are atoms of the same element. Therefore, an isotope of carbon-12 must be the same element but have a different mass number. The only answer choice that satisfies this criteria is answer choice (B). Carbon-14 is an isotope of carbon-12.
