Not all atoms of an element have the same mass, and atoms of different elements can have the same mass. In this course we learn about isotopes and isobars, why the atomic mass of chlorine is 35.5 u, and how some isotopes are used in medicine and industry.
3 lessons, about 1 h 15 min3 simulations10 problems6 practice questionsPython in every lesson
Created by StemPy LabIsotopes are atoms of the same element, with the same atomic number and different mass numbers, so they have the same number of protons and electrons and a different number of neutrons. Hydrogen has three isotopes: protium, deuterium and tritium. Isotopes have the same chemical properties, because chemical properties depend on the electrons, and slightly different physical properties that depend on mass. An isotope is named with the element and the mass number, such as carbon-12 and carbon-14.
The atomic mass of an element is the average of the masses of its isotopes, weighted by how much of each there is. This is why many atomic masses are not whole numbers, such as 35.5 u for chlorine. The average is the sum of the mass number times the percentage for every isotope, divided by one hundred, and it is nearer to the mass of the more common isotope. The same idea gives the missing percentages when the average is known.
Isobars are atoms of different elements with the same mass number and different atomic numbers, such as calcium-40 and argon-40, and they have different chemical properties because they have different electrons. Isotopes have the same atomic number and different mass numbers. Some isotopes have unstable nuclei and are radioactive. They have important uses: uranium-235 as nuclear fuel, cobalt-60 against cancer, iodine-131 for the thyroid gland, carbon-14 for finding the age of old objects and americium-241 in smoke detectors. They must be used under strict safety rules.
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