In this case, the 5 isotopes (with their relative percentage abundances) are: zirconium-90 Again you can find these relative abundances by measuring the lines on the stick diagram. This time, the relative abundances are given as percentages. The 5 peaks in the mass spectrum shows that there are 5 isotopes of zirconium - with relative isotopic masses of 90, 91, 92, 94 and 96 on the 12C scale. Our answer of 10.8 allows for the fact that there are a lot more of the heavier isotope of boron - and so the "weighted" average ought to be closer to that. A simple average of 10 and 11 is, of course, 10.5. Notice the effect of the "weighted" average. The average mass of these 123 atoms would be 1330 / 123 = 10.8 (to 3 significant figures).ġ0.8 is the relative atomic mass of boron. The total mass of these would be (23 x 10) + (100 x 11) = 1330 23 of these would be 10B and 100 would be 11B. Suppose you had 123 typical atoms of boron. The example coming up should make that clear. The relative atomic mass (RAM) of an element is given the symbol A r and is defined as: The relative atomic mass of an element is the weighted average of the masses of the isotopes on a scale on which a carbon-12 atom has a mass of exactly 12 units.Ī "weighted average" allows for the fact that there won't be equal amounts of the various isotopes. In this case, the two isotopes (with their relative abundances) are: boron-10 You can find the relative abundances by measuring the lines on the stick diagram. The tallest peak is often given an arbitrary height of 100 - but you may find all sorts of other scales used. The relative sizes of the peaks gives you a direct measure of the relative abundances of the isotopes. The carbon-12 scale is a scale on which the mass of the 12C isotope weighs exactly 12 units. That means that the mass/charge ratio (m/z) gives you the mass of the isotope directly. We are assuming (and shall do all through this page) that all the ions recorded have a charge of 1+. Notes: Isotopes are atoms of the same element (and so with the same number of protons), but with different masses due to having different numbers of neutrons. The two peaks in the mass spectrum shows that there are 2 isotopes of boron - with relative isotopic masses of 10 and 11 on the 12C scale. Note: If you need to know how this diagram is obtained, you should read the page describing how a mass spectrometer works. Monatomic elements include all those except for things like chlorine, Cl 2, with molecules containing more than one atom. It also looks at the problems thrown up by elements with diatomic molecules - like chlorine, Cl 2. It shows how you can find out the masses and relative abundances of the various isotopes of the element and use that information to calculate the relative atomic mass of the element. Use this information to calculate the A r of chlorine.This page looks at the information you can get from the mass spectrum of an element. A sample of chlorine gas was made up 75% chlorine-35 and 25% chlorine-37. The equation used to calculate the A r of an element is:Ĭhlorine has two isotopes – chlorine-35 and chlorine-37. Relative abundance is how much there is of each isotope in comparison to the full amount of the element in the world. The relative atomic mass of an element is calculated using the mass and relative abundance of each isotope present in a sample of the element. Relative atomic mass (A r) is a weighted average of the masses of all of the isotopes of an element. As their masses differ, isotopes may also differ in their physical properties. The different number of neutrons does not affect their chemical reactions, so isotopes will have similar chemical properties. Each of the three isotopes has an atomic number of 6 so each has 6 protons carbon-12 has 6 neutrons, carbon-13 has 7 neutrons and carbon-14 has 8 neutrons. For example, carbon can exist in three different forms with three different mass numbers – carbon-12, carbon-13 and carbon-14. This means that isotopes have the same atomic number, but different mass numbers. Isotopes are atoms of same element, which have the same number of protons but a different number of neutrons.
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