The relative atomic mass is worked out using the following formula, illustrated for two isotopes, where the abundances are given in percentage values. ![]() In any sample of chlorine, 75 per cent of the atoms are 35 Cl and the remaining 25 per cent are 37 Cl. But the relative atomic mass of chlorine is not 36. The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes – because if there is much more of one isotope then that will influence the average mass much more than the less abundant isotope will.įor example, chlorine has two isotopes: 35 Cl and 37 Cl. Calculating relative atomic mass from isotopic abundance The A r values also allow you to work out that three oxygen atoms have the same mass as two magnesium atoms.Ĭhlorine’s A r of 35.5 is an average of the masses of the different isotopes of chlorine. They also tell you that hydrogen atoms have 12 times less mass than a carbon atom. These values tell you that a magnesium atom has twice the mass of a carbon atom, and 24 times more mass than a hydrogen atom. The atomic mass ( ma or m) is the mass of an atom. Rare lithium-6 (mass of 6.015 Da) has only 3 neutrons, reducing the atomic weight (average) of lithium to 6.941. Atoms with an A r that is more than this have a larger mass than a carbon atom. (January 2020) Stylized lithium -7 atom: 3 protons, 4 neutrons, and 3 electrons (total electrons are 14300 th of the mass of the nucleus).Atoms with an A r of less than this have a smaller mass than a carbon atom.To convert the mass of the iron bar to the number of moles of iron, use the mass of one mole of iron atoms as a conversion factor. ![]() Therefore, the mass of one mole of iron atoms is 55.8 g. The average mass of an iron atom is 55.8 u. Carbon is taken as the standard atom and has a relative atomic mass ( A r ) of 12. Use the periodic table to find the molar mass of iron. Atoms have such a small mass it is more convenient to know their masses compared to each other.
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