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To a reasonable approximation neutrons and protons have a mass of 1 atomic mass unit, but they are not exactly 1, and neither are their masses exactly the same.įor A level purposes, we make the assumption that their masses are both exactly 1 on the C-12 scale, and so when you work out a relative isotopic mass, you can just count the number of protons plus neutrons. ![]() Note: When we talk about Cl-35 or Mg-24, the numbers quoted are strictly speaking the mass numbers and count the numbers of protons + neutrons. ![]() The relative isotopic mass is the mass of the isotope relative to 1/12 of the mass of a carbon-12 atom.įor example, an atom of Mg-24 is twice as heavy as an atom of C-12, and so is given a relative isotopic mass of 24. On the C-12 scale, the C-12 isotope is given a mass of exactly 12 units, and the masses of all other isotopes are measured on the same scale. The Cl-37 hast two more neutrons than Cl-35. So, for example, there are two isotopes of chlorine: Cl-35 and Cl-37. Isotopes are atoms of the same element (and so with the same number of protons and electrons), but with different masses due to having different numbers of neutrons. This isn't the first term on the syllabus list, but it makes sense to talk about it first. This statement wants you to be able to define various various important mass terms in chemistry. There is more about this in the next section. We commonly talk about the masses of atoms, ions, molecules and so on as being measured on the C-12 scale. ![]() That means that an atomic mass unit is one-twelfth of the mass of an atom of the carbon-12 isotope. The masses of atoms are all measured relative to the mass of a carbon-12 isotope on a scale where this isotope has a mass of exactly 12 unified atomic mass units. This statement just wants you to know what is meant by a unified atomic mass unit which has often in the past just been called an atomic mass unit. Chemguide: CIE A level chemistry support: Learning outcomes 2.1Ĭhemguide: Support for CIE A level Chemistry
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