periodic table molar mass vs atomic mass

If you look at the Periodic Table you will find that 35.5 is the figure quoted as the relative atomic mass of chlorine. The 'average' mass of one atom is therefore The total mass of the four atoms would be Suppose you had four typical chlorine atoms - 3 atoms of Cl-35 and 1 atom of Cl-37. That is an average which takes account of the different proportions of the various isotopes. Relative atomic mass is a weighted average (often called a weighted mean) of the masses of the isotopes. An average of 35 and 37 is 36, but that doesn't allow for the fact that there are three times as many Cl-35 atoms as Cl-37. If you have a sample of chlorine it will contain unbelievably vast numbers of chlorine atoms, and it is useful to be able to give an average value for the mass of a chlorine atom. This is an example where the numbers are so easy that you might well be expected to remember them.Ĭhlorine has two isotopes, Cl-35 and Cl-37, and ordinary chlorine contains these in the ratio of 3 atoms of Cl-35 to every 1 atom of Cl-37 (to a good-enough approximation for our purposes). 'Weighted average' is also called 'weighted mean'. Relative atomic mass is given the symbol A r. (I'm using the version of the definition which I find easier!) The relative atomic mass is the weighted average of the masses of the isotopes on a scale on which the mass of a carbon-12 atom is exactly 12 units. Use whichever seems more obvious to you while you are trying to understand this, but for exam purposes learn whichever is in your syllabus.

The relative isotopic mass is the mass of the isotope on a scale on which the mass of a carbon-12 atom is exactly 12 units.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. There is an alternative phrasing for this which some people find easier to understand.

The relative isotopic mass is the mass of the isotope relative to 1/12 of the mass of a carbon-12 atom. Most people start with relative atomic mass but it makes sense to talk about relative isotopic mass first. Weighing atoms in a standard mass unit like grams would be daft! For example, it would take about 6 x 10 23 hydrogen atoms to weigh 1 g. You can't use a familiar mass unit like a gram because atoms are so small.

The masses of atoms are measured relative to the mass of a C-12 atom. They have the same number of protons but different numbers of neutrons. Isotopes are atoms which have the same atomic number but different mass numbers. The fact that they have varying numbers of neutrons makes no difference whatsoever to the chemical reactions of the carbon. These different atoms of carbon are called isotopes. They all have the same number of protons, but the number of neutrons varies. For example, there are three kinds of carbon atom 12C, 13C and 14C. The number of neutrons in an atom can vary within small limits. The importance of the C-12 isotope in chemistry calculations This page explains the atomic mass scale based on the mass of an atom of the C-12 isotope. Relative atomic mass, relative molecular mass and relative formula mass Relative atomic mass and relative formula mass

Periodic table molar mass vs atomic mass