The term 'formula weight' has been replaced by 'molar mass'. So, calcium carbonate contains 40% calcium, 12% carbon and 48% oxygen by mass. So, the formula weight of calcium carbonate is: 40+12+48=100. In calcium carbonate, we have one atom of calcium, one of carbon and three of oxygen. The formula for calcium carbonate is CaCO 3 and from the Table above, the relative atomic mass of calcium is 40, of carbon is 12 and of oxygen is 16. Let's do a slightly more complicated one. The proportion of chlorine in sodium chloride is: 35.5/58.5 x 100% = 60.7% by mass. So, the proportion of sodium in sodium chloride is: 23/58.5 x 100% = 39.3% by mass. If we add these together we get the formula weight of a sodium chloride molecule: 23+35.5=58.5. The weight (relative atomic mass) of sodium is 23 and of chlorine is 35.5.

There is one atom of sodium and one of chlorine. Sodium chloride again: the atomic ratio of sodium to chlorine is 1:1. It also means we can weigh stuff out in the right proportions to make different compounds. This is useful because with this knowledge we can calculate how much, by weight, of each element is present in compounds. A sodium atom is 23 times as massive as a hydrogen atom and a sulfur atom is twice as massive as an oxygen atom (see the Table above and you'll get the idea). The basic point is that the atomic weight tells you the relative masses of atoms. Although called atomic weights, they are not really weights because they are ratios and therefore dimensionless but the term atomic weight is kept for historical reasons. There have been other definitions as well, but they are all the same to within about 1%. These days, atomic weights are not based on hydrogen, but on one-twelfth of carbon-12. In other words, if you have 1 gram of hydrogen and 40 grams of calcium, there would be the same number of atoms in each. This meant that calcium, with an atomic weight of 40, is 40 times as massive for the same number of atoms as hydrogen. In the old days, atomic weights were based on hydrogen with an atomic weight of 1. One molecule of calcium carbonate, CaCO 3, contains one atom of calcium, one of carbon and three of oxygen.īasic Chemistry 3: Atomic weights (also known as relative atomic masses)

One molecule of sodium chloride contains one atom of sodium and one of chlorine.

For example, common table salt is sodium chloride, NaCl. Some useful elemental symbols in the context of cement follow, with their approximate atomic weights:Ī chemical formula is more than just a convenient short form of the name of a chemical - they indicate its composition. Symbols are usually based on the Latin names, so they don't always resemble the English names especially for elements known in antiquity (eg: Au-gold, aurum). If you missed out on chemistry at school, or it just seems a long time ago, the following short notes might be useful.Ĭhemical symbols are abbreviated forms of the names of chemical elements, eg: Ca (calcium) Si (silicon) K (potassium). Some very basic chemistry will help a lot with understanding how cement is made and how it works. A few basic chemistry notes Bemused by chemistry? Read on.

Ca element chemistry