8/16/2023 0 Comments Ag charge with chloride![]() How do you know if an ion is positive or negative? It is an ammonium salt (containing NH 4 + ions - you will need to learn that one). It is an acid in solution (which will contain H + ions), Some of these rules have exceptions, but you won't meet them at this introductory level - so we will ignore the exceptions for now. Once you know the charges on the ions making up the compound, working out the formula is easy. ![]() Incidentally, the number 1 is never written into a formula - if there is no number after a symbol, then there is one of it. In the second formula, Na 2O, the 2 only applies to the sodium. So in the first formula, MgCl 2, there are 2 chlorines, but only 1 magnesium - the 2 doesn't apply to the magnesium as well. A number written smaller and subscripted (set lower) in a formula counts the number of atoms or ions immediately before it. Notice the little numbers in these formulae. You need to have two sodium ions to balance the charges on the oxide ion. So the formula for magnesium chloride is MgCl 2.Īs another example, if you knew that the charge on a sodium ion was +1, Na +, and the charge on an oxide ion was 2-, O 2-, then it is easy to see that the formula for sodium oxide is Na 2O. To balance the charges you would need 2 chloride ions for every magnesium ion: 2 negative charges to balance the 2 positive charges. The magnesium chloride has to be electrically neutral overall. Suppose you knew that magnesium ions had a 2+ charge, Mg 2+, and chloride ions had a 1- charge, Cl. Suppose, then, that you wanted to write the formula for the ionic compound, magnesium chloride. There are a few simple generalisations, and a small amount of learning. Knowing what those charges are is going to take up quite a lot of the rest of this page. There have to be equal numbers of positive and negative charges in the compound. Secondly, all these ionic compounds are overall electrically neutral. There is a simple bit of guidance to help you there, and I will give you that in a minute. You will need to do a tiny bit of learning, but once that is done, the process is easy.įirst of all, of course, you need to know that the compound is ionic. Trying to learn all of them would be a ridiculous waste of time, and really difficult. There are potentially thousands of ionic compounds whose formulae you could possibly be asked to write or recognise - although a relatively small number will turn up again and again. How writing formulae for ionic compounds works You will need to know about ionic bonding and have access to a Periodic Table such as the one you can download from this site. You can't succeed without this most basic chemistry tool. This is a key bit of chemistry, and the truth is that if you can't be bothered to do it properly, you might as well give up chemistry here and now. It is essential that you take our time over this, and don't leave the topic until you feel reasonably competent at writing these formulae. This page explains how to work out the formulae of the simple ionic compounds that you will meet at this level. How to write formulae for simple ionic compounds
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