Hello friends, welcome to this video. In this video we will study writing chemical formula. The chemical formula of a compound is a symbolic representation of its composition. The chemical formula of different compounds can be written easily. But for this we should know the symbols and their combining capacity. We have studied symbols of elements. Let us now learn about the combining capacity of elements. The combining power or capacity of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atoms of another element to form a chemical compound. Let us understand valency by an example. Imagine if an octopus is to hold some humans in such a way that its eight arms are used with both arms of humans how many humans can an octopus hold. If we represent octopus as O and humans with edge symbols can you write a formula for this combination? The octopus can hold up to four humans. Therefore we can write its formula as O H4 where four represents the number of humans captured by the octopus. Here you can see the valencies of some simple and polyatomic ions. Let us now look at the rules for writing chemical formula. One, the valencies or charges on the ion must balance. For example the formula of hydrochloric acid compound is HCl where the valency of both hydrogen H plus and chlorine Cl minus is one which is balanced in this formula. Two, when a compound consists of a metal and a non-metal the name or symbol of the metal is written first. For example calcium oxide C A O where oxygen is non-metal and is written on the right whereas calcium is metal and is written on the left. Three, in compounds formed with polyatomic ions the number of ions present in the compound is indicated by enclosing the formula of ion in a bracket and writing the number of ions outside the bracket. For example the formula for ammonium sulfate is NH4 2 SO4. Here the number of polyatomic ions ammonium NH4 plus is 2. So we write it in a bracket. Similarly if the number of polyatomic ions is 1 such as sodium hydroxide N A O H bracket is not needed here. Let us now practice writing the formulae of various compounds. First we will see some examples of writing the formulae of simple compounds. The simplest compounds formed from two different elements are called binary compounds. For example when writing formulae for molecular compounds such as hydrogen sulfide we first consider the constituent elements hydrogen and sulfide. We can write their symbols H and S and valencies 1 and 2 respectively. After that valencies of compound atoms can be written by crossover such as H2S. But if the compound is composed of ions of different elements then how shall we write its formulae? Let us understand this by an example. Magnesium chloride compounds are formed by combining magnesium, mg2 plus and chloride, Cl minus ions. To find its formula we write the symbol of cation, mg2 plus first followed by the symbol of anion, Cl minus. Then their charges are Christrost to get the formula that is mgCl2. Thus we see that in the molecule of mgCl2 that is magnesium chloride has two chloride ions, Cl minus for each magnesium ion and g2 plus. Note that in the formula the charges on the ions are not indicated. Let us now look at some examples of writing the formulae of a compound with polyatomic ions. We do not use a bracket when there is only one ion present in a compound containing polyatomic ions. For example in one molecule of sodium carbonate there are two ions of sodium ion and a plus having one atom and only one ion of polyatomic carbonate ion CO3 to minus. Therefore we do not use brackets for the polyatomic ion CO3 to minus in its formula any to CO3. When the formula contains two or more ions of the same ion we use brackets for them. For example the formula for calcium hydroxide is CAOH in bracket 2 and not CAOH2. Here the bracket around OH with a subscript 2 indicates that there are two hydroxyl OH groups joined to one calcium atom. In this video we learned to write chemical formulae. In the next video we will study molecular mass and mole concept.