when we name a molecule we need for that name to convey a specific molecular formula for ionic compounds we don't have to specify how many of each ion are in the molecule when stating the name because this is implied by their common charges when we say calcium chloride we know there is one calcium and two chlorides because that is the ratio that is required to make a neutral compound but when we are looking at covalent compounds there can be several different possible combinations between two elements so we need a way to specify how many atoms of each element are in the molecule molecular compounds are comprised of non-metals so when two elements form a compound in some ratio it is called a binary molecular compound let's say for example that we are looking at carbon and oxygen the nature of covalent bonds makes it possible for both co and co2 to exist these are totally different compounds with totally different properties so we cannot just call them both carbon oxide we need a way to differentiate between them unambiguously in the name we will still name the more metallic element or the one further to the left on the periodic table first and it will remain simply the name of the element the less metallic element will be named next and it will bear the suffix i'd but in addition to this we will use prefixes for each element to indicate how many atoms of that element there are if there is only one atom of the first element we typically don't use a prefix but when there is one atom of the second element we use the prefix mono so co will be carbon monoxide when there are two atoms of an element we use the prefix dye so co2 will be carbon dioxide here is a list of the prefixes we will use using these rules what would be the names of the following compounds we would have sulfur hexafluoride dinitrogen tetroxide and iodine hepta fluoride notice that sometimes if the element begins with a vowel we might drop the last vowel in the prefix so it's dinitrogen tetroxide rather than dinitrogen tetra oxide we should be able to go from the name to the molecular formula as well what compounds do these names refer to phosphorus pentachloride would be pcl5 carbon tetrachloride would be ccl4 and dinitrogen trioxide would be n2o3 and that's a lot more molecules we can now name you