Naming Molecular Compounds and Hydrates

Added: 2026-05-15

[00:00:03]all right so we are going to switch gears so we've already talked about how to name ionic compounds um we're going to switch over and look at how to name those molecular compounds which remember are made up of two non-metals um so the name and formula molecular compound describes the type and number of atoms in a molecule of that compound so it'll tell you which atoms there are and how many of each there are as a general rule the most metallic element is going to appear first in the name this is the one to the furthest left of the periodic table or further down but remember we're still dealing with those non-metals that are in that top right corner to the right of the stair step um if both elements are in the same group the one close to the bottom gets named first it's like we were dealing with carbon and oxygen carbon is further to the left so that would be listed first but like still for an oxygen sulfur is further down so sulfur would come first um and we're still going to be doing the same thing as ionic where the second element is going to have the root of the element name and we're going to change the ending to IDE so one thing that's different is that each element is going to be given a prefix describing the number of atoms in a compound whenever we're dealing with only non-metals we can't deal with charges because they would all be negative so we use prefixes to tell us how many of each atom we have um the only time you're not going to see a prefix is if there's only one of the first element it will not have a prefix so like the prefix for one is mono so like if nitrogen is our first element we're just going to call it nitrogen instead of mononitrogen however if there's one of the second element it would have that prefix of minus the only time you do not see a prefix is whenever the first element only has one um you do need to know the prefixes to use um this table shows you the first 10 prefixes um so mono means there's one die is to try is three um Tetra is one that people tend to mess up but Tetra is four so remember your tetrahedral shape had four atoms attached henta is five hexa is six Hector is seven OCTA is eight Nona is nine and deca is ten so make sure that you know the appropriate prefixes that you'll need to use as we go through and do some of these um and whenever you're writing molecular formulas um the symbols for the elements are going to be listed in the order that the elements appear in the name so you're not going to rearrange them or anything and the prefix indicates the number of atoms in each element so the number of atoms we write as a subscript to the right of each symbol so the prefix tells us which subscript we need to do um and if the number is one we don't have to put anything um and one thing with molecular formulas is we do not have to crisscross we do not have to reduce any of that stuff that we did during ionic all right so let's look at um several examples of these and we'll name them and write the formulas um so for the first one we have n2o4 n is nitrogen so we name the first one as is it has a subscript of two so we need the prefix that matches with two so this is dinitrogen and then o4 o remember we changed the ending to Ides that will be oxide and it has a subscript of four it's the prefix for four is Tetra and a lot of times you'll see that it will lose that like a at the end if it begins with a vowel so that is dinitrogen tetroxide for the next one NO2 n is nitrogen there is only one of them so we do not need a prefix for the first element because we don't start with mono and then O2 we said o would be oxide and the prefix for 2 is die so this would be nitrogen dioxide um looking at the next one H2O you guys more commonly know this is water but it's actual chemical name so H is hydrogen there are two of them so it's dihydrogen and then oxygen is oxide there's only one of those oxygens so it does have that prefix of mono so water's actual chemical name is dihydrogen monoxide I'm looking at the next one we have n2o6 N2 would be dinitrogen 06 o is oxide 6 is hexa so this is going to be hex oxide um next one p308 p is phosphorus three is Tris this can be triphosphorus and then O8 um 8 is octase this is going to be OCT oxide on the flip side if we have the name and we are writing the formula um so here we have tetracarbon DEC iodide so tetracarbon Tetra means four and carbon is C so that's C4 and then DEC iodide iodide is I and deca is 10. and remember we don't crisscross or reduce or anything like that looking at the next one diphosphorus tetrafluoride diphosphorus phosphorus p dies to tetrafluoride fluoride is fluorine which is f Tetra is four and last one here we have Tri silver heptachloride trisulfur Tris III and sulfur is s this S3 heptachloride chloride is CL and hepta is seven all right so we are also going to include hydrates um a hydride a hydrate is a specialized compound so it's a compound that contains water of hydration so it tends to be an ionic compound that is bonded or attached to water molecules um with these they can be heated enough to actually boil off the water producing an anhydrous powder so you can dehydrate it and get rid of the water or we can add water to one of these anhydrous powders to recreate a hydrate um whenever naming a hydrate or writing the formula you're going to write the name of the Onyx compound so write the ionic compound first using the ionic compound rules we did last week you're going to have a DOT almost like you're multiplying and then you're writing the number of water molecules that there are and you're going to know how many water molecules there are because it will have a prefix to tell you how many Waters there are so for example copper II sulfate we would write the formula for copper two sulfate or copper II sulfate pentahydrate copper is cu the two is its charge sulfate is so4 where the negative two charged when you crisscross those twos reduce to give you the cuso4 then we put the dot in pentahydrate means that there are five water molecules on the flip side you can do the same thing if we have cocl2.6h2o name the ionic compound first name the cocl2 So Co is Cobalt CL is chloride you can find the charge of cobalt and then six H2O H2O is going to be listed as hydrate whenever it's in this formula and 6 is hexus this is Cobalt 2 chloride hexahydrate all right so let's do a couple of examples of hydrates together um here we have ngco3.7 H2O so we want to focus on the ionic compound first the ngco3 mg is magnesium remember anytime you have more than two capital letters you have polyatomic so CO3 that's carbonate and remember magnesium is not a transition metal so we don't need Roman numerals and then 7h2o 7 is hepta and remember H2O is representing a hydrate so this is heptahydrate so that's magnesium carbonate heptahydrate um go in the other direction if we have the name and writing the formula we have barium hydroxide octahydrate so we want to write that on compound first we want to write out barium hydroxide first barium is ba and where it is on periodic table it has a positive two charge hydroxide remember is your polyatomic that does end in ide so that's o h with a negative one charge when we crisscross we get ba o h i remember we're crossing over 2 so we need to put the o h in parentheses and then octahydrate we put our DOT to show that they're connected and options eight so there are eight Waters attached here looking at the next one we have feso4.7 H2O um we want to deal with ionic compound first so Fe is iron remember that is a transition metal so we need Roman numerals so4 is sulfate and remember to find the charge of iron when you look at the charge of sulfate on our polyatomic list which is negative two mean the iron has to be positive two based off of the subscripts so that's Iron II sulfate 7 is hepta and H2O is hydrate so that's Iron II sulfate heptahydrate and the last one we have sodium sulfate decahydrate um sodium sulfate sodium is in a so then group one so it has positive one charge sulfate is so4 with an A2 charge so we crisscross that na2so4 and then Deca hydrate remember Deca is 10 so there are 10 Waters attached to that so anytime you see hydrate it is representing water so don't forget that you're still using and kind of combining the ionic rules and then using the prefixes from molecular to tell you how many hydrates there are