in this video we're going to focus on writing formulas for ionic compounds at first we're going to go over a few basic examples then we'll cover examples that contain polyatomic ions and even transition metals so before we begin we need to know the charges of common ions elements in the first column of the periodic table like hydrogen sodium potassium rubidium even lithium these metals they form positive one charges when they become ions elements in the second column like magnesium calcium these ions typically form plus two charges and then to the right you have group three elements like aluminum gallium in group 3a these elements form positive 3 charges next to aluminum you have elements such as carbon silicon germanium we don't have to worry about the charges for these elements because it can vary and typically you won't see them very often in ionic compounds by themselves at least now the next group you need to know that's nitrogen phosphorus these typically form negative three charges and then you have elements like oxygen and sulfur which usually form ions with negative two charges and then you have the halogens like fluoride chloride bromide and iodide these ions they form negative one charges so make sure you're familiar with the charges of certain elements now let's say if we want to write the formula for an ionic compound like sodium chloride the first thing that you want to do is you want to write the ions that are in this compound sodium we said has a plus one charge as an ion and chloride has a minus one charge if the charges are the same if they're one to one you can simply write the ions together these two ions they're going to combine into this compound nacl and that's all you got to do to write the formula if the charges are the same now let's try another example calcium sulfide it might be good to have a periodic table in front of you you can go to google images and print one and you want to have it next to you where you can see what the charges are calcium is found in the second column of the periodic table it's in group 2 and so as an ion it's going to form a two plus charge sulfide is in group six a or group sixteen and those elements typically form charges of negative two so because the charges are the same even though the sign is different these two ions are going to combine in a one-to-one ratio and so it's going to be cas calcium sulfide let's try one more example aluminum nitride aluminum is found in group 3a or group 13 of the periodic table and so it's going to have a plus 3 charge nitrogen is in 5a or group 5a and it has a minus 3 charge so because the two have the same charge magnitude so to speak we can write them in a one-to-one ratio it's a ln now what should we do if the magnitude of the charges are different if it's not the same so consider the example lithium oxide lithium is found in the first column of the periodic table it's an alkali metal and it has a plus one charge oxide is a calcagen it's in group 6a and so it has a negative 2 charge so what do we do in this case if the charges are different you can use a crisscross method where the charge is going to become a subscript so the charge on lithium will become the subscript of oxygen and the charge on oxygen will become the subscript of lithium and you could ignore any negative charges because the subscripts will always be positive so the formula is going to be li 2 o one now you really don't need to write the one so we can simply leave it as li2o if there's no number it's always assumed to be a one so that's why we don't need the one so this is the formula for lithium oxide that's what you need to do to write it so for the sake of practice let's try another example gallium bromide so feel free to pause the video and try this example yourself so what do you think the formula for this compound will be so let's find the charge on each element gallium as an ion is going to form a plus three or three plus charge bromide is going to have a negative one charge so using the crisscross method it's going to be ga1 br3 so we could ignore the one and leave it as ga br3 so that's the formula for gallium bromide consider this ionic compound magnesium phosphide go ahead and write the formula for this ionic compound so let's start with magnesium magnesium is found in group 2a of the periodic table it's an alkaline earth metal and so it has a plus 2 charge phosphide it's in group 5a just like nitrogen and so it's going to have a negative 3 charge so using the crisscross method it's going to be mg3 p2 and so as you can see it's not very difficult to write the formulas for ionic compounds now before we move on into examples containing polyatomic ions you need to know what the polyatomic ions are in their formulas no3 minus is nitrate that's with an ate no2 minus is called nitrite with an ite so these are polyatomic ions they're ions that contains many atoms this is nitride nitrite is a monoatomic ion it's an ion consists of one atom notice that if it's monotonic it has the end in ide but if you see eight or i typically that represents a polyatomic ion po4 three minus what is the name of this polyatomic ion also go ahead and name these ones as well po4 three minus is called phosphates with an ate po3 3 minus is phosphite and p3 minus is phosphide so p3 minus contains a single atom so it's a monoatomic ion and so it has the end in ide notice that eight usually has one more oxygen than i now what are the formulas for the following ions sulfate sulfite and sulfide sulfate is so4 two minus so fight so3 two minus and sulfide is s two minus try these let's say if you have perchlorate chlorate chlorite hypochlorite and finally chloride what are the formulas for these ions perchlorate is clo4 minus chlorate is clo3-1 chlorite is clo2 -1 and hypochlorite is clo minus chloride doesn't have any oxygen atoms it's simply cl with a negative one charge now what are the names for these ions o h minus o h minus is known as hydroxide cn minus is cyanide c2h3o2 minus is acetate this is oxalate co32 minus is carbonate hco3 minus is bicarbonate or hydrogen carbonate cro42 minus that's chromate and this one is dichromate nh4 plus is ammonium and mno4 minus is permanganate so those are some other ions that you should know as well so now let's go over some examples how can we write the formula for potassium sulfate so potassium is an alkaline metal that has a plus one charge sulfate is a polyatomic ion that's so4 two minus you simply need to know that unless your teacher is going to give you a polyatomic ion sheet you need to have this memorized so now the one is going to move to the right the 2 is going to move to the left so it's going to be k2 so4 times 1 so we really don't need to write the 1 so we can leave it as k2 k2so4 so this is the formula for potassium sulfate try this one strontium phosphate strontium is an alkaline earth metal in group two and it has a plus two charge phosphate this is something you have to know it's po4 three minus so the two is going to move here and the three is going to go there so it's going to be sr3 po4 two whenever you have multiple polyatomic ions you need to enclose it within a parenthesis try this one barium nitrate so like strontium barium is also a group two alkaline earth metal so it's going to have the same charge plus two nitrate is no3 minus one so using the crisscross method it's going to be ba1 no3 times 2. whenever you have multiple polyatomic ions make sure to enclose it within the parenthesis now we really don't need a one so we can simplify it to ba and o32 so that's the formula for barium nitrate now let's try some other examples with transition metals many transition metals contain variable charges for example iron metal can be in the plus two state or the plus three oxidation state copper for example has two common charges plus one and plus two so when you're given the name of an ionic compound that contains a transition metal you're going to see a roman numeral for example let's say if we want to write the formula for iron 2 sulfide the roman numeral tells you what the charge is so you need to know what it represents if you see this this represents a one this symbol represents two this represents three if you see a v v represents five if you see an i let's say to the left of the v that represents four now if you see the i to the right of the v you have to add instead of subtract so it's like five plus one it's six so what do you think this represents a v with two eyes on the right so that's five plus two that's seven so you need to know or be familiar with the roman numeral system so we have iron two sulfide that means that the iron ion has a plus two charge sulfide we know has a minus two charge because both of these ions have the same charge magnitude we can simply write it in a one-to-one ratio it's simply fes but even if you were to use the crisscross method you would get fe2s2 notice that both of these numbers are reducible we can divide both numbers by a common number so it's going to turn into fe1 s1 which we can write it like this try this one rn3 sulfide so this time iron has a plus three charge instead of a plus two charge and sulfide is the same so the form is going to be fe2 s3 so the roman numeral simply tells you the charge of the element let's try another example try this one copper to nitrite so copper has a plus two charge as specified by the roman numeral nitrite you simply need to know this it's no2 with a negative one charge so using the crisscross method it's going to be cu 1 but we don't need to write the one times no2 with a 2 and so that's the answer for copper 2 nitrate here's another one copper 1 phosphite try that so copper has a plus one charge phosphite is po3 with a negative three charge so this is going to be cu3 po3 times one since we have a one next to the po3 we do not need a parenthesis for this example so this is the answer that's copper one phosphite try this one vanadium five dichromate so vanadium can have an oxidation state of five dichromate is cr2o7 with a negative 2 charge so the 5 is going to move to the right and the 2 is going to move to the left so it's going to be v2 and then in parenthesis cr207 times 5. so that's vanadium five dichromate go ahead and try this challenge problem lead 4 oxide this is going to be the last example for today so lead has a plus 4 charge oxide has a negative two charge so using the crisscross method we're gonna see that we're gonna have a formula that's pb2 o4 but notice that two and four are even numbers which means that we can reduce it by two and we should so if you get a situation like this make sure you reduce it if we divided by two we're going to get pb1 o2 which means the final answer is simply pbo2 so if you can make sure to reduce these subscripts until you get the lowest whole number ratio and so that is it for this video thanks for watching and have a great day