so we've seen now that atoms contain three primary subatomic particles there are protons neutrons and electrons so the positively charged protons are the particles that give an element its identity for example the reason carbon is carbon is that it has six protons if you change the number of protons then it's no longer carbon but the neutrons can vary and that will change the mass and that creates different isotopes well the electrons can also vary and this will change the charge this is what creates ions so for example the sodium that you eat in your diet the sodium found in the ocean the sodium found in the ground in the earth crust those sodium atoms are not neutral atoms they are in fact sodium ions so a sodium ion has a one positive charge the way you would write that is you'd write the symbol you put that charge up here as a superscript and notice by the way if it's a 1 we don't include the 1 so let's think about this for example what does that mean well if it's got a 1 positive charge it must have one less electron than the number of protons so your first instinct might be Oh what what's one positive it's got one more positive but again you can't give it one more positive you can't give it a proton else it's no longer sodium so if it's got a 1 positive charge and must have actually lost an electron so that still gives it one more positive than it does electrons so if you think about a sodium plus ion how many protons and electrons must have let's look over here at the periodic table will find sodium it's right here and sodium we see is number 11 so that means sodium must have 11 protons well how many electrons must have again even though it's positive that doesn't mean that you add a proton to make it positive because we did that it would transmute the sodium into magnesium if it's got a 1 positive charge that means it's got one more positive one more proton than electron so it must only have ten electrons again if it was neutral these would be equal but because it's positive it must have one more positive than it does negative so this is what differentiates ions from atoms in that they have a charge and again the reason these charges form is that the number of electrons is either greater or less than the number of protons so I'll say that one more time you cannot change the number of protons again if you change the protons you change the identity of the element so whenever you're forming ions whether they're positively charged or negatively charged you only change the number of electrons not the protons so a positively charged ion is called a cation and again if it's positively charged it must have fewer electrons than in the neutral atom and a negatively charged ion is called an anion and these have more electrons than in the neutral atom well one way that I think about this is to remember batteries so if you think about a battery it could be like a a D battery for example or a car battery every battery has two terminals it has a positive terminal and a negative terminal well the positive terminal is called the cathode and the negative terminal is called the anode well it's the same prefix where cathode and cation are both positive anode and anion are both negative let's try a few examples go ahead and pull out your chapter 4 handout and I'll do the first one with you just to get you started so CL negative so chlorine with a negative charge so let's find chlorine on our periodic table and it's right over here it's number 17 so that tells us it's got 17 protons but if it has a one negative charge what does that indicate well that tells me it's got one more negative than it does positive so normally in a neutral chlorine it would have 17 protons and 17 electrons but this one has one more negative so it must have 18 electrons now a very common mistake I see sometimes from students is they'll see that negative sign and they immediately want to subtract but actually in this case the negative sign tells us to add an electron so make sure you don't take one away and make it 16 LS you would have one more positive ok well go ahead and pause the video and take your best shot at BC and D and then come back here and hit play when you're ready to check your answers okay Oh two negative so oxygen is right over here number eight so that means it's got eight protons and if it's to negative well it must have two more negatives than normal so instead of eight negatives it must have ten negatives how about aluminum three plus aluminum is right over here number thirteen so aluminum has 13 protons it's got a three positive charge so that must mean it has three more positives than it does negatives so would normally have 13 negatives if it was neutral this one must only have 10 negatives which you notice by the way is the same as Oh two - these have the same number of electrons because again it's the protons that give the atom its identity and how about phosphorus three negative well phosphorus is right over here number fifteen so must have 15 protons and if it's three negative and must have three more negatives than it does positives so it must be 18 negative okay let's try another example this is number six off your chapter for a handout I'll do the first one with you so write the symbol of the following ions so a says 20 protons and 18 electrons so which element has 20 protons well look at your periodic table find number 20 and it's right here it's calcium CA so it's gonna be ca and what would the charge be well it's got 20 protons again those have a positive charge and it's got 18 electrons that have negative charges so it's got two more positives than it does negative so 20 compared to 18 so two more positives so it must have a two positive charge okay go ahead and pause the video try B and C and come back and hit play to check your answers all right sixteen protons 18 electrons so find number 16 over here on your product table and it's right here so it is sulfur so sulfur s what's the charge going to be well it's got 16 positives and it's got 18 negatives so it's got two more negatives so it must be two negatives and letter C 26 protons 23 electrons well the element with 26 protons is iron right here Fe and what would the charge be well it's got 26 positives 23 negatives so it's got three more positives so it must be three positive now notice by the way the way I'm writing these charges I always write the number first and then the charge so make sure you don't write these as like plus 2 or minus 2 or plus 3 this is technically incorrect because when you write it in this format this is known as an oxidation state or an oxidation number you're trying to indicate an ionic charge you always write the number first so 2 plus not plus 2 or 2 minus not minus 2 so this is correct and this is technically incorrect okay let's do one more for practice with ions this is going to look very similar to the table you filled out before when we were dealing with neutral isotopes but now we're going to deal with charged ions so I'll do the first couple of columns with you to get you started so the symbol is chlorine-37 with a 1 negative charge let's fill out the protons the neutrons electrons mass number and the charge so how do we find the protons well again the protons is determined by the element CEL so chlorine will find chlorine it's right here and it's number 17 so that tells me it's got 17 protons how about the neutrons well we can find the neutrons from the mass number the mass number is 37 and remember the mass number is the number of protons added to the number of neutrons so we've got 37 protons and neutrons and 17 protons well we can find the neutrons by the difference so 37 minus 17 there must be 20 neutrons how about the number of electrons well we need to know the charge for that we see the charge is negative I'll put a 1 negative here how many electrons would have happen well if it's got a 1 negative charge that means it's got one more negative than it does positives well the protons remember how the positive charge the electrons are negative if it's got one more negative and instead of 17 it must have 18 okay let's do column two together so 12 protons 13 neutrons 10 electrons so 12 protons will find number 12 on our periodic table and it is right here magnesium so it must be magnesium what's the mass number going to be well getting the mass number is protons plus neutrons so 12 plus 13 so 12 plus 13 tells me it must have a mass number of 25 so magnesium 25 and how about the charge well we want to compare positive protons to negative electrons and this one has two more positives so it must be two positive for the charge and I'll write that up here make sure you do include the mass number and the charge on these isotopic symbols okay well go ahead and pause the video try the next four columns and then come back and hit play when you're ready all right 82 protons mass number 208 charge of 4 + well 82 protons tells me it's element 82 which is PV lead so put lead what are the neutrons gonna be well again it's gonna be the mass number of 208 subtract the protons 82 so 126 and I'll put that mass number in here - how about the electrons well the charge is for positive so I'll put that in so that means it must have four more positives than it does negatives so if it's got 82 positive protons it must only have 78 negative electrons so again make sure you don't add four electrons sometimes students will say 82 and they'll make this 86 but if you make it four more that would actually make it four negative so four plus means it's got four more positively charged protons a column for calcium for T with a two plus charge so calcium find calcium calcium is right here number 20 so must have 20 protons how about the number of neutrons what again neutrons is the mass number forty - the protons so 40 minus 20 so it must have 20 neutral neutrons how about electrons well for that we need to know the charge and the charge is two positives so I'll put two positive again don't write plus two that is incorrect for a charge so two plus tells me it's got two more positively charged protons and it does negatively charged electrons so must be 18 because 20 is 2 more than 18 column 5 126 neutrons 80 electrons three positive charge so how many protons would I have well if the charge is three positive it must have three more positives than it does negative so must have three more protons and electrons so it'd be 83 what is element 83 what's right here is bi business so bi what's the mass number well again it's protons plus neutrons so 83 plus 126 so 209 so the mass number is 209 and the charges 3 plus and the last one 15 protons 18 electrons 33 mass number number 15 is right here it's phosphorus the mass number it told us was 33 so to find the neutrons you would take 33 subtract those 15 protons and you would get 18 neutrons and the charge you compare the positive protons to the negative electrons and we see it's got three more negative electrons and it does protons so must be three negatives so phosphorous 33 with a three negative charge