If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:3:45

Worked example: Identifying isotopes and ions

Video transcript

an isotope contains 16 protons 18 electrons and 16 neutrons what is the identity of the isotope and I encourage you to pause the video and see if you can figure it out and I'll give you a hint you might want to use this periodic table here alright so I'm assuming you've had a go at it so the element is defined by the number of protons it has so if someone tells you the number of protons you should be able to look at a periodic table and figure out what element they are talking about so because it is 16 protons well we can go right over here to the atomic number what has 16 protons well anything that has 16 protons by definition is going to be sulfur right over here so I could write a big s now the next thing we might want to think about is the mass number of this particular isotope remember an isotope all sulfur atoms are going to have 16 protons but they might have different numbers of neutrons so the Sulphurs that have different number of neutrons those would be different isotopes so in this case we have 16 protons and we have 16 neutrons so if you add the protons plus the neutrons together you're going to get your mass number so 16 plus 16 is 32 now let's figure out if there's going to be any charge here well the protons have a positive charge the electrons have a negative charge if you have an equal amount of protons and electrons then you wouldn't have no charge but in this case we have a surplus of electrons we have two more electrons than protons and so we have a surplus of the negative charged particles we and we have two more we're going to have a negative two charge and we write that as two minus so this is actually an ion it has a charge so this is the isotope of sulfur that has a mass number of 32 the protons plus the neutrons are 32 and it has two more electrons than protons which gives it this negative charge let's do another example where we go the other way where we are told we are given some information about what isotope and really what ion we're dealing with because this has a negative charge and then we need to figure out the protons electrons and neutrons well the first thing that I would say is well look they tell us that this is fluorine as soon as you know what element we're dealing with you know what it's atomic number is when you look at the periodic table and you can figure out the number of protons remember atomic number is number of protons and that's what defines the element that's what makes this one fluorine so let's go up to the our periodic table and we see fluorine right over here has an atomic number of nine that means any fluorine has nine protons so let's scroll back down so just because it is fluorine we know we have nine protons now what else can we figure out well we know we have a negative charge right here and this this is you could use as a negative one charge and so we have one more electron than we have protons and so since we have nine protons we're going to have 10 electrons and then finally how many neutrons well remember the neutrons plus the protons add up to give us this mass number so if you have nine protons well how many neutrons you have to add to that to get to 18 we're going to have to have nine neutrons nine plus nine is 18