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Ionization energy trends in periodic table

Ionization energy increases across the period and decreases down the group. Let's learn why. Created by Sal Khan.

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Video transcript

so let's talk a little bit about a word you might have heard and that is ion let's talk about what it is and then it will talk about trends in the periodic table on on I guess how hard it is to make something an ion in particular how hard it is to make something a positive ion so an ion is just an atom or molecule that has charge and it'll have charge if the protons are not equal to the electrons neutrons are all obviously also a constituent of atoms but neutrons are neutral what you're gonna get your charge from where your protons or electrons so you're going to have a net charge if you're a number of number of protons and this is for an atom or molecule a molecule is just a bunch of a bunch of atoms bonded together if the number of protons does not equal the number of electrons and you can have positive ions if the protons are more than the number of electrons protons are positive electrons are negative and you can have negative ions if the number of electrons are greater than the number of protons for example for example if you just had hydrogen in its neutral state has one proton and one electron but if you were to take one of those electrons away then hydrogen would have a positive charge and essentially it would just be in its most common isotope it would just be a proton by itself and so when we talk about a positive ion like this where our protons are more than our electrons the number of protons are more than the number of electrons we call these cations cations cation once again just another word for positive ion likewise we can have negative ions so say for example fluorine so fluorine gains an electron it's going to have a net negative charge it's gonna have a charge of negative one and a negative ion we call an anion and the way that I remember this is you know a kind of means the opposite or the negation or something so this is a negative I we're negating you could somehow think we were negating the ion so with that out of the way let's think about how hard it will be to ionize different elements in the periodic table in particular how hard it is to turn them into cations and to think about that we'll introduce an idea called ionization energy ionization ionization energy energy and this is defined this is defined as the energy required energy required to remove an electron to remove an electron so it could have even been called cat ionization energy because you really it's the energy required to remove an electron and and and make the overall atom more positive so let's think about the trends and we already have a little bit of background on the different groups of the periodic table so for example if we were to focus on especially you know we could look at Group one and we've already talked about hydrogen's a bit of a special case in Group one but if we look at everything below hydrogen if we look at the alkali look at the alkali and metals here we've already talked about the fact that these are very willing to lose an electron why because if they lose an electron they get to the electron configuration of the noble gas before it so if lithium loses an electron then it has an outer shell electron configuration of helium it has two outer electrons and that's kind of you know we typically talk about the octet rule but if we're talking about characters like lithium or helium they're happy with two because you can only put two electrons in that first shell but all the rest of them sodium potassium etc etc if you take an electron away from them then their outermost shell will all of them their outermost shell they're going to have the electron configuration of the noble gas before it and for sodium on down that outer shell is going to have that perfect eight lithium if you remove an electron it would get to helium it will have two electrons in its outer shell so you could imagine that the ionization energy right over here the energy to require the the energy required to remove electrons from your alkali metals is very low so let me just write down this is so when I say low I'm talking about low ionization energy low now what happens as we move to the right of the periodic table in fact let's go all the way to the right on the periodic table well if we go here to the noble gases the noble gases we've already talked about they're very very very stable they don't want no one they don't want their electron configurations messed with so it would be very hard I mean you know neon on down has their eight electrons that cocked up rule helium has two which is full for the first shell and so it's very hard to remove an electron from here and so it has a very high ionization energy low energy easy to remove electrons or especially the first electron and then here you have a high ionization energy and you have trouble seeing that H so this is high high ionization energy and that's the general trend across the periodic table as you go from left to right you go from low ionization energy to high ionization energy now what about trends up and down the periodic table well well then any group if we even if we look at the alkali if we look at the alkali metals right over here if we're down at the bottom if we're looking at if we're looking at say cesium right over here that electron in the one two three four five six in the six shell that's going to be further from that one electron that lithium has in its second shell so it's going to be it's going to be further away it's not going to be as closely bound to to the nucleus I guess you could say so this is going to be even that that one electron is going to be even easier to remove than the one electron in the outermost shell of lithium so this one has even lower even lower even lower and that's even going to be true of the noble gases out here that xenon that it's electrons in its outermost shell even though it has eight valence electrons they're further away from the nucleus and so they're little that the energy required to remove them is still going to be high but it's going to be lower than the energy from from say neon or helium so this is low so once again ionization energy low to high as we go from left to right and low to high as we go from bottom to top or we could say a general trend that if we go from the bottom left to the top right we go from low ionization energy very easy to remove an electron from these characters right over here to high ionization energy very hard to move remove an electron from these characters over here