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AP.Chem:
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Video transcript

ionization energy refers to the energy that's required to remove an electron from a neutral atom so if we look down here this a represents a neutral atom meaning equal numbers of protons and electrons and since the positively charged nucleus is going to attract those negatively charged electrons it's going to take energy to pull an electron away from that attractive force of the nucleus and so that's your ionization energy if you take away an electron you no longer have equal numbers of protons and electrons you have one more proton than you do electrons so you get a +1 charge here so you form an ion and so ionization energy is always going to be positive so it always takes energy to pull an electron away so a positive value for ionization energy and our units are kilojoules per mole and in this video we're only going to be talking about the first ionization energy so ie one like that let's look at some actual ionization energies for elements in Group one and so we can see here some elements in Group one and so for hydrogen right it would take 1312 kilojoules per mole of energy to pull an electron away from hydrogen for lithium it would take about 520 kilojoules per mole to take an electron away and we can see as we go down here the number decreases so sodium be 496 potassium would be 419 so there's a clear trend as we go down a group in the periodic table right there is a definite decrease in the ionization energy so it must be easier to pull an electron away so let's see if we can figure out the reason why and we're going to study in detail here these two elements so hydrogen and lithium so let's go ahead and look at these diagrams here we're going to fill them in for hydrogen and lithium and so for our first diagram we will put hydrogen so hydrogen has atomic number of one so there's one proton in the nucleus so plus one charge in the nucleus and in a neutral atom there's one electron so go ahead and draw on hydrogen's one electron right here like the electronic configuration would be 1s1 so that one electron is in an S orbital in the first energy level alright so this this negatively charged electron feels an attraction for this positively charged nucleus and so to pull it away you must add energy so if you add 1000 3 or 12 kilojoules per mole of energy you can pull that electron away and if you do that you'd be left with just a positive one charge of nucleus and no electrons around it and so you no longer have a neutral atom you have an ion you have H+ because you have a positive charge of 1 the nucleus and zero electrons so H+ so that's the concept of ionization energy here let's look at lithium right so down here we'll we'll draw lithium lithium has atomic number of 3 so 3 protons in the nucleus and in a neutral atom 3 electrons right so the electron configuration is 1s2 2s1 so there are two electrons in the first energy level and they're in an S orbital so I'm going to go ahead and draw those in here so these two electrons I just drew represent the two electrons in the first energy level and the second energy level there's one more electrons I'm going to put that electron down here like that so for lithium if we were to take an electron away the one that's most likely to leave would be this outermost electron here the one in the 2's orbitals so if you apply 520 kilojoules per mole of energy right you can pull away that electron and so if you did that you'd be left with a plus 3 charge in the nucleus right and you would have your you'd still have your electrons in the in the 1s orbital so I'm go ahead and put all those in there but you've taken away that that outer electron and so therefore you have a lithium cation here you'd have Li plus 1 because because you have three positive charges in the nucleus and only two electrons now so 3 minus 2 gives you plus 1 the electron configuration for the lithium cation would therefore be 1s 2 because we pulled away that outer electron in the 2's orbital so this is the picture for the ionization of of hydrogen and lithium and we're going to examine some of the factors that affect the ionization energy and so first we'll talk about you're charged so let me go ahead and write a nuclear charge here so the idea of nuclear charge is the more positive charges you have in your nucleus the more of an attractive force the electron would feel and so therefore the harder it would be to pull that electron away so in general you can think about increased nuclear charge that would want to increase the ionization energy because again there's a greater attractive force for the electrons so let's look at these two situations and let's think about hydrogen first so hydrogen has a plus 1 charge in the nucleus and this 1 electron here would be would be pulled to the nucleus by that positive charge if we look at lithium right plus 3 in the nucleus right so that's a greater nuclear charge so just thinking about nuclear charge alone right you would think oh this electron might be pulled in even more than with hydrogen because plus 3 is is greater than plus 1 and so just thinking about nuclear charge with these two things right that's what seemed to indicate that lithium would have a grid that lithium is outer electron would have a greater attractive force for the nucleus and so therefore you might think it might take more energy to pull that electron away so just thinking about nuclear charge we might think an increase in the ionization energy alright next let's talk about electron shielding so electron shielding or you can also call it electron screening so the idea of electron shielding is is the inner shell electrons are going to shield the outer electrons from the positive charge of the nucleus and let's look at lithium for an example of that so we have these two inner shell electrons are going to repel the outer shell electrons so this electron to blue is going to is going to repel this electron in green and this electron in blue is going to repel this electron in green and so they're going to they're going to shield that outer electron in green from that positive 3 charge because electrons repel other electrons like charges repel other light charges and so that's the idea of electron shielding or electron screening and so thinking about just this factor right for lithium for lithium these two inner shell electrons are going to shield are going to shield that outer shell electron they're going to there's going to be a force in the opposite direction if you will and so that means that it would be easier to take that outer electron away so due to the repulsive force of those electrons and so if you just think about electron shielding or electron screening by itself right you it'd be easier to take away lithium's outer electron due to the shielding effect and so therefore you would need less energy so a decrease in the ionization energy for just thinking about this factor now nuclear charge and electron shielding go hand-in-hand and and one way to relate those we think would be to think about what's called the effective nuclear charge so I'm going to go ahead and write to the effective nuclear charge so Zef is equal to the nuclear charge which is Z minus the effect of the shielding electrons and so this is uh this is one way to think about it this is a very simplistic way of doing the math here so let's look at hydrogen first and calculate the effective nuclear charge that this electron experiences well there's a there's a +1 charge in the nucleus all right so that's the nuclear charge Z and there are zero shielding electrons so 1 minus 0 is of course plus 1 so this outer electron experiences an effective nuclear charge of plus 1 for lithium right there are three protons in the nucleus so Z would be plus 3 and there are two shielding electrons these two inner shell electrons here so if you plus three minus 2 so the effective nuclear charge would be a plus 1 so if you think about it the effective the effective nuclear charge that hydrogen's electron feels is about the same as AZ lithium's outer electrons so they're because they both have an effective nuclear charge of plus 1 so the fact that lithium has this electron shielding or electron screening right that kind of cancels out this effect of the nuclear charge and so these two things kind of cancel out now of course this is a very very simplistic way of calculating the effective nuclear charge and in reality for lithium if you do the more complicated way if they actually get a value of approximately 1.3 so we can say that lithium's effective nuclear charge is close to as close - positive one even though it's it's it's a little it's a little bit more accurate to say it's around 1.3 and so for our purposes on the electron shielding for lithium cancels out that increased nuclear charge and so we have to look at the last factor to understand this trend and the last factor is the distance of that outer electron from the nucleus all right so let's think about that so for hydrogen right this this electron is pretty close to the nucleus and the closer it is the more of an attractive force it has for the nucleus so once again in physics Coulomb's law it's distance dependent the closer you are the more of an attractive force you will feel so that electron feels a very strong attractive force so it's hard to pull that electron away for lithium right this outer shell electron is on average right a further distance away from the nucleus and and so therefore it doesn't have as much of an attractive pull towards the nucleus there's not as great of an attractive force so it's easier to pull that outer electron away if it's easier to pull that outer electron away that of course mean a decrease in the ionization energy so because of distance we can say that it's easier to pull that outer electron away from lithium because it's further away from the nucleus and so thinking about all three factors at once right the nuclear charge and the electron shielding effect sort of cancel each other out and so we can just think about the distance factor to explain the trend that we see in groups for ionization energy
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