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Current time:0:00Total duration:14:42

Video transcript

let's see what's the trend of metallic nature in a periodic table basically how does the metallic nature vary as we go across a period and down a group and if you're wondering what's metallic nature then I hope you recall that a metal is an element that can easily give away its outermost electron for example sodium is a very good metal it has one electron in its valence shell and it will participate in chemical reactions to give that away because by doing so it can attain a complete octet just like the noble gas the nearest noble gas neon and therefore will become more stable so metals like to give away their valence electrons and become more stable so if there is an element that can easily give away its outermost electron then it is going to be more metallic in nature whereas if there is another element that requires some effort to give away its outermost electron then is going to be less metallic in nature so that's the trend that we are looking for but before we begin just trying to find out what makes it easy or hard to give away this outermost electron well actually you will recall that the electrons are held in their position because of the pulling force by the nucleus see nucleus has a positively charged protons and they will attract the negatively charged electrons right opposite charges attract each other so because of this pulling force the electrons are held in their position so this pulling force is more then will be hard to give away this electron but if this pulling force is less they'll be easier to give away this electron and the element will be a better metal well then what does this pulling force depend on well it depends on two things first the effective nuclear charge and second the distance from the nucleus now let's understand each of these factors in little detail so the first one is effective nuclear charge this basically means effectively how many protons are pulling our electron if the number of protons pulling is more then the pulling force is going to be higher if the number of protons pulling is less then the pulling force is going to be lower for example you know if you have one magnet pulling an eye nail the pulling force is less compared to if you had two magnets right what about this term effective what does this mean so let's understand this let me draw the valence electrons somewhere below over here now imagine that this is being pulled by the protons over here there are 11 protons in the nucleus of sodium atom so there will be the pulling force will be due to 11 protons but is that it no see there are some electrons in between and these electrons will be repelling our valence electron see these electrons are negatively charged just like our valence electrons and light charges they repel each other so there are 10 electrons inside so they will be repelling our valence electrons so our valence electron will be feeling a repulsive force by 10 electrons okay so so it's experiencing a pull by 11 protons and a repulsion by 10 electrons so we can say that effectively this electron is being pulled by one proton right let me actually replace this effectively I can say that 11 minus 10 is 1 this electron is experiencing a pull by 1 proton so this becomes its effective nuclear charge the effective nuclear charge that this electron experiences well in reality these calculations are going to be slightly complicated which we will be talking about in future videos but for the sake of this video this level of simplicity is okay fine now let's talk about the distance from the nucleus so see if the if the electron is closer to the nucleus then the force that will experience is going to be stronger whereas if this if this electron is farther away okay then the force is going to be slightly weaker for example if we take the analogy of magnets again so if this magnet if I take this magnet little further away you can imagine that the pulling force that little experience will suddenly reduce so the force also depends on the distance from the magnet right so the pulling force that this electron experiences it depends on how closer or how farther away from the nucleus this electron is okay and you know what the distance from the nucleus is somewhat related to the number of shells number of shell in which the electron is so generally speaking in general if this electron is in the higher shell that means it will be farther away from the nucleus currently this electron is in the third shell if this electron were in the fifth shell it would have been farther away from the nucleus generally speaking now let's see how the metallic nature varies as we go across a period so the two factors that decide this are one the effective nuclear charge and two the distance from the nucleus now let's focus on the period number two the elements of period number two let me bring these elements closer well let's not talk about neon for now because we know that neon is a noble gas it's not gonna lose electrons so what's the point of talking about it let me hide this okay now first let's see how the effective nuclear charge varies as we go across this period so first of all let's talk about lithium so lithium atomic number is three it has got three protons in the nucleus and three electrons if we focus on this valence electron it is being pulled by three protons but that's not it it's being pushed by repelled by these two inner electrons also like charges repel so this electron is being pulled by three being repelled by two so technically speaking this electron is being pulled by one proton effectively right so it's effective nuclear charge the effective nuclear charge that this electron experiences is going to be one now let's talk about beryllium so beryllium atomic number is four it has four protons at its nucleus and four electrons if we focus on this valence electron okay so this is being pulled by these four protons and it's being repelled by these two inner electrons well this electron is in the same shell as this valence electron so this will also provide some repulsion but you know what electrons in the same shell they provide very little repulsion so to keep calculations simpler I'm going to ignore this okay so this electron is being pulled by four and is being repelled by these two so effectively speaking this electron is being pulled by four minus two is two protons so it's effective nuclear charge is going to be - now let's talk about boron the next element Boros atomic number is 5 it will have 5 protons and five electrons now can you pause the video tell me what will be the effective nuclear charge experienced by this electron pause and try now if you have tried it less see you see this electron is being pulled by these 5 protons and being repelled by these two inner electrons see these two electrons will also be repelling but only little bit they are in the same shell right so for simpler calculation at this level let's ignore them so this electron is being pulled by five repelled by two five minus two effectively we can say that it's being pulled by 3 so it's effective nuclear charge is going to be three now as you can see the effective nuclear charge is increasing as we go across a period now that would mean that as we go across they pull on the valence electron that increases this atom will have a higher pull on the valence electron this than this one than this one now that would mean that this atom finds it harder to lose this electron than this than this and that would mean that this atom is less metallic than this one than this one okay so as we go across a period the effective nuclear charge it increases and because of this the metallic character this decreases now let's talk about the effect of distance from the nucleus so in the video of atomic radius we have already seen that the atomic radius reduces as we go across a period and that's also happening because the nuclear charge is increasing and with increasing nuclear charge it is able to pull the shells much more close up okay now if the distance between the nucleus and the valence electron is decreasing then that would mean that the pull between them has increased now if the pull has increased it becomes harder and harder to give away the valence electron that means the metallic character has reduced so what we see is that as we go across the period the distance between the valence electron and the nucleus is reducing because of which the metallic character is again reducing okay so because of both of these variables the metallic character reduces so therefore we can say conclusively that as we go across a period the metallic character reduces now let's see what happens as we go down a group so here I have elements of group number one over here I haven't shown hydrogen because technically hydrogen does not belong to group number one and we have spoken about why in the video of modern periodic table so go ahead and watch that first over here let us see how the distance between the nucleus and the valence electron varies as we go down a group we'll come back to effective nuclear charge little later so for this I have drawn the electronic configuration of lithium sodium and potassium the first three elements of this group and from the video of atomic radius you might recall that the atomic radius increases as we go down a group well you can see over here there are more shells there are newer shells getting added between the valence electron and the nucleus so therefore their atomic radius the distance between them increases now with this we can say that the pull or the attraction that the valence electron feels is going to reduce we saw that with the help of magnets also if two magnets are getting far away then the attraction the pull between them is reducing the same thing will happen over here now that would mean that it will become very easy for potassium to give away its electron compared to sodium compared to lithium so that means as we go down a group it becomes easier to give away the valence electron that means the metallic character is increasing at as we go down so therefore in summary we would say that as we could have a group the distance between the valence electron and the nucleus increases and therefore the metallic character also increases now let's talk about the effective nuclear charge so lithium has an atomic number of 3 that means it has 3 protons so this valence electron is experiencing a pull by 3 protons but it's also being repelled by these two electrons so 3 are pulling it and 2 are repelling it so effectively speaking 3 minus 2 1 this electron experiences a nuclear charge of so it's effectively a charge will be won now let's talk about sodium so sodium ceramic number is 11 that means it has 11 proton now this way lys electron will be pulled by 11 protons but also being repelled by 2 plus 8 10 electrons so being pulled by 11 being repelled by 10 so 11 minus 10 is 1 so effectively speaking this electron is also being pulled by one proton it's effective nuclear charge is one similarly if you look at potassium its atomic number is 19 it has 19 protons this valence electron will be pulled by 19 but it will be repelled by 2 plus 8 10 plus 8 18 pulled by 19 repelled by 18 19 minus 18 is due 1 so effective nuclear charge for this electron is going to be 1 now if you notice over here the effective nuclear charge remains the same as we go down now this means that metallic character should not change well that's not actually true see calculating effective nuclear charge is not that straightforward and we will look at how to calculate effective nuclear charge in future videos but for now I looked up on Wikipedia and turns out this is how the effective nuclear charge varies it increases actually as we go down a group now you would say that this means that our valence electron of potassium this will experience a higher effective nuclear charge means a more stronger pull than the previous atom right now this would mean that it would be harder to give away this electron there for potassium should be less metallic than sodium and sodium should be less metallic than lithium right so basically the effective nuclear charge should actually is actually increasing as we go down a group and because of this the metallic character should reduce now there is a problem should the metallic character decrease or increase well see it turns out that the change in effective nuclear charge this is very small as we go down so therefore due to it the decrease in metallic character is small compared to the since the distance increases by a lot as we go down and therefore the increase in metallic character should be more so if you see you know net net as we go down a group the metallic character will be increasing okay now let's summarize the video in this video we spoke about the periodic trend of metallic character and metallic character can be measured by the ability of an element to give away its electron now this depends on two variables the effective nuclear charge and the distance between the nucleus and the valence electron now as we go across a period the effective nuclear charge increases meaning it becomes harder to give away the valence electron also the distance reduces meaning it becomes even more hard to give away the valence electron so as we go across a period it becomes harder to give away the electron meaning the metallic character reduces now as we go down a group and this is where you need to be careful as we go down the effective nuclear charge increases so it becomes harder to give away the electron but the distance this also increases making it easier to give away the electron so what happens well it turns out that distance increases a lot so it becomes much more easier to give away the electron so therefore net-net as we go down it becomes easier to give away the electron and therefore the metallic character increases