The last periodic property that we have is electronegativity. Now what is electronegativity basically means? The definition for electronegativity is it is the power of an atom to attract the shared pair of electron towards itself in a covalently bonded compound. Right? So now what you have to see is that if I have a covalently compound, something like this. Now the one which has a higher electronegativity would just tend to pull the shared pair of electron towards itself. Now you should remember this that it will not absorb the electrons totally, it would just shift the electrons towards itself. So that is the basic difference if you find out between electron gain enthalpy and electronegativity. In electron gain enthalpy there was a complete transfer of electron that was taking place but in electronegativity it is just the shifting of the shared pair of electron towards itself. That is electronegativity. Now, electronegativity depends on the size and the nuclear charge. Smaller the size higher would be the electronegativity. So if I generalize, can I say that from moving from left to right in a period my electronegativity value would increase and if I go down the group my electronegativity value will decrease. Right? So, why? Because when you are moving from left to right in a period the sizes decrease and decrease in size is causing the electronegativity increase. Now if I take the example once again, because over here, the nucleus of this atom and nucleus of this atom, where do you think that the shared pair of electron is lying closer. The shared pair of electron is lying closer over here so it can pull the electron towards itself. So I can say that the electronegativity of this particular atom is more as compared to the other one. Now what is the consequence of the shift that happens? There is a partial negative charge that would be created on this and a partial positive charge which would be created over here. So these partial charges are the consequence of electronegativity. Now in electronegativity there are 114 elements as we know and remembering the electronegativity is again a very difficult task. So there is one jugaad where you can just memorize the entire electronegativity in one line, which says F, O, N, Cl; then leave all the elements. Then it comes as C, H and then leave everything, it comes as metals. So this is what you memorize and using this you can find out the electronegativity of anything. If you take a closer look, over here nitrogen and chlorine are placed just slightly above each other. That has been deliberately done. It is not some misanimations or miscalculations. It is deliberately done. Why? Because the electronegativity value of nitrogen and chlorine are almost similar, so that is why I am not making them equal. I have just placed them slightly above. I cannot place chlorine right above nitrogen because then that would be not fair with nitrogen. Now, when you leave all the elements after nitrogen, then comes carbon and hydrogen and finally it is metals. Now for metals whenever you are comparing I told you before also that any element whose name ends with ‘ium’ is a metal and the metals would have the lowest electronegativity possible. You will never read a question where they will ask you to compare the electronegativity of two substances because you will not be able to do that. So, out of this only you will be asked a question. Right?
Now electronegativity, the factors on which electronegativity is depending, I told you it’s the atomic radius and the nuclear charge. There is one more factor on which electronegativity can depend, i.e. the cationic charge. Higher cationic charge means that it is short of electrons and it would want to pull the electron towards itself so that it can lower down the positive charge on itself. So if I take the example of Fe3+ and Fe2+ out of the two which one would have a higher electronegativity? Yes, without checking anything you can just say Fe3+ because the cationic charge is more, would have a higher electronegativity. Now if I give you a deeper understanding why this cationic charge or a higher cationic charge has more electronegativity value if you recall our ionic radius. The one which has a higher cationic charge, the size is smaller. When the size is smaller, what happens? It would have a higher electronegativity value.
Last factor on which electronegativity depends is the neighboring atoms. Now suppose if I have an example as CH3I and I have CF3I and if I ask you that out of the two examples which carbon would be more electronegative, how would you say that. Now over here you have to see what atoms are surrounding the atom in question. Now over here the structure of CH3I would be represented like this and for CF3I the structure would be something like this. Now over here there are 3 atoms of hydrogen which are lesser electronegative as compared to carbon and only iodine is the one which is having a higher electronegativity value as compared to carbon. But over here there are three very high electronegative elements attached to the carbon which pull the electron density from the carbon and now making the carbon short of electrons so the electronegativity of carbon in CF3I would be more as compared to CH3I. So see how the surrounding atoms make a huge impact on the electronegativity. So these are the factors on which the electronegativity value depends. In some of the books you would come across many formulas related to Pauling’s way of finding out the electronegativity or the Mulliken’s way of finding out the electronegativity, but in the day to day life or in solving the questions these two scales are not actually required because nowhere they will ask you the exact value of the electro negativities because whatever values of electronegativity we are finding they are not the absolute values. They are just the relative values. Because they have used the bond enthalpies and we know that we don’t have any accurate method of finding out the bond enthalpies. So that is why the electronegativity values are just relative. The formula for Pauling’s scale is this and for Mulliken’s scale is this. Now, using the formulas is not a big deal but practically they are not used for solving any questions or anything so that is why the relative values you would have to know the factors on which the electronegativity depends, because that would give you the relative values. Now over here if you just take a closer look that the halogens are the ones which have the highest electro negativities and the alkali metals that is the group for elements would be the ones which would have the lowest electro negativities. Now, the ones which have the lowest electro negativities that mean they would be the ones which would be wanting to give out electrons and the ones which are wanting to give out the electrons or make the electrons shift towards the more electronegative ones are the ones which are called as electropositive elements, and the ones which attract the shared pair of electrons are the ones which are called electronegative elements. So this is all about electronegativity that you should know.
Now if we talk about the metallic and the non-metallic character so here we would have to see that how do we judge which one is a good metal and which one is a good non-metal. So for this many books say that the element which releases more number of electrons upon ionization is the one which is a good metal. But I would say that the one which removes an electron very easily is the one which is a better metal. So if I take the example of sodium, magnesium and aluminum, out of the three which one would remove its electron easily and where would it be difficult to remove the electron? Yes, the first electron would be removed very easily in the case of sodium as compared to aluminum, so sodium would be a better metal as compared to aluminum. Although aluminum can remove 3 electrons but that is not what metallic character is.
Now similarly, if I take oxygen and fluorine and if I want to know which is a better non-metal I would have to just check, out of the two which one can accept the electron rather easily and we know that fluorine can accept an electron rather easily as compared to oxygen so fluorine is a better non-metal as compared to oxygen. So you just have to see the ease by which an electron can be given out or the electron can be taken in. That is all.
Now, if I generalize the periodic trends then from moving from left to right in a period the metallic character would decrease and the non-metallic from left to right would increase. So this is what you should know about the metallic and non-metallic character. Now after this there is a very peculiar relationship or rather I should say there are 3 pairs which almost have the same values or the same characteristics or the same periodic properties. Now which are they? The first pair is lithium and magnesium. Now, since in the periodic table they are placed diagonally to each other they are said to be in a diagonal relationship. Similarly, there are two other pairs. The second one being beryllium and aluminum and the third one being boron and silicon. So these 3 pairs have the same atomic radius, or if I say they have similar atomic radius because of which all the properties that we have just discussed have the same or similar values because of which they are said to be having anomalous behaviors as compared to any other element present in their respective groups.
Now the last trend that we have to study from the period table is the chemical reactivity. Now over here the elements on the extreme left hand side love to react with the elements on the extreme right hand side. Why is that? Because the elements on the extreme left hand side have a very low ionization enthalpy, so their electrons are removed very easily. And the elements on the extreme right hand side are the ones which have a very high negative electron gain enthalpy value which can easily absorb the electrons which are coming in from this side of the periodic table elements. Now we don’t take into consideration the noble gases because they are non-reactive. So, that is how we can generalize and say that the elements of extreme left hand side react with the elements of the extreme right hand side. Now if I start moving inside then as you move to the center the chemical reactivity starts decreasing so it is only on the extreme ends that the chemical reactivity is very, very high. So now we have seen that how this plain and simple looking periodic table packs a punch where so many dynamic things are happening at the same time. So many data can be found out even without knowing the actual values we can still find out the approximate values just by knowing the trends. And trust me on this that you would always be needing to correlate these properties with the periodic table till the time you will be studying chemistry. Because these are the five fundamental properties that you should always and always know.