 The modern periodic table you would have seen this somewhere in your textbook See, this is a table where we arrange elements in the increasing order of their atomic numbers for example 1, 2, 3, 4, 5, 6, 10 and so on and so forth and also over here We group the elements based on their similar chemical properties all the elements in one group or basically one column They will have similar chemical properties same for this one and so on and so forth But it took us some years of effort and trials to arrive at this current periodic table For example our earlier periodic table the Mendeleev periodic table was based on elements being arranged in the increasing order of their atomic mass See, this is mass of hydrogen. This is mass of lithium and not the atomic number So let's see in this video how we decided why we decided to use atomic number and not atomic mass And how did that help us solve the earlier problems? So the earlier scientists including Mendeleev they had no clue about electrons, protons or atomic number So they could not have arranged the elements based on atomic number They only knew about atomic mass and that's what they did They arranged the elements based on increasing order of the atomic mass and they saw that after a period The properties of the elements were repeating Then again after chlorine the properties of the elements started to repeat Potassium had same properties as sodium, lithium, calcium had same properties as magnesium and beryllium and so on and so forth In fact, they were so surprised and overwhelmed that Mendeleev even gave a law based on this Mendeleev said that the chemical properties of elements are periodic function of atomic mass Now see at this point in time Mendeleev has no clue about how is the chemical property of an element or an atom? Related to its atomic mass, like think about this How is the atomic mass deciding the chemical property? He had no idea and because of this He could not answer some of the limitations or challenges that appeared for example There were certain elements that could not fit the periodic table in the order of their atomic mass such as cobalt and nickel See cobalt's atomic mass is 58.9, which is heavier than nickel, which is 58.6 So technically speaking cobalt should come here and nickel should come here, nickel should be placed before cobalt But actually cobalt was placed before because cobalt's chemical properties were similar to the elements below it in this particular group See, I haven't drawn the periodic table completely because I was running out of space But there will be some elements over here below cobalt also So cobalt's chemical properties were similar to the elements of this group And that's why cobalt was placed before nickel even though it was heavier and mandlive could not explain this And also about isotopes see isotopes are atoms of the same element meaning they have same chemical properties But they have different atomic masses such as carbon 12 atom and carbon 14 atom Both are carbon only both have same chemical properties But different masses so should we should they be placed separately or in the same place? And there were some other limitations also So only if mandlive knew how the chemical properties of atoms or elements are being decided he would have been able to answer this But this answer comes after certain years So the year is 1913 and by now we know about electron protons and atomic number And our physicist our brilliant physicist Henry mostly he does an experiment to prove that The atomic number is a fundamental property of an atom not the atomic mass Now i'm not going to show you his experiment, but i'll explain you his inference See this basically means that the chemical properties of an atom They depend on the atomic number of the atom not the atomic mass And now this is well understood See by chemical property Basically i mean how the atom is going to react in a chemical reaction right And by now we know that in a chemical reaction atoms just exchange electrons That's all that happens in a chemical reaction different atoms come together and exchange some electrons gain lose or share electrons And so the number of electrons that they gain or lose or share that depends on the total number of electrons So we can say that in a sense the chemical properties of the atom They are dependent on the number of electrons in the atom They're dependent on that And we now know that in a neutral atom the number of electrons Is equal to the number of protons and the number of proton is also called as the atomic number The atomic number So now you see how the chemical properties are dependent on the atomic number And they have nothing to do with atomic mass see atomic mass Let me write it down atomic mass This is basically a number of protons plus number of neutrons. Okay That's it And this has no relationship with Number of electrons Basically, there is no direct relationship between number of electrons and the sum So you can see that how atomic mass is not dependent on or how atomic mass does not decide the chemical properties of an atom Now based on this finding mostly suggesting to change the periodic law he suggests that The chemical properties of elements are periodic function of not atomic mass but atomic number And based on this he's suggesting to go back and change the periodic table also And that's what we did We replace the atomic mass from the periodic table and we arrange the elements based on their atomic numbers increasing order of their atomic number And that's how we got our modern periodic table And yes, this table was able to solve almost all of the limitations of our old periodic table Finally the position of cobalt and nickel was settled See cobalt has a higher atomic mass So according to the old periodic law, it should come after nickel But cobalt was showing chemical properties similar to the elements of this group So it should be in this group, right? Now if you arrange cobalt and nickel based on their atomic numbers You will see that cobalt has a lesser atomic number And therefore it should come before nickel in this rightful group where its chemical properties are aligned So that got settled Next thing was the position of isotopes. See isotopes are basically atoms of the same element But they have different atomic masses For example, one atom of carbon has 12 units of mass and the other atom of carbon has 14 units of mass So if you arrange the atoms according to the old periodic law, you will place them in separate positions because they have different atomic masses, right? But technically speaking, they are both carbon atoms. They have the same chemical properties and therefore they should be placed in the same place So what do we do about that? Well, now that since we are arranging elements based on atomic number, we don't have to worry about this Because both the atoms have the same atomic number even though they have different atomic masses So therefore both will occupy the same place So this is how the modern periodic table solved few of the limitations of the old periodic table There is one limitation that is still pending and is still not solved by the modern periodic table Which is the position of hydrogen See even in the old periodic table, we were not sure where to place hydrogen Because hydrogen shows some chemical properties that are similar to group one element And some chemical properties that are similar to group 17 elements But yet it is different than both of these group elements, okay? So we still don't know where to place it even with the new periodic law. We don't have a definitive answer for this Some people do place it in the group one along with the group one elements because hydrogen has one valence electron Just like the elements of group number one Now let's summarize the video in this video. We spoke about the modern periodic law I hope you remember what it was and who gave it men leave or mostly And how it solves the limitations of the old periodic table And I also hope that you understood the explanation of how chemical properties are related to the atomic number