KCET 2026 Chemistry Oneshot | Classification of Elements Oneshot Explained for KCET

Added: 2026-05-05

[00:00:00]Hello champions, welcome to the channel.

[00:00:01]In today's video, we are going to do a quick revision of the chapter classification of elements and periodicity in properties, okay? This is the third chapter of PU 1 and it is not very very important from KCT point of view, but yes, there are certain basic things that you need to know from this chapter. So, let's get started and see what we are going to cover. So, first we'll discuss about periodic law and the modern periodic table. Then we'll talk about S, P, D and F block elements, the periodic trend. This is the most important from where questions can be asked and we'll be discussing some those points from where KCT, like those important points from where KCT often ask questions. So, first let's talk about the genesis of the periodic classification, means how the periodic table was formed. So, there like you see in the atomic model, there was first Thomson model, then Rutherford model.

[00:00:47]So, science is a process where there are continuous discovery. Similarly, in the periodic table also, the first attempt was given by Dobereiner. And what he did, he gave us a law of triads.

[00:00:58]According to him, he arranged the elements in the form of or in the set of three and the mass of the middle element is the average mass of the other two elements. So, here are three triads that was given to us by Dobereiner. That was lithium, sodium, potassium, calcium, strontium, barium and chlorine, bromine and iodine. You have to remember these three examples. Next we have here this one, there was one more cylindrical table, but this is not important, so you can just skip it. Now, coming to law of octaves. So, according to this law of octaves, this was given by Newlands that the property of the eighth element will be similar to that of the first element, like the music saregama padanisa, that concept he has taken. But the demerit here was it was applicable or it was feasible only till calcium. After that, this rule does not follow. Next we have Mendeleev periodic law and here according to this Mendeleev periodic law, it says that the property of the elements are the periodic functions of their atomic weights. Now, Mendeleev periodic law has a lot of good things as well as some limitations, so we will see them first. So, first of all, um if you see what are the good things about this, he predicted element. This is one of the best thing that you can ever think that he Suppose there was something where you find that okay, the next element, he was arranging the atoms in the increasing order of their atomic weights. Now, suppose after X, there was an atom Y, but the properties of Y was not fitting what he was imagining or what he was expecting. So, what he said that okay, after X, there will be one more element which is yet to be discovered and he left that place blank.

[00:02:32]So, that is the best thing or that is the most most important thing of Mendeleev periodic table. He left gaps for the undiscovered element and later he called eka-aluminum. So, he believed that there will be one element which will have some properties and that element was called as eka-aluminum by Mendeleev and later it formed gallium and eka-silicon also he predicted which was later named as germanium.

[00:02:55]Next there are some demerits also, like anomaly handling, that means he was placing the elements according to the atomic weight, but somewhere where the atomic weight was not matching, he didn't obey this rule. Like for example, iodine was placed before tellurium despite lower atomic weight, okay? So, tellurium is having lower atomic weight, so first tellurium should come, then iodine was iodine should come, but to match the properties, iodine was kept before tellurium. Next, prediction accuracy. He predicted the density, melting point, formula of the oxides and chlorides for even gallium and germanium which were undiscovered, okay? And that were confirmed later. You can see here, this is what he predicted for gallium, the atomic mass should be 68 and we found it was almost 70, which is very close. Similarly, the density you can see it was predicted so accurately, melting point, the formula of the oxide chloride formula, everything he predicted very very accurately for gallium and germanium.

[00:03:52]Next we will see modern periodic law.

[00:03:54]So, due to these demerits of Mendeleev's periodic law, we have modern periodic table and according to modern periodic table, the physical and the chemical properties of the elements are the periodic functions of their atomic numbers and not atomic weights as said by the Mendeleev, okay? And who gave us this modern periodic law? It was Moseley and he did it x-ray powder spectra showed the atomic number and based on this x-ray spectra, he gave us this modern periodic table. And uh it is not that Mendeleev doesn't like How does the problems that was there in Mendeleev periodic table was solved by modern periodic table? See, Mendeleev anomaly was iodine was placed before tellurium. Similarly, cobalt was placed before nickel was resolved by arranging Z and not mass.

[00:04:41]Now, when you take the factor as atomic number and not atomic mass, then this problem was very easily solved.

[00:04:49]Next in the modern periodic table, we have seven periods and 18 groups and they are numbered 1 to 18. Now, we will see a more about them in the upcoming slides. Now, here we'll talk about IUPAC nomenclature for the elements with atomic number more than 100. So, just a minute.

[00:05:06]Yeah. So, when the elements are having atomic number more than 100, if it is zero, we use this nil and the symbol used is n. For one, it is un, the symbol used is u. For two, it is bi, the symbol used is b. For three, it is tri, symbol used is t. For four, it is quad, symbol used is q. For five, the symbol word is used pent and symbol is p. For six, it is hex and h. For seven, it is sept and seven sorry, s. For eight, it is oct and o. For nine, it is en and e. Now, how to use this? Combine the roots for each digit and add ium at the end. For example, if you want 101, so for one, it is what? Un, okay? Then for zero, it is nil. So, un plus nil plus again un, then you have to add ium. So, the IUPAC name becomes unnilunium. And how will be the symbol used here? For one, it is u, then for zero, it is n and then again it is u. So, u n u will be the symbol here, okay?

[00:06:04]Next we will see about the electronic configuration and the periodic table.

[00:06:08]This is very important. There are seven periods, okay? The In the first period, there are only two elements, that is hydrogen and helium and the 1s orbitals are filled. Now, in the second period, we have eight elements, that is lithium to neon and here you can see already 1s electrons are filled, then we have 2s also filled and 2p is also filled, okay?

[00:06:30]Now, you can see 2p can hold six electrons, 2s can hold two electrons, so total eight electrons, eight elements are filled. Now, in third period also, we have 18 eight sorry, again we have eight elements from sodium to argon and here we have from 3s to 3p. Now, coming to the fourth period, we have 18 elements, okay? Remember, 2 8 8, then we have 18 elements in the fourth period.

[00:06:54]And here it is from potassium to krypton. Here 4s, 3d and 4p is filled.

[00:07:00]So, you can see 4s has two electrons, 3d 10 and 4p is six. So, 10 plus six plus two, that is 18. Next again, in period five, we have 18 elements. Here it is from rubidium to xenon and here 5s, 4d and 5p is filled. Next we have 32 elements here in the period six.

[00:07:19]It is from cesium to radon. Here 6s, 4f, 5d and 6p is filled. Then the last orbit the last period, that is seventh has also 32 elements and here we have francium and the this period is not complete, so that is why francium onwards it is written. Okay, periodic period number is equal to the highest principal quantum number. So, if any configuration is given and you want to know its period, so for example, I'll take argon, okay? So, argon electronic it it is having 18 electrons. So, what will be its electronic configuration? We already know how to write from the structure of atom chapter. So, 1s2 2s2 2p6 3s2 3p6 will that work? 6 7 8 9 10 11 12 plus six 18, right? Now, you see the highest n value is three. That means it will belong to third period, okay?

[00:08:08]So, that is how we can calculate. You just need to write the electronic configuration. The highest n value will give the period number.

[00:08:15]Now, we'll talk about the four blocks in our periodic table, S, P, D and F block elements.

[00:08:21]S block it represents group one and group two elements, okay? The first two columns of the periodic table in the leftmost part and its configuration is ns1 for group one and ns2 for group two.

[00:08:33]Now, this is called as alkali metals, the group one and alkaline earth metals are group two elements. Now, they are highly reactive, low ionization enthalpy, forms ionic compounds and never found free in nature because they are highly reactive. Now, talking about the P block elements, it is from group 13 to group 18 and here we have the configuration ns2np1 to ns2np6 and they represent boron to noble gases and these are also called as representative elements. Now, here we have metals, nonmetals and metalloids.

[00:09:04]Noble gases is ns2np6, that is the group 18 and group 17 is called as halogens and they are highly electronegative.

[00:09:13]Now, nonmetallic character increases from left to right. As we move towards the right, the nonmetallic character increases.

[00:09:20]Talking about D block element, I think you know about this in much detail because we have a dedicated chapter for this D and F block in class 12. So, groups three to group 12 comes under D block element. The electronic configuration is n-1d1-10 and ns0-2 and the transition elements 3d, 4d, 5d and 6d elements are present here. All of them are metals, they are colored ions, they have variable oxidation states, paramagnetic and are used as catalyst. Now, coming to the F block elements, they are lanthanides plus actinides. The general configuration is n-2f1-14, n-1d0-1 and ns2, okay? Now, cerium to lutetium we have 4f and thorium to lawrencium we have 5f. That is inner transition elements. Now, all of them are metals.

[00:10:04]Actinoids are radioactive. Lanthanoid is lanthanide contraction and they are placed at the bottom of the periodic table. Now, we'll talk about metals, nonmetals, and metalloids. So, 78% of all the known elements are actually metals. Okay? Now, they are present in the left side of the periodic table and group 1 and group 2 are completely metal and group 3 to group uh that is 12. Right? That is d block and your group 1 and group 2. They are completely metals. Now, they are solid at room temperature, high melting point and boiling point, good conductors of heat and electricity, malleable and ductile, and they form basic oxides. These are the basic properties of metals that we have studied in 10th class also. Now, what is a metalloid? These are border metals between metals and nonmetals.

[00:10:48]Means they are neither completely metal nor completely nonmetal. They lie in between. Now, they are diagonal uh uh band in the periodic table. Here, PT stands for periodic table. They Remember this example. This is very important.

[00:11:01]Silicon, germanium, arsenic, and then you have uh antimony, and uh tellurium.

[00:11:06]These are the five elements which are actually metalloids. In 2024, there was a question. Very simple. Four options were given. Which is uh a metalloid?

[00:11:15]That was a question. So, remember to uh you memorize these five elements. They are show the properties of both. They are used in semiconductors, important in electron uh electronics and are also called as semimetals. Now, coming to nonmetals, less than 20 elements are nonmetals. They are present on the right side of the periodic table. They are solid or gas at the room temperature.

[00:11:36]They have low melting and boiling point.

[00:11:38]Poor conductors. They are brittle when they are solid and generally form acidic oxides. Okay?

[00:11:44]Now, we'll talk about the periodic trend. This is the most important part from where the questions are being asked. So, first of all, if I talk about the radius, we have two types of radius, covalent radius and metallic radius. So, the covalent radius is found in a covalent bond, whereas metallic radius is found in a metallic bond. The definition of this is not that important, but the trend is very important. Across the period, what happens? The atomic size decreases. What is the reason for that? As we move along the period, the number of shells remain the same, but the number of protons increases. So, that is why the nuclear charge increases. And since the cell number remains the same, the force of attraction increases and the electrons are pulled closer. Now, coming to down the group, what happens? When down the group, the uh size of the atom increases. This is because new shells are added and the shielding effect uh gets lower and that is why the size is increasing. So, about ionic radius, what we can talk? The cation is always size is less than the parent atom. Like, for example, if I uh compare Cl, Cl uh plus, Cl, and Cl minus, then the size will be in this order. The cation will always be less than the parent atom. And the anion will be more than the parent atom. And if it is isoelectronic, same electron count, then whichever is having larger Z value, means more number of protons, it will have the smaller size. So, O2 minus, F minus, Na plus, Mg2 plus, if you see, all of them will have total of 10 electrons.

[00:13:11]Okay? All of them have electrons 10. But oxygen has eight protons, fluorine has nine protons. Um Right? Sodium has 10 protons and magnesium has 12 protons. So, more is the number of protons, more will be the force of attraction and size will be least there. Okay? Now, let's talk about ionization enthalpy. So, it is the energy required to remove one electron from the outermost shell. Okay? And it is a endothermic process. Means that uh value is always a positive value. You need energy here. Now, across the period, the ionization enthalpy increases as the size decreases. And down the group, the ionization enthalpy decreases as the size increases. Next, if you compare IE1, IE2, and IE3, the that IE3 will be greater because once you remove one electron, the number of protons increases. Right? So, now there are less number of electrons and more number of protons. So, the electrons are held more tightly. So, removal of electron becomes very, very difficult here. Next, we have anomalies here. That is very important from KCT point of view. If you talk about boron and beryllium. Okay?

[00:14:16]Hydrogen uh in the second period, we have uh hydrogen, helium, then we have lithium, beryllium, and boron. Right?

[00:14:23]And if I see along the period, what happens? The ionization enthalpy should increase. Right? But what we find is the ionization enthalpy of beryllium is actually more than that of boron. And the reason for here is the electronic configuration. Beryllium electronic configuration is 1s2 2s2, whereas for boron, it is 1s2 2s2 2p1. Now, here you are going to remove electrons from 2s orbital. Now, remember 2s orbitals are very closer to the nucleus. So, they are held tightly. So, that is why what happens? The enthalpy of boron is more.

[00:14:54]Similarly, if I compare oxygen and nitrogen. Okay? Nitrogen electronic configuration is I'll just do the outer uh electronic configuration. Or let me write down full. 1s2 2s2 2p3. And for oxygen, it is 1s2 2s2 and 2p four. Okay?

[00:15:14]Now, here what happens? In 2p3, the electrons are completely uh like it is half-filled configuration, very stable, tightly attracted towards the nucleus.

[00:15:24]Whereas in 2p4, that one electron which is extra is can be removed easily. So, that is why the nitrogen value will be higher than that of oxygen, even though oxygen comes towards the right of the nitrogen. Okay? Now, uh in noble gases, the ionization enthalpy is maximum because they are completely filled shell. And in alkali metals, they have the minimum because in alkali metals, group 1, there is only one electron in the valence shell which can be easily removed. Next, we have electron gain enthalpy. So, this is the energy released when one atom gains an electron. So, across the period, what happens? There is more negative As we move along the period, the electron gain enthalpy keeps on decreasing. Okay? Uh like keeps on increasing. Means the negative value keeps on increasing as the effective nuclear charge increases. Now, down the group, it becomes less negative. Okay?

[00:16:15]Along the period, it becomes more and more negative. And down the group, it becomes less and less negative. Now, in noble gases, they have the largest positive value because in that case, already they are very stable. They don't want to accept any electron. So, it is a positive value here. Now, halogens, group 17, they have the most negative electron gain enthalpy, but there is an exception. Now, see. What happens?

[00:16:37]Fluorine, chlorine, bromine, and iodine.

[00:16:39]It says that down the group, it becomes lesser negative. Right? Suppose here it is minus two, then it should have minus one. Less negative it is becoming. That means what? But the thing is in case of fluorine and chlorine, there is an exception. The chlorine electron gain enthalpy is more than the fluorine.

[00:16:54]Means it is more negative. The reason being is fluorine is a smaller atom and there is a lot of repulsion, interelectronic repulsion is there. Due to which, it cannot accept electrons.

[00:17:05]So, its electron gain enthalpy is less negative. Okay? Now, here you should also remember comparing oxygen and sulfur. Sulfur will have more electron gain enthalpy than that of oxygen. The same reason what we have understood here.

[00:17:20]Now, n is equal to two level, small, crowded. Addition of electron faces more repulsion than in than n is equal to three one. Now, talking about electronegativity, it is the qualitative measure of the ability of an atom to attract a shared pair of electrons.

[00:17:35]Okay? Now, uh across the period, it increases and down the group, it decreases. You just need to know the trends. Okay? You don't have to remember this value. You can just see how along the period it is increasing.

[00:17:48]Next, we'll talk about the periodic trends. Just summarize it. In atomic radius, along the period, it uh it increases. Sorry, decreases. And down the group, it increases. Now, ionization enthalpy, along the period, increases.

[00:18:01]Down the group, decreases. Electron gain enthalpy, across the period, it is more negative. Down the group, it is less negative. Across the period, electronegativity is more. Down the group, the electronegativity is less.

[00:18:12]Metallic character, along the period, it is uh keeps on decreasing. And down the group, it keeps on increasing. Nonmetallic character, along the period, it increases. And down the group, it decreases. Okay?

[00:18:26]Now, we will talk about the chemical reactivity and oxides. So, reactivity is maximum at extremes of a period and minimum at the center. So, what you will observe is in the left side, that is alkali metals, they are easily they are losing electrons. They have very low ionization enthalpy and can form cations very, very easily. And if you see on the right side, that is extreme right, you will exclude the noble gases because they are inert. Then we have halogens, that is group 17. They can easily gain electrons and they have high negative electron gain enthalpy and can readily form anions. And they are also highly reactive. But the middle elements, that is the transition group 3 to group 12, these are intermediate and the reactivity is in between. They form amphoteric oxides. Here, we get basic oxides. Here, we get acidic oxides. And here, we mainly get amphoteric oxides.

[00:19:14]You can see the examples here. Now, nature of the oxides, as we discussed, on the right hand side, the sodium, it will be uh basic in nature. And on the right side, it will be acidic in nature.

[00:19:24]And Al2O3, it is an amphoteric oxides.

[00:19:26]And also, we have some neutral oxides like CO, NO, and N2O. So, these examples are important. You need to remember them. Now, if I talk about the valence and the oxidation state, so you can see group 1 has a valency of 1. Group 2 valency is 2. Group 13, 3. Group 14, 4.

[00:19:42]Group 15, 5. And uh group 16, uh is six, their valence electron. Okay?

[00:19:48]Means how many electrons are present outside. And what is their valency? You can see for 15 it can be three or five, for 16 it is two or six, for 17 one or seven, and for 18 it is zero or eight.

[00:20:00]Now, here are some important reactions that you should remember like you can see SiBr4 gives like Si and Br is formed. And here we have Al2S3, it is made up of aluminum and sulfide. Now, there are some anomalous property of the second period elements, okay? Now, you can see lithium and beryllium differ from the rest of their group and there also we have some diagonal relationship between lithium, magnesium, and beryllium, aluminum. Now, what is the reason for that? That is because the first element of the group is small in size. It has high charge to radius ratio. Only four valence orbitals are there, so their maximum covalency is four. So, if you find this nitrogen, oxygen, carbon, they have certain different property than all other elements of the same group. And there is diagonal relationship between lithium, magnesium, and beryllium, aluminum.

[00:20:49]Diagonal relationship means these elements lie in the diagonal and they have some properties similar to each other. Next, let's do a quick revision here. So, you should know there are total 114 known elements. We have seven period. Period Period one to sorry. Yes, period one has two elements, period two and three has eight elements. Period four five has 18 elements and six and seven has 32 elements. Next, we have 18 groups and chlorine is the most electronegative It is the element with most negative electron gain enthalpy. Most electronegative is definitely fluorine, but most negative electron gain gain enthalpy is of chlorine. Now, fluorine has the highest electronegativity.

[00:21:33]Sodium is the one with the notable metallic radius 186 picometer. Hydrogen Sorry, helium and neon are noble gases.

[00:21:41]They have maximum ionization enthalpy.

[00:21:44]And gallium and germanium are those elements which were undiscovered and were later like were predicted by Mendeleev. So, gallium is called as eka-aluminum and germanium is called as eka-silicon. Okay?

[00:21:57]So, now let's see some exam tips that you should very very important revision.

[00:22:01]Chlorine has more negative electron gain enthalpy than fluorine. Please always remember from here questions are asked.

[00:22:06]The ionization order, ionization enthalpy, lithium. After that, instead of beryllium we have a boron here. There is a change, okay? Then carbon, oxygen, nitrogen. And again here we have a change, okay? That is because of half-filled and here because of s-orbital. This point is very very important. Next, ionization enthalpy of first is definitely ionization enthalpy of sodium is less than that of magnesium. And but IE2 of sodium is more than that of magnesium.

[00:22:33]After Na loses one electron, it comes to a noble gas core. So, that is why the second ionization enthalpy of sodium is much more than the first one, okay?

[00:22:42]Next, isoelectronic species, they have same electron count. Larger the positive charge, smaller will be the size. Now, metallic character increases down the group and across the period it decreases. Electron gain enthalpy of oxygen is less than sulfur and similarly fluorine is less than chlorine. This point I'm repeating again and again because it is very important.

[00:23:02]Mendeleev's periodic table is based on atomic mass, whereas modern periodic table is based on atomic number. The period will be equal to the highest value of n and group will be equal to the valence electrons. You can easily From the valence electron you can predict the group of the atom. So, I will just show you how.

[00:23:18]See, from the For example, if the valence electron is three or five, then the group number will be 15. If it is two or six, then it is 16. One or seven, then it is 17. If it is only one, then it is group one. Only two, it is group two. If it is only three, it is group 13. Only four, it is group 14. Okay? So, children, this is all about the classification of elements and periodicity in properties. There are certain exceptions that you should remember. Otherwise, this is a very very easy chapter. Thank you so much and stay tuned for the next video where we are going to solve the PYQs of this chapter.

[00:23:49]Thank you so much for watching and all the best.