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Classification PERIODIC CLASSIFICATION OF ELEMENTS (i) Classification is the method of arranging similar atoms together and separating them from dissimilar atoms. If Lithium is first element then Sodium is the eighth element and both have similar chemical properties. (ii) If Fluorine is first element then Chlorine is the eighth element and both have similar chemical properties. Need For Classification Of Elements To make the study of large number of elements easier it is reduced to the study of small group of elements by classification of elements in different groups. Many scientists like Dobereiner, Newland etc. laid the foundation of classification of elements on the basis of which the modern classification is developed. Dobereiner’s Classification or Dobereiner’s Triads Dobereiner observed triads. Triads were group of three elements having similar properties. He stated that, when elements are arranged in the increasing order of atomic masses, the atomic mass of the middle element of the triad is equal to the arithmetic mean of the atomic masses of the other two elements and these three elements possess similar chemical properties. Example Element Atomic Mass Lithium 7 Sodium 23 Potassium 39 Element = 35.5 Bromine 79.9 Iodine 127 1. 2. 3. It was found that the Law of Octaves was only applicable upto calcium, as after calcium every eighth element did not possess properties similar to the first element. It was assumed by Newland that only 56 elements existed in nature and no more elements would be discovered in the future. But later on, several new elements were discovered, whose properties did not fit into then law of octaves. In order to fit elements into his periodic table, Newland adjusted two elements in the same slot, but also put some unlike elements under the same note. Mendleev’s Classification 7 + 39 2 Mendleev in 1869 arranged elements in the order of increasing atomic masses and the properties were repeated after regular intervals. Mendleev’s Periodic Law It states that the properties of the elements are the periodic function of their atomic masses. Atomic Mass Chlorine Anomalies Of Newland’s Law Of Octaves = 35.5 + 127 2 He was able to make only three triads out of the then known elements. But all the elements could not be arranged in such triads. Newland’s Law Of Octaves Newland arranged elements in order of increasing atomic masses. He stated that, when elements are arranged in such a way, the properties of eighth element (starting from a given element) are the same as the properties of first element. Elements are arranged in horizontal rows with seven elements in each row. Example Newland’s Table Of Elements Li Be B C N O F Na Mg Al Si P S Cl K Ca Periodic Table A periodic table is the arrangement of the known elements in rows and coloumns according to their properties in a tabular form. Mendleev’s Periodic Table Mendleev’s periodic table consisted of eight groups (vertical columns) and six periods (horizontal rows). At that time 63 elements were known and when they were arranged in the order of increasing atomic masses, the properties of elements were repeated after regular periods. He also left some gaps in the periodic table for the undiscovered elements and predicted the properties of those elements by studying the properties of neighbouring elements. Principles On Which Mendleev’s Periodic Table (i) Increasing order of the atomic masses. (ii) Similarities in the properties of the elements of same group. Achievements Of Mendleev’s Periodic Table 1. In his periodic table the elements with similar chemical properties were grouped together due to which the order of atomic masses for some elements was wrong i.e, decreasing order instead of increasing. Like Cobalt (atomic mass 58.93) placed before Nickel (atomic mass 2. 3. 58.71) and Tellurium (atomic mass 127.6) placed before Iodine (atomic mass 126.90). Mendleev left some gaps in his periodic table as he predicted the existence of some new elements that had not been discovered at that time. Later on those elements were adjusted in the table without disturbing the original table. When the noble gases were discovered, they could be placed in a new group without disturbing the existing order. arranged in groups (columns) and periods (rows). Periods In periods, elements are arranged in order of increasing atomic number. The number of elements in a period is fixed by the maximum number of electrons. A new outer shell is there in each period. Sixth period is the longest & complete period as it contains Lanthanide Series. Period No. Of Elements 1st 2(Shortest Period) 8 2nd 3rd 8 18 4th 18 5th 32(Longest Period) 6th 23(Incomplete) 7th There are two more rows at the bottom of the periodic table. These rows consist of fourteen elements after lanthanum (called lanthanides) and fourteen elements after actinium (called actinides). These are placed separately in the periodic table to save space and to avoid unwanted sidewise expansion of the periodic table. Groups Anomalies In Mendleev’s Periodic Table 1. Not Good For The Prediction Of New Elements The atomic masses do not increase in a regular manner in going from one element to the next. So it was not possible to predict how many elements could be discovered between two elements especially when we consider the heavier elements. 2. The Position Of Isotopes Could Not Be Explained Mendleev was not able to place the elements in the periodic table which are having isotopes. (Isotopes are the atoms having same atomic number but different atomic masses. 3. Position Of Hydrogen Hydrogen is placed in group 1. However it resembles the elements of group 1 as well as the elements of group 17. Therefore the position of hydrogen was not correctly defined. Modern Periodic Law The modern periodic law was proposed by Moseley. It states that the properties of the elements are the periodic function of their atomic numbers. All the anomalies of Mendleev’s periodic table were removed in modern periodic table. When elements are arranged according to increasing atomic number, there is a sequence or periodicity in the electronic configuration of the elements. The cause of periodicity is the recurrence of similar electronic configurations. The properties of the elements get repeated after intervals of 2, 8, 8, 18, 18, 32 because similar electronic configurations recur only after these intervals. The elements having similar electronic configuration show similar properties. Modern Periodic Table The modern form of periodic table is based on atomic number of elements. It was prepared by Bohr. The arrangement of electrons in modern periodic table is based on their atomic number and electronic configuration. The elements are In groups, elements which are having same number of valance electrons are placed in one group. Outer shell electrons are identical in each group. The elements within a group show: (i) Similar properties due to similar electronic configuration. (ii) A gradation of properties due to slowly varying attraction between nucleus and the valence electrons. (iii) According to the new recommendations of International Union of Pure and Applied Chemistry (IUPAC) there are 18 groups numbered 1 to 18. The group 1, 2 and group 13 to 17 contain Normal elements. Group 18 elements are known as Noble gases or Inert gases & elements in group 3 to 12 are Transition element. Elements from 93 onwards are purely synthetic and are called Transuranium elements. These are all radioactive in nature. Trends In Groups And Periods 1. Valance Electrons Valence electrons are the electrons present in outermost shell of an atom. Trend Of Valence Electrons In Groups All elements of a group of the periodic table have the same number of valance electrons. Group Group Group Group Group Group Group 1 2 13 14 15 16 17 1 2 3 4 5 6 7 valance valance valance valance valance valance valance electron electron electron electron electron electron electron To Find Out That A Particular Element Belongs To Which Group (i) The group number of elements having up to two valence electrons is equal to the number of valence electrons. (ii) The group number having more than two valence electrons is equal to the number of valence electrons +10. Example Element Number Hydrogen Number Of Valence Electrons 1 Group 1 Helium Aluminum Silicon Phosphorous Sulphur Chlorine Argon 2 3 4 5 6 7 8 + + + + + + 10 10 10 10 10 10 = = = = = = Trend Of The Size Of Atom In Groups The size of the atoms increases on going down in a group, because in moving top to bottom in a group, a new shell of electrons is added in each element. Due to the increase in number of shells, the size of the atom increases. 2 13 14 15 16 17 18 Trend Of Valence Electrons In Periods On moving from left to right in a period the valence electrons increase in a linear order. In every period, the first element has one valence electron and the last element has eight valence electrons. Example In the second period Lithium has one valence electron and Neon, the last element of the period has eight valence electrons. 2. Valency Number of electrons lost, gained or shared by an atom of an element to become stable is called valency. Trend Of Valency In Groups All the elements in a group have same valency. It is due to the presence of same number of valence electrons in the atoms of elements in a group. Example Group Valency 1 1 2 2 13 3 14 4 15 3 16 2 17 1 18 0 Trend Of Valency In Periods On moving left to right in a period, the valency first increases then decreases. For example, in the second period, the valency first increases from 1 to 4 then decrease to 0. It is because the number of electrons increases in a linear order. Example Li Be B C N O 1 2 3 4 3 2 Element in same period have different valences. 3. F 1 Ne 0 Size Of Atom It is the distance from the centre of the atom to its outermost shell. So it is also called the radius of atom. Example In group 1, size of the last element Francium is largest. Trend Of The Size Of Atom In Periods On going left to right in a period, the size of atoms decreases. It is because on going left to right, the atomic number of elements increases, i.e. number of protons and electrons increases. Increased numbers of electrons are added to the same shell in a particular period. Due to the increase in the positive charge on the nucleus, (effective nuclear charge) the increased number of electrons in the valence shell, are attracted more towards nucleus and therefore the size decreases. Example In the second period Lithium is biggest in size but Fluorine is smallest in size. Note Neon even being the last element of the period is not having the smallest size. Rather its size is larger due to the presence of Wander Val forces of noble elements. So the elements having the smallest sizes will be from the halogen family i.e., the 17th group elements. 4. Electropositive Nature It is the ability to form positive ions or the ability to loose electrons easily. Trend Of The Electropositive Nature Of Atom In Groups The electropositive nature increases on going down in a group. This is due to the fact that when an electron shell is added at each step while moving down in a group the size of the atom increases. The valence electrons become more and more away from the nucleus and the hold of the nucleus on the outermost electrons become less, so, the atom can easily loose these electrons to form positive ions and become electropositive. Example In group 14, Carbon, the first member is electropositive as well as electronegative due to valency as 4. The electropositive character increases on going from top to bottom, shown by the last member Lead of this group, which is most electropositive. Trend Of The Electropositive Nature Of Atom In Periods Electropositive nature of an element decreases on going from left to right in a period. This is due to the fact that the size of the atoms decreases on moving from left to right due to the increase in the effective nuclear charge. So, the valence electrons become more and more close to the nucleus and the hold of the nucleus on the outermost electrons increases. In this situation it becomes difficult for an atom to loose electrons to form positive ions and become electropositive. Example In second period Lithium is the most electropositive whereas the electropositive nature decreases on moving towards right showing that Fluorine is not at all electropositive. 5. Metallic Nature The electropositive nature of an element shows that it is a metal. Trend Of Metallic Nature In Groups The metallic character increases on moving down in a group due to increase in the electropositive nature of elements. Example In group 14 Carbon the first member is a non metal. The metallic character increases on going from top to bottom, shown by the last member Lead of this group, which is a typical metal. Trend Of Metallic Nature In Periods The metallic nature decreases on moving left to right in a period. It is due to the decrease in the electropositive nature of the elements. Example In second period Lithium is the most electropositive hence it is a metal whereas the electropositive nature decrease on moving towards right showing that Fluorine is a non metal. 6. Electronegative Nature It is the ability to form negative ions. Or it is the ability to gain electrons easily. Trend Of Electronegative Nature Of Atom In Groups The electronegative nature decreases on going down in a group. This is due to the fact that when an electron shell is added at each step while moving down in a group the size of the atom increases. The valence electrons become more and more away from the nucleus and the hold of the nucleus on the incoming electrons in the outermost shell decreases. Therefore it becomes difficult for the atom to gain electrons to form negative ions. Example In group 17, Fluorine, the first element is most electronegative but Iodine the last element is least electronegative. Trend Of Electronegative Nature Of Atom In Periods The electronegative nature increases on moving left to right in a period. This is due to the fact that the size of the atoms decreases on moving from left to right in a period due to the increase in the effective nuclear charge. So, the valence electrons become more and more close to the nucleus and the hold of the nucleus on the outermost electrons increases. In this situation it becomes easier for the atom to gain the incoming electrons and form negative ions. Example In second period Lithium is not electronegative whereas the electronegative nature increases on moving towards right showing that Fluorine is the most electronegative. 7. Non Metallic Nature The electronegative nature of an element shows that it is a non metal. Trend Of Non Metallic Nature In Groups The non-metallic character decreases on moving down in a group due to the decrease in the electronegative nature of elements. This is because due to the increase in the size of the atoms the hold of the nucleus on the incoming electrons becomes weaker and it becomes difficult for the atom to gain electrons. Example In group 17, Fluorine, the first member is most non-metallic but Iodine the last element is least non-metallic. Trend Of Non Metallic Nature In Periods Non-Metallic character of elements increases on moving left to right in a period, i.e. electropositive character decreases and electronegative character increases. This is because, due to decrease in the size of the atom its ability to gain electrons increases therefore the non-metallic character continuously increases. Example In second period lithium is metallic whereas the electronegative nature increases on moving towards right showing that fluorine is non metallic. 8. Acidic And Basic Nature Of Oxides The acidic nature of an oxide is related to its non metallic nature whereas as the basic nature of an oxide is related to its metallic nature. It means that metals generally form basic oxides and non metals generally form acidic oxides. Trend Of Acidic/Basic Nature Of Oxides In Groups On moving top to bottom in a group the acidic nature of the oxides decreases whereas the basic nature of the oxides increases. It is due to the fact that on moving down in a group the metallic character increases hence basic nature of oxides increases whereas the non metallic character decreases hence the oxides become less acidic. Example In 1st group Francium the last element makes the most basic oxide whereas the in 17th group Fluorine the first element forms the most acidic oxide whereas the last element Astatine forms the least acidic oxide. Trend Of Acidic/Basic Nature Of Oxides In Periods On moving left to right in a period the acidic nature of the oxides increases whereas the basic nature of the oxides decreases. It is due to the fact that on moving left to right in a period the metallic nature of the elements decreases hence the basic nature of oxides decreases whereas non metallic nature increases hence the acidic nature of the oxides increases. Example For example on moving from left to right in 2nd period, Lithium forms basic oxide whereas Fluorine forms acidic oxide. Advantages Of Long Form Of Periodic Table (i) The periodic table has made the study of chemistry systematic and easy. (ii) It is easier to remember the properties of an element if it’s position in the periodic table is known. (iii) The type of compounds formed by an element can be easily predicted by knowing its position in the periodic table. (iv) Many elements have been discovered with the help of periodic table. (v) The atomic masses of many elements have been corrected on the basis of their position in the periodic table. (vi) This classification is based on the atomic numbers which is the more fundamental property of elements. (vii) The Lanthanides and actinides which have properties different from other groups are placed separately at the bottom of the periodic table. (viii) The position of placing isotopes is fully justified. jhkljhkl