➡️INTRODUCTION
• There are 118 elements known at present and it is very difficult to study the properties of all these elements separately.• Around the year 1800, only 30 elements were known.
• Scientists started looking for some pattern in their properties on the basis of which they could study the elements with ease.
• Therefore, the elements have been classified into groups based on similarities in their properties.
➡️WHAT IS CLASSIFICATION ? WHY IS IT ESSENTIAL?
Grouping or Sorting of elements based on their similarities and differences is known as classification.
Advantage:- It makes the study easier.
➡️DOBEREINER'S TRAIDS [1829]
• Johann Wolfgang Dobereiner was a German chemist.
• 30+ elements were known at Dobereiner's time .
• His law:- "When elements are arranged in increasing order of their atomic masses in a group of three(triads) , then the arithmetic mean of mass of 1st and 3rd element is found approximately equal to the mass of central atom"
• Limitations of Dobereiner's triads:- He could not arrange all the elements known at that time into triads. He could identify only 3 such triads.
➡️NEWLANDS LAW OF OCTAVES [1864]
• John Newlands, was an English scientist.
• Till Newlands time 56 elements were known.
• His law:- "If elements are arranged by the increasing order of their atomic masses, property of every eighth element is similar to its respective first one, just like the notes of music(octaves)"
• He started his table, with hydrogen and ended at Thorium.
• Limitations of Newlands law of octaves:-
(1) It was applicable only till Calcium i.e. after Calcium,every 8th element didn't posses the same properties of the 1st element.
(2) He assumed that only 56 elements, existed. But later other elements came to place and didn't fill into his laws.
(3) In order to fill elements into his table, he fit elements like Co and Ni under the same slot and that too in the same columns as F, Cl and Br which have very different properties than these elements.
(4) Fe, which resembles Co and Ni in properties was kept far away from these elements.
➡️MENDELEEV LAWS [1869]
• In 1869, Mendeleev, a Russian scientist arranged the then known 63 elements on the basis of similarities in properties. Among similarities, he concentrated on the compounds formed with oxygen and hydrogen.
• He arrange the elements on the similarities in formula of compound (oxides and hydrides) formed by these elements.
• His observations:
(1) Most of the elements that got a place in his periodic table were arranged in the order of increasing atomic masses.
(2) There occurs a periodic reoccurrence of elements with similar physical and chemical properties.
• He arrange the elements on the similarities in formula of compound (oxides and hydrides) formed by these elements.
• His observations:
(1) Most of the elements that got a place in his periodic table were arranged in the order of increasing atomic masses.
(2) There occurs a periodic reoccurrence of elements with similar physical and chemical properties.
• Mendeleev formulated a Periodic Law that stated that "The properties of elements are periodic function of their atomic masses".
• Mendeleev's Periodic Table contains vertical columns called "groups" and horizontal rows called "periods".
• Mendeleev's Periodic Table contains vertical columns called "groups" and horizontal rows called "periods".
(1) He corrected atomic masses of certain elements for example atomic mass of beryllium was corrected from 14.9 to 9.01.
(2) He left 3 gaps for undiscovered elements and named them as eka-Boron, eka-Aluminium,
and eka-silicon and also predicted their properties later those elements were discovered as Scandium, Gallium, and germanium respectively. Also predicted properties were found to be correct.
(3) Nobel gases, which were discovered later, could be placed in his table without disturbing the actual form.
• Limitations of Mendeleev's periodic law:-
(1) Position of isotopes could not be explained.
(2) Position of Hydrogen could not be assigned properly.
(3) Wrong order of atomic masses of some elements could not be explained.
(4) Many of the empty gaps remain unexplained.
(2) Position of Hydrogen could not be assigned properly.
(3) Wrong order of atomic masses of some elements could not be explained.
(4) Many of the empty gaps remain unexplained.
➡️MODERN PERIODIC TABLE [1913]
• In 1913, Henry Mosely showed that the atomic number of an element is a more fundamental property than it's atomic mass.
• Mendeleev's Periodic Table was modified and atomic number was adopted as the basic of Modern periodic table.
• Modern Periodic Table law states that *Properties of elements are a periodic function of their atomic number*.
• The prediction of properties of elements could be made with more precision when elements were arranged on the basis of increasing atomic number.
• The Modern Periodic Table helps overcome the 3 limitations of Mendeleev's Periodic Table.
• Mendeleev's Periodic Table was modified and atomic number was adopted as the basic of Modern periodic table.
• Modern Periodic Table law states that *Properties of elements are a periodic function of their atomic number*.
• The prediction of properties of elements could be made with more precision when elements were arranged on the basis of increasing atomic number.
• The Modern Periodic Table helps overcome the 3 limitations of Mendeleev's Periodic Table.
• Position of elements in the modern periodic table:
(1) The Modern Periodic Table has 18 vertical columns known as "groups" and 7 horizontal rows known as "periods".
(2) The arrangement is based on their electronic configuration.
(3) There is an irregularity when it comes to the position of H. It can either be placed in group 1 since the electronic config. of H is similar to alkali, or it can be placed in group 7 with halogens because it forms diatomic molecules.
(4) The atoms of different elements with the same number of occupied shells are placed in the same period.
(5) The position of an element in the periodic table tells us about it's chemical reactivity.
(6) The valence electrons determine the kind and number of bonds formed by an element.
(1) The Modern Periodic Table has 18 vertical columns known as "groups" and 7 horizontal rows known as "periods".
(2) The arrangement is based on their electronic configuration.
(3) There is an irregularity when it comes to the position of H. It can either be placed in group 1 since the electronic config. of H is similar to alkali, or it can be placed in group 7 with halogens because it forms diatomic molecules.
(4) The atoms of different elements with the same number of occupied shells are placed in the same period.
(5) The position of an element in the periodic table tells us about it's chemical reactivity.
(6) The valence electrons determine the kind and number of bonds formed by an element.
• Trends in the modern periodic table:-
(1) Valeney: It is determined by the number of valence electrons present in the outer shell if its atom.
Variation of valency in a period: On moving from left to right in each short period, the valency of elements increases from 1-4 and decreases to 0.
(1) Valeney: It is determined by the number of valence electrons present in the outer shell if its atom.
Variation of valency in a period: On moving from left to right in each short period, the valency of elements increases from 1-4 and decreases to 0.
Variation of valency in a group: Since the number of valence electrons of all elements in a groups is same, all elements in a group have same valency.
(2) Atomic Size: It refers to the radius of an atom.
Variation in a period: On moving left to right, the size of atoms decrease.
Variation in a group: On going down, the size of atoms increase.
• Metallic & Non-metallic properties:
(1) In the Modern Periodic table, a zigzag line separates metals from non-metals.
(2) Metalloids: The border-line elements like B, Si, Ge, As, Sb, Te and Po exhibit properties of both metals and non-metals.
(3) Metals: They are found on the left side of the periodic table.
Variation in a period: On moving left to right, the metallic character of elements decrease.
(2) Atomic Size: It refers to the radius of an atom.
Variation in a period: On moving left to right, the size of atoms decrease.
Variation in a group: On going down, the size of atoms increase.
• Metallic & Non-metallic properties:
(1) In the Modern Periodic table, a zigzag line separates metals from non-metals.
(2) Metalloids: The border-line elements like B, Si, Ge, As, Sb, Te and Po exhibit properties of both metals and non-metals.
(3) Metals: They are found on the left side of the periodic table.
Variation in a period: On moving left to right, the metallic character of elements decrease.
Variation in a group: On moving down, the metallic character of elements increases.
(4) Non-metals: They are found on the right side of the periodic table.
Variation in a period: On moving left to right, the non-metallic character increases.
Variation in a group: On moving down, the non-metallic character of elements decreases.
(5) Oxides: Oxides of metals are basic and of non-metals are acidic in general.
Variation in a period: On moving from left to right, the basic nature of oxides decreases, the acidic nature of oxides increases.
Variation in a group: On moving down, the basic nature of oxides increases, the acidic nature of oxides decreases.
Variation in a period: On moving left to right, the non-metallic character increases.
Variation in a group: On moving down, the non-metallic character of elements decreases.
(5) Oxides: Oxides of metals are basic and of non-metals are acidic in general.
Variation in a period: On moving from left to right, the basic nature of oxides decreases, the acidic nature of oxides increases.
Variation in a group: On moving down, the basic nature of oxides increases, the acidic nature of oxides decreases.
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