Periodic Classification of Clements – CBSE Notes for Class 10 Science
1. Need for classification of elements:
Increase in discovery of different elements, made it difficult to organise all that was known about the elements. To study the large number of elements with ease, various attempts were made. The attempts resulted in classification of elements into metals and non-metals.
2. Dobereiner’s triads: –
Johann Wolfgang Dobereiner, a German chemist, classified the known elements in groups of three elements on the basis of similarities in their properties. These groups were called triads.
(i) Characteristics of Triads:
(a) Properties of elements in each triad were similar.
(b) Atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.
(ii) Examples of Triads:
(iii) Limitations: Dobereiner could identify only three triads. He was not able to prepare triads of all the known elements.
3. Newlands’ Law of Octaves:
John Newlands’, an English scientist, arranged the known elements in the order of increasing atomic masses and called it the ‘Law of Octaves’. It is known as ‘Newlands’ Law of Octaves’.
(i) Characteristics of Newlands’ Law of Octaves:
(a) It contained the elements from hydrogen to thorium.
(b) Properties of every eighth element was similar to that of the first element.
(ii) Table showing Newlands’ Octaves:
(iii) Limitations of Newlands’ law of Octaves:
(a) The law was applicable to elements up to calcium (Ca).
(b) It contained only 56 elements.
(c) In order to fit elements into the table, Newlands’ adjusted two elements like cobalt and nickel in die same slot and also put some unlike elements under same note.
4. Mendeleev’s Periodic Table : Dmitri Ivanovich – 5 ’ Mendeleev, a Russian demist, was the most important contributor to the early development of a periodic table of elements wherein the elements were arranged on the basis of their atomic mass and chemical properties.
(i) Characteristics of Mendeleev’s Periodic Table:
(a) Mendeleev arranged all the 63 known elements in an increasing order of their atomic masses.
(b) The table contained vertical columns called ‘groups’ and horizontal rows called ‘periods’.
(c) The elements with similar physical and chemical properties came under same groups.
(ii) Mendeleev’s Periodic Law : The properties of elements are the periodic function of their atomic masses.
(iii) Achievements of Mendeleev’s Periodic Table:
(a) Through this table, it was very easy to study the physical and chemical properties of various elements.
(b) Mendeleev adjusted few elements with a slightly greater atomic mass before the elements with slightly lower atomic mass, so that elements with similar properties could be grouped together. For example, aluminium appeared before silicon, cobalt appeared before nickel.
(c) Mendeleev left some gaps in his periodic table.
He predicted the existence of some elements that had not been discovered at that time. His predictions were quite true as elements like scandium, gallium and germanium were discovered later.
(d) The gases like helium, neon and argon, which were discovered later, were placed in a new group without disturbing the existing order.
(a) No fixed positions was given to hydrogen in the Mendeleev’s periodic table.
(b) Positions of Isotopes of all elements was not certain according to Mendeleev’s periodic table.
(c) Atomic masses did not increase in a regular manner in going from one element to the next.
5. Modem Periodic Table: Henry Moseley, gave a new ! property of elements, ‘atomic number’ and this was I adopted as the basis of Modem Periodic Table.
(i) Modem Periodic Law: Properties of elements are a periodic function of i their atomic number.
(ii) Position of elements in Modem Periodic Table:
(a) The modem periodic table consists of 18 groups and 7 periods.
(b) Elements present in any one group have the same number of valence electrons. Also, the number of shells increases as we go down the group.
(c) Elements present in any one period, contain the same number of shells. Also, with increase in atomic number by one unit on moving from left to right, the valence shell electrons increases by one unit.
(d) Each period marks a new electronic shell getting filled.
(iii) Table showing Electronic Configuration of First 20 Elements:
Trends in the Modern Periodic Table:
(i) Valency: Valency of an element is determined by the number of valence electrons present in the outermost shell of its atom.
• Valency of elements in a particular group is same.
• Valency of elements in a period first increases from one to four and then decreases to zero.
(ii) Atomic Size: Atomic size refers to the radius of an atom.
In a period, atomic size and radii decreases from left to right.
In a group, atomic size and radii increases from top to bottom.
(iii) Metallic and Non-metallic Properties:
(a) The tendency to lose electrons from the outermost shell of an atom, is called metallic character of an element.
(b) The tendency to gain electrons from the outermost shell of an atom, is called non-metallic character of an element.