Check CBSE Class 10 Science Notes for Chapter 5 – Periodic Classification of Elements for quick revision before Board Exam 2021. These notes are according to the revised CBSE Class 10 Science Syllabus. All the topics have been explained in a simple way. It will take only about 10-15 minutes to revise the whole chapter with the help of these quick revision notes.
Revision Notes for CBSE Class 10 Science Notes for Chapter 5 Periodic Classification of Elements:
Need for classification
It is difficult to study a number of element individually and to know their properties and uses. Therefore, they have been classified into groups on the basis of similarities in their properties. Thus, classification is needed to easily understand the properties of a large number of elements arranged in a group.
Early attempts at classification of elements
It showed that when the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.
Limitations of Dobereiner’s Triads
Dobereiner’s could identify only three triads from the elements known at that time. Thus, he failed to arrange all the known elements in the form of triads, even having similar properties.
Newland’s Law of Octaves
It states that when elements are placed in order of increasing atomic masses, the physical and chemical properties of every 8th element are similar to that of the first element.
Limitations of Newland’s Law of Octaves
1.Newland’s Law of Octaves was applicable only upto calcium, as after calcium every eighth element did not possess properties similar to that of the first.
2.Properties of the elements which were discovered later did not fit into the Law of Octaves.
3.In a few cases, Newlands adjusted two elements in the same slot to fit them in his table.
4.He also grouped unlike elements under the same slot.
Example: Cobalt and nickel are in the same slot and are placed in the same column as fluorine, chlorine and bromine which have different properties than these elements.
Mendeleev’s Periodic Table
It is based on the fact that the physical and chemical properties of the elements are the periodic functions of their atomic masses. This fact is named as Mendeleev’s Periodic Law.
Achievements of Mendeleev’s Periodic Table
1.This table had gaps to give place for some undiscovered elements.
2.There was a zero group in the table to accommodate the noble gases.
3.It corrected the atomic masses of certain elements. For example, the atomic mass of beryllium was corrected from 13.5 to 9.
Limitation of Mendeleev’s Periodic Table
1.No fixed position could be given to hydrogen in this Table.
2.It did not explained the position of isotopes.
3.In a few cases the trend of increasing atomic masses was not followed. For example, Cobalt (Co) has higher atomic weights but was placed before Nickel (Ni) in the periodic table.
Modern periodic table
This table is based on the Modern Period Law which states that “The physical and chemical properties of elements are the periodic function of their atomic numbers.”
All The elements in this table were arranged in order of increasing atomic numbers.
Elements in the Modern Periodic Table are arranged in 18 vertical columns called groups and 7 horizontal rows called periods.
- Group 1 elements are known as alkali metals.
- Group 2 elements are known as alkaline earth metals.
- Group 17 elements are known as halogens.
- Group 18 elements are known as noble gases.
Periodicity of elements: The recurrence of properties of the elements, after a certain regular intervals, when they are arranged in the increasing order of their atomic numbers, is called periodicity.
Cause of periodicity: It is due to the repetition of the same outer shell electronic configuration at a certain regular intervals.
The Modern Periodic Table removed various anomalies of Mendeleev‘s Periodic Table:
(a) Position of hydrogen is fixed as it is kept in the group with the elements of same valence electrons.
(b) In the Modern Periodic table elements are arranged in the increasing order of their atomic number, so there was no need to place more than one elements in one slot.
(c) The atomic numbers of cobalt and nickel are 27 and 28 respectively. Hence, cobalt with a lower atomic number is placed before nickel in the modern periodic table.
(d) All isotopes of an element have same atomic number but different atomic masses. Therefore, all the isotopes are placed in the same position in the modern periodic table.
Trends in the Modern Periodic Table
Valency: The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom
→ On moving from left to right in a period, the valency first increases from 1 to 4 and then decreases to 0.
→ On moving from top to bottom in a group, the valency remains same because the number of valence electrons remains the same.
Atomic size: Atomic size is determined by the distance between the centre of the nucleus and the outermost shell of an isolated atom.
→ On moving from left to right in a period, the atomic size decreases due to the increased effective nuclear charge that pulls the valence electrons closer to the nucleus..
→ On moving from top to bottom in a group, the atomic size increases due to the addition of an extra shell at each step.
Metallic character: It is determined by the tendency of an atom to lose its outermost (valence) electrons.
→ On moving from left to right in a period, metallic character of elements decreases because due to the increase in nuclear charge the tendency to lose valence electrons decreases.
→ On moving from top to bottom in a group, metallic character of elements increases due to the increase in atomic size that makes it easy to lose the valence electrons.
Non-metallic character: It is determined by the tendency of an atom to gain electrons.
→ On moving from left to right in a period, non-metallic character of elements increases because due to the increase in nuclear charge the tendency to gain the electrons increases.
→ On moving from top to bottom in a group, non-metallic character of elements decreases because due to the increase in atomic size the nuclear pull decreases. Due to this the tendency to gain the electrons decreases.