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# NCERT Solutions for Class 10th Science

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## Chapter 5. Periodic Classification of Elements

Chapter-5, Periodic classification of Elements

Q1: Define Modern Periodic law. Who gave the concept of this law?

How does electronegativity of an element vary on moving from?

1. Left to right in a period.
2. From top to bottom in a group.

Q2: An element A has 4 electrons in the outermost shell of its atom and combines with another element B having 7 electrons in the outermost shell of its atom. The compound formed does not conduct electricity. What is the nature of the chemical bond in the compound? Give the electron dot structure of its molecule.

Q – What is Dobereiner;s triads? Give one example.

A -  Dobereiner show that when three elements in a triad were written in 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. Such a triad is known as Dobereiner;s triads. For example – Lithium, sodium and potassium.

Q – What is Newland’s Law of Octaves?

A – Every eighth element had properties similar to that of the first. This law is known as Newland’s Law of Octaves.

Q –

Q – What is the limitation of Newland’s Law of ctaves?

A –

(i)It was found that the law of octaves was applicable only up to calcium.

(ii)No more elements would be discovered in the future instead of 56 known elements, it was assumed by Newlands. But did not match the properties of new discovered elements to this law.

(iii)Newland’s Law of Octaves worked well with lighter elements only.

Q – Write three achievements of Mendeleev’s Periodic table.

A -  three achievements of Mendeleev’s Periodic table.

(i)There could be possible to classify all elements.

(ii)He prepared this periodic table on the basis of atomic masses of the elements.

(iii)Mendeleev left some gaps in his periodic table

Q - Write the law for Modern Periodic table.

A - The physical and chemical properties of elements are the periodic function of their atomic numbers in Modern Periodic table.

Q – Distinguish between Modern and Mendeleev’s Periodic table.

A –

 Modern Periodic table Mendeleev’s Periodic table Elements are arranged in increasing order of their atomic numbers. There are 18 column and 7 raw. Isotopes are kept at the same place of their corresponding elements as they have same atomic numbers.  There are no anomalies in the modern periodic table. Elements are arranged in increasing order of their atomic masses. There are 8 column and 6 raw. There is no position explained for isotopes.   Mendeleev’s periodic table had a number of anomalies or defects.

Q – Why do you think the noble gases are placed in a separate group?

A – Noble gases like helium (He), neon (Ne) and argon (Ar) were discovered very late because they have inert and present in extremely low concentrations in our atmosphere. They have completed their octane and their valency are zero.

Q – What is the atomic radius of hydrogen?

A - 37 pm (Picometre).

Q- How does the atomic radius changes as you go from left to right in a period?

A-The atomic radius decreases in moving from left to right along a period. This is due to an increase in the nuclear charge which tends to pull the electrons closer to the nucleus the size of the atom.

Q-How does the atomic size vary as you go down a group?

A-The atomic radius decreases in moving from left to right along a period. This is due to an increase in the nuclear charge which tends to pull the electrons closer to the nucleus the size of the atom.

Q- Name them

(a) Three elements that have a single electrone in their outer most shells.

(b) Two elements that have two electrons in their outer most shells.

(c) Three elements with filled outermost shells.

Ans:

(a)  lithium, sodium, potassium.

(b)  magnisium, calcium.

(c) helium, neon, argon.

Q-An atom has electronic configuration 2,8,7.

(a) What is the atomic number of this elements?

(b) To which of the following element would it be chemically similar.

N(7), F(9), P(15) Ar(18)
A-chlorine

B-17

C-F(9) because its electronic configuration 2,7.

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