# NCERT Class 10: Science-Intext Solutions

Reviewed By:
Krishna Kant Majee
M.Sc., B.Ed.

### Chapter 05-Periodic Classification of Elements-Intext Solutions

#### NCERT Book Page Number 81

Q.1. Did Dobereiner’s triad also exist in the column of Newland’s octaves? Compare and find out.
Ans. Yes, Dobreiner’s triads also exist in the columns of Newland’s octaves. One such column is Li, K, Na.

Q.2. What were the limitations of Dobereiner’s classification?
Ans. Limitation of Dobreiner’s classification : All known elements could not be classified into groups of triads on the basis of their properties.

Q.3. What are the limitations of Newland’s law of octaves?
Ans. Limitations of Newland’s law of octaves :
(i) It was not applicable for the elements that had atomic masses higher than calcium (Ca) because after calcium every eighth element did not passes properties to that of first.
(ii) Newland’s assumed that only 56 elements existed in nature and no more elements would be discovered in future. But later several new elements were discovered. e.g. Noble gases — He, Ne, Ar, etc.
(iii) Cobalt and nickel were placed in the column of fluorine, chlorine and bromine, having completely different properties than these elements.

#### NCERT Book Page Number 85

Q.1. Use Mendeleev’s period table to predict the formulae for the oxides of the following elements :
K, C, Al, Si, Ba.

Ans. $K_{2}O, CO_{2}, SiO_{2}, BaO.$

Q.2. Besides gallim, which other elements have since been discoverd that were left by Mendeleev in his periodic table (any two)?
Ans. (i) Scandium (ii) Germamium.

Q.3. What are the criteria used by Mendeleev in creating his periodic table?
Ans. (i) Atomic mass of elements.
(ii) Physical and chemical properties of elements.

Q.4. Why do you think the noble gases are placed in a separate group?
Ans. Inert gases lie He, Ne, Ar, Kr, Xe were discovered very late. When noble gases were discovered they could be placed in a new group without disturbing the existing order and this is one of the strengths of Mendeleev’s periodic table.

#### NCERT Book Page Number 90

Q.1. How could the Modern periodic table remove various anomalies of Mendeleev’s periodic table?
Ans. (i) Since Modern periodic table is based on atomic number, the position of placing isotopes at one place is justified.
(ii) Modern periodic table separate metals non-metals and metalloids clearly.
(iii) Correct the wrong order of atomic masses of certain elements.

Q.2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Ans. Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements and since chemical properties are due to valence electrons, they show same chemical reactions.
Q.3. Name
(a) three elements that have a single electrons in their outermost shells.
(b) two elements that have two electrons in their outermost shells.
(c) three elements with filled outermost shells.
Ans. (a) Lithium (Li), sodium (Na) and potassium (K) have a single electron in their outermost shells.
(b) Magnesium (Mg), Calcium (Ca) and strontium (Sr) have two electrons in their outermost shells.
(c) Neon (Ne), Argon (Ar) and Xenon (Xe) have filled outermost shells.

Q.4. (a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is their any similarity in the atoms of these elements?
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Ans. (a) Atoms of lithium, sodium and potassium has one electron in their outermost or valence shell. They are alkali metals and are very reactive.
(b) The atoms of ‘He’ and ‘Ne’ have filled outermost shells. That is why they are inert.

Q.5. In the modern periodic table, which are the metals among the first ten elements?
Ans. Lithium (Li) and Beryllium (Be).

Q.6. By considering their position in the periodic table, which one of the following elements would you expect to have maximum metallic characteristic? Ga, Ge, As, Se, Be
Ans. Since Be lies to the extreme left hand side of the periodic table, Be is the most metallic among the given elements.