NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification Of Elements

Q1 )

Which of the following statements is not a correct statement about the trends wlien going from left to right across the periods of Periodic Table.

• (a) The elements become less metallic in nature.
• (b) The number of valence electrons increases.
• (c)The atoms lose their electrons more easily.
• (d)The oxides become more acidic.

Answer :

Option (C) the atoms lose their electrons more easily because their size increases from left to right

Q2 )

Element X forms a chloride with the formula $XC{l}_2,$ which is solid with a high melting point. X would most likely to be in the same group of the periodic table as

• (a) Na
• (b) Mg
• (c) Al
• (d) Si

Answer :

(B) Since elementX forms a chloride XCl2,so valency of X is 2.Now out of all, the valency 2is mg

Q3 )

Which element has

• (a) two shells, both of which are completely filled with electrons ?
• (b) the electronic configuration 2, 8, 2 ?
• (c) a total of three shells, with four electrons in its valence shell ?
• (d) a total of two shells with three electrons in its valence shell.
• (e)twice as many electrons in its second shell as in its first shell ?

Answer :

• (a) Neon(2,8
• (b) Magnesium
• (c) Silicon(2,8,4)
• (d) Boron(2,3)
• (e)Carbon(2,4)

Q4 )

(a)What property do all elements in the same column of the Periodic Table as boron have in common ?
(b) What property do all elements in the same column of the Periodic Table . as fluorine have in common ?

Answer :

(a) The property that all the elements in the same column of the periodic table as boron have is: “the same number of valence electrons (3). Hence, they all have valency equal to 3.”
(b)
All elements in the same column of the Periodic Table as fluorine have in common seven electrons in their valence shell and they all are belong to group thirteen.

Q5 ) An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element ?
(b) To which of the following elements would it be chemically similar ? (Atomic numbers are given in parentheses.)
N (7), F (9), P (15), Ar (18)

Answer :

(a) The atomic number of the given element is 2 + 8 + 7(= 17).
(b) It would be chemically similar to fluorine [F(9)] because its electronic configuration is 2, 7

Q6 )

The positions of three elements A, B and C in the periodic table are shown below :
(a) State whether A is a metal or non-metal.

Answer :

• (a)? Since the valency of group 17 elements is 1 and all these elements accept electrons, thus A is a non-metal.
• (b) C is less reactive than A because as we move down in a group, the reactivity of non-metals increases.
• (c) C is smaller in size than B because B and C both are related to the same period and the size decreases as one moves from left to right in a period.
• (d) A will form anion because it is a non-metal

Q7 ) Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the periodic table. Write the electronic configuration of these two elements. Which of these will be more electronegative ? Why ?

Answer :

Electronic configuration of nitrogen -2,5
Electronic configuration of phosphorus = 2, 8, 5
Nitrogen will be more electronegative because outermost shell is nearer to nucleus and therefore nucleus will attract electrons more strongly. In a group of the periodic table, electron attracting tendency decreases as we move from top to bottom.

Q8 ) How does the electronic configuration of an atom relate to its position in the Modern Periodic Table ?

Answer :

The position of an atom/element in the periodic table is related to the number of electrons in the outermost shell. In a given column or group, all elements have the same number of electrons in their outermost shells. For example, in group 2, all elements have two electrons in their outermost shell. In group 15, all elements have 5 electrons in their outermost shells.

Q9 ) In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic number 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium ?

Answer :

The electronic configuration of elements with :
Atomic number 12 = 2, 8, 2
Atomic number 19 = 2, 8, 8, 1
Atomic number 20 = 2, 8, 8, 2
Atomic number 21 = 2, 8, 9, 2
Atomic number 38 = 2, 8, 18, 8, 2
Elements with atomic number 12 i.e., magnesium (Mg) and 38 i.e., strontium (Sr) will have similar physical and chemical properties as element with atomic numbers 20 i.e., calcium (Ca).

Q10 ) Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and the Modern Periodic Table.

Answer :

Mendeleev's Periodic Table
Elements are arranged in the increasing order of their atomic masses.
> There are 8 Groups.
> Each groups are subdivided into sub group 'a' and 'b'
> Groups for Noble gas was not present as noble gases were not discovered by that time.
> There was no place for isotopes.



Modern periodic table
> Elements are arranged in the increasing order of their atomic numbers.
> There are 18 Groups.
> Groups are not subdivided into sub-groups.
> A separate group is meant for noble gases.
> This problem has been rectified as slots are determined according to atomic number.

Q1 ) Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.

Answer :

Yes, Dobereiner’s triads existed in the columns of Newland’s Octaves.For example, the Dobereiner’s triad includes the elements lithium (Li), sodium (Na) and Potassium (K) in it's part. The same elements existed in second column of Newlands' classification too.

Q2 ) What were the limitations of Dobereiner’s classification ?

Answer :

Dobereiner could find only three triads i.e total of 9 elements only.It failed to arrange all the then known elements in the form of triads of elements having similar chemical properties.Thus, Dobereiner’s classification was not able to classify most of the elements known at that time.

Q3 ) What were the limitations of Newlands’ law of octaves ?

Answer :

(i) It was found that the Law of Octaves was applicable only up to calcium, as after calcium every eighth element did not possess properties similar to that of the first.
(ii)It appears that Newland assumed that only 56 elements existed in nature and no more elements would be discovered in the future. But after discovery of more elements, his Law could no longer hold true and several new elements existed whose properties did not fit into Newlands’ law of Octaves.

(iii)Newland adjusted two elements under the same slot inspite of their different properties. For example: Cobalt and Nickel were put together in just one slot and that too in the column of elements like fluorine, chlorine, and bromine which were very different properties from these elements.

(iv) Iron (Fe) which resembles the elements like cobalt and nickel in properties, was placed far away from these elements

Q4 ) Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements : K, C, Al, Si, Ba.

Answer :

The general formula for oxides of 1st, 2nd, 3rd and 4th group are $R_{2}O,RO,R_2{O}_{3}andR{O}_2$ respectively in the Mendeleev’s Periodic Table, where R denotes the element.
Here we have:

• Potassium (K) belongs to the 1st group so, the formula of its oxide will be $K_{2}O$.
• Barium (Ba) belongs to the 2nd group, so the formula of its oxide would be BaO.
• Aluminium (Al) belongs to the 3rd group, so the formula of its oxide would be $A{l}_2{O}_3$
• Carbon (C) and Silicon (Si) belong to the 4th group,so the formula of its oxide would be $C{O}_{2}andSi{O}_2$ respectively.
.

1. K ? $K_{2}O$.
2. C? $C{O}_2$
3. Al ? $A{l}_2{O}_3$
4. Si ? $Si{O}_2$
5. Ba ? $BaO$

Q5 ) Besides gallium, which other elements have since been discovered that were left by Mendeleev in his periodic table ? (any two)

Answer :

Scandium and Germanium

Q6 ) What were the criteria used by Mendeleev in creating his Periodic Table ?

Answer :

Mendeleev used the relationship between the atomic masses of the elements and their physical and chemical properties.He proposed that the chemical properties of elements are the periodic function of their atomic masses. And thus, he arranged the elements in the increasing order of their atomic masses and also placed elements with the same chemical properties in the same group.

Q7 ) Why do you think the noble gases are placed in a separate group?

Answer :

Noble gases are chemically inert and all of them show similar properties and are present in the atmosphere in extremely low concentrations. Therefore, owing to their similar inert behavior and similar electronic configuration with similar properties, they are placed in a separate group in the Periodic Table.

Q8 ) How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?

Answer :

(i) The modern periodic table is based on atomic number, while Mendeleev’s periodic table was based on atomic mass.So there was no need for keeping more than one element in one slot.
(ii) The isotopes of an element have the same number of protons (or atomic number). So they are alloted the same position in the modern periodic table.Hence the position of isotopes is defined in modern periodic table

(iii)In Modern periodic table elements are classified according to their electronic configuration into different blocks. Elements having same valence electron are kept in same group.Elements having same number of shells were put under the same period.Thus, various anomalies (irregularity) which were seen in Mendeleev’s table due to the arrangement according to atomic masses were overcome by the Modern Periodic table.

(iv)Hydrogen has been alloted special position, i.e., it is placed at the top of alkali metals in the first group.

Q9 ) Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?

Answer :

Beryllium (Be) and Calcium (Ca).
Electronic configuration of both Be and Ca are similar , i.e. two electrons in outermost shells.

Be?( 2,2)

Ca?(2, 8, 8, 2)

Also basic oxides $BeO$and $CaO$ are formed when both Be and Ca react with oxygen respectively.

Q10 ) Name:
(a) Three elements that have a single electron in their outermost shells.

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

(c) Three elements with filled outermost shells.

Answer :

(a) Lithium : (2, 1)
Sodium :(2, 8, 1)

Potassium : (2, 8, 8, 1)

(b) Beryllium :(2, 2)
Calcium : (2, 8, 8, 2)


(c) Helium :(2); Neon :(2, 8)
Argon : (2, 8, 8)

Q11 ) Explain the following:
(a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there 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?

Answer :

(a)
Lithium, Sodium and Potassium have following similarities:

(i) They all have valence electrons equal to 1 so they all belong to same group, i.e. Group 1st in Periodic Table.

(ii) They all are highly reactive and form ionic bonds with non-metals.

(iii) They all are highly electropositive..

(iv)All of these react with water to form alkalis.

(b)

Following properties are common in the atoms of Helium and Neon:

(i) Both have completely filled outermost orbit and have stable electronic configuration.

(ii) Both belong to the same group, i.e. zero group in Modern Periodic Table.

Q12 ) In the Modern Periodic Table, which are the metals among the first ten elements?

Answer :

Lithium and Beryllium

Q13 ) 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.

Answer :

Beryllium (Be) shows maximum metallic characteristics.
Reason:

Metallic character of elements depends upon the position in the periodic table. In the periodic table, the elements placed on the left show maximum metallic characteristics. Since Beryllium(Be) occupies the most left position in comparison to other elements, hence it shows maximum metallic characteristics.