NCERT Solutions Class 10 Science Chapter 5

Q.1 Atomic number of an element is 19.
(i) Is it a metal or non-metal?
(ii) Will the size of this element be greater than that of sodium?

(i) The electronic configuration of the given element is 2,8,8,1. Therefore, it has 1 electron in the valence shell. Hence, it is a metal.
(ii) The element with atomic number 19 is potassium. Since it is placed below sodium in periodic table, it is larger in size as compared to sodium.

Q.2 What change will be observed in atomic size when we move from top to bottom in group 2. Give reasons also.

As we move from top to bottom in a group, new shells are being added and the distance between the nucleus and the outermost shell goes on increasing and size of the atom increases. Therefore, when we move down in group 2, the size of atom increases.

Q.3 An element (X) belongs to second period and first group of the periodic table. Find:
(i) the number of valence electrons present in its atom.
(ii) the valency of the element.
(iii) whether element (X) forms acidic or basic oxide.

(i) The number of valence electrons in element X is one (because it belongs to group 1).
(ii) As element X has one electron in outermost shell, its valency is one.
(iii) Since element (X) belongs to group one, it is a metal. It will form basic oxide.

Q.4 When a metal M reacts with a non-metal X, it forms a compound of MX type.

(a) Identify the group in which element M and X are placed in modern periodic table.
(b) Write the number of valence electrons present in element M and element X.
(c) Write an example of MX type of compound.

(a) The metal M belongs to group1 (alkali metals) and X belongs to group 17 (Halogen group).
(b) The number of valence electrons present in M is one and the mumber of valence electrons present in X is 7.
(c) NaCl or KCl

Q.5 Element P belongs to group 2 of modern periodic table and element Q belongs to group 17 of modern periodic table. Write the formula of:

(i) the compound formed by elements P and Q.
(ii) the oxide of element P.
(iii) the chloride of element P.

(i)The valency of element P is 2 (because it belongs to group 2) and that of element Q is 1 (because it belongs to group 17). The formula of compound will be PQ₂.
(ii) PO
(iii) PCl₂

Q.6 What was the drawback of the law of octaves?

The law of octaves was applicable only upto calcium out of the total 56 elements known.

Q.7 The positions of three elements A, B, and C in the periodic table are shown here.

(a) State whether A is a metal or non-metal
(b) State whether C is more reactive or less reactive than A.
(c) Will C be larger or smaller in size than B?

(a) The group 17 is placed in right hand side of the periodic where non-metals are placed. So, A is a non-metal.
(b) The chemically reactivity of non-metals decreases on going down the group of the periodic table. So, C is less reactive than A.
(c) On going from left to right in a period, the atomic radii decreases. So, C is smaller than B.

Q.8 By considering their position in the periodic table, which one of the following elements would you except to have the maximum metallic character?
C , O, F, B, Be

The maximum metallic character is found in elements on the extreme left side of the periodic table. Out of the above given elements, Be will have the maximum metallic character because it is on the extreme left side in the periodic table.

Q.9 What does each period in the periodic table indicate?

Each period in the periodic table indicates the number of shells.

Q.10 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?
N (7), F (9), P (15), Ar (18).

(a) Atomic number of this element is 17.
(b) F will chemically be similar to this element because F has similar valence electrons.

Q.11 Name two elements you would choose to show chemical reactions similar to magnesium. Write the basis for your choice?

Calcium and Strontium would show chemical reactions similar to magnesium because they have same number of valence electrons like Mg.

Q.12 Write the characteristics of the groups.

Characteristics of the groups are: –
(i) Valence Electrons: – All the elements of a group of the periodic table have the same number of valence electrons.
(ii) Size of Atoms: – On moving down the group atomic size of the atom increases due to increase in number of shells.
(iii) Metallic Character: – On moving down the group metallic character of elements increases.
(iv) Valency: – Since the number of valence electrons in a group is the same, so all the elements in a group have the same valency.
(v) Non-metallic Character: – On moving down the group non-metallic character of elements decreases.

Q.13 Write the merits of the modern periodic table.

Merits of the modern periodic table are: –
1.It is based on the atomic number of elements which is the most fundamental property of elements.
2. It explains the reason for the periodicity in properties of elements.
3. It helps us understand why elements in a group show similar properties but elements in different groups show different properties.
4. It tells us why the properties of elements are repeated after 2, 8, 18 and 32 elements. There are no anomalies in the arrangement of elements in the modern periodic table.

Q.14 Consider the following elements:Na, Ca, Al, K, Mg, Li
(a) Which of these elements belong to the same period of the periodic table?
(b) Which of these elements belong to the same group of the periodic table?

The electronic configurations of the following elements are:
Na: – 2, 8, 1
Ca: – 2, 8, 8, 2
Al: – 2, 8, 3
K: – 2, 8, 8, 1
Mg: – 2, 8, 2
Li: – 2, 1
(a) Na, Al and Mg elements belong to third period and Ca and K elements belong to the fourth period of the periodic table because Na, Al and Mg elements have same number of shell (three shells) and Ca and K elements have same number of shells(four shells).
(b)The elements which have same number of valence electrons belong to the same group. So, Li, Na and K having 1 valence electron belong same group and Mg, Ca having 2 valence electrons belong same group.

Q.15 Write the limitations of Mendeleev’s classification of elements.

1. The position of isotopes could not be explained.
2. Wrong order of atomic masses of some elements could not be explained.
3. Correct position could not be assigned to hydrogen in the periodic table.

Q.16 For each the following triads, name the element with the characteristics specified below:

Ans-

Q.17 Write the merits of Mendeleev’s classification of elements.

Merits of Mendeleev’s classification of elements:
1. Mendeleev’s classification was first successful classification of elements which included all the elements.
2. It predicted the properties of elements which were not known at that time.
3. When noble gases were discovered, they could be placed in a new group without disturbing the existing order.

Q.18 Write the demerits of Newlands’ law of octaves.

Demerits of Newlands’ law of octaves: –
(i) This law was applicable upto calcium only.
(ii) Newlands’ assumed that only 56 elements existed in nature and no more elements would be discovered in future.
(iii) In order to fit elements into his table, Newland put even two elements together in one slot and that too in the column of unlike elements having very different properties.

Q.19 What are the advantages of the periodic table?

1. The periodic table has made the study of chemistry systematic and easy.
2. It is easier to remember the properties of an element if its position in the periodic table is known.
3. The type of compounds formed by an element can be predicted by knowing its position in the periodic table.

Q.20 An element X belongs to group 2 and another element Y belongs to group 15 of the periodic table:-

(a) What is the number of valence electrons in X?
(b) What is the valency of X?
(c) What is the number of valence electrons in Y?
(d) What is the valency of Y?

Ans-

(a) Valence electrons of X = 2
(b) Valency of X = 2
(c) Valence electrons of Y = 5
(d) Valency of Y = 3

Q.21 What happens to

(i) Metallic character of the elements as we move from left to right in a period.
(ii) Atomic size when we move from top to bottom in a group.

Ans-

(i) When we move left to right in a period metallic character decreases.
(ii) Atomic size increases when we move from top to bottom in a group.

Q.22 Explain why?

(i) All the elements of a group have similar chemical properties.
(ii) All the elements of a period have different chemical properties.

Ans- Chemical properties of the elements depend upon the valence electrons, it means the elements which have the same number of valence electrons in their outermost shells, will have similar chemical properties.
(i) So all the elements of the group have similar chemical properties owing to the same number of valence electrons.
(ii) But the elements of a period have different valencies, hence they show different chemical properties.

Q.23 Element X forms a chloride XCl₂, which is a solid with a high melting point. X would most likely be in the same group of the periodic table as:
(a) Na (b) Mg (c) Al (d) Si

Ans- Since element X forms a chloride XCl₂, so the valency of X is 2. Now, out of Na, Mg, Al, and Si, the element of valency 2 is Mg. So, X would be in the same group as that of Mg because all the elements of the same group have same valency.

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?

Ans- Electronic configuration of Nitrogen is 2, 5.

Electronic configuration of Phosphorus is 2, 8, 5.
Nitrogen is more electronegative than phosphorus due to its small size.

Q.25 How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?

Ans- The properties of the elements depend upon the valence electrons. On arranging the elements in the increasing order of their atomic numbers, the elements with the same number of valence electrons and hence, showing similar properties occur at regular intervals. Hence, the chemical properties of the elements show repetition.

Q.26Out of F and Cl, which is bigger in size and why?

Ans- Fluorine and chlorine belong to same group. When we move down the group the size of atom increases due to increase in number of shells. Therefore, the size of chlorine atom will be larger as compared to the atom of chlorine.

Q.27 Write the electronic configuration of Ca in terms of K, L, M, N shells.

(Atomic number of Ca is 20)

Ans- The electronic configuration of Ca is

Q.28 Define groups.

Ans- The vertical columns in the long form of the periodic table are called groups.

Q.29 In the Modern Periodic Table, which are the metals among the first ten elements?

Ans- Li and Be are the metals amongst first ten elements of the Modern Periodic Table.

Q.30 Which period is the longest period in the modern periodic table?

Ans- 6^th period is the longest period and it has 32 elements.

Q.31 In the following diagram for the first three periods (showing only group 1,2, 13 to 18)of the periodic table, five elements have been represented by the letters a, b, c, d and e (which are not their chemical symbols):

(i) Select the letter which represents a halogen.
(ii) Select the letter which represents a noble gas.
(iii) What type of bond is formed between a and b?
(iv) What type of bond is formed between c and d?
(v) Which element will form a divalent anion?

Ans-

(i) The halogens are placed in group 17 of the periodic table. So, letter ‘b’ represents a halogen.
(ii) The noble gases are placed in group 18 of the periodic table. So, letter ‘e’ represents a noble gas.
(iii) ‘a’ and ‘b’ both are non-metals. So, the bond formed in between ‘a’ and ‘b’ will be covalent.
(iv) ‘c’ is metal and ‘d’ is non-metal. So, the bond formed in between ‘c’ and ‘d’ will be electrovalent.
(v) ‘d’ element will form a divalent anion because it has 6 valence electrons and it wants to 2 more electrons to completes its octet.

Q.32 What is the basis of classification of elements?

Ans- Elements are classified on the basis of similarities in their properties.

Q.33 How many periods are present in Modern periodic table?

Ans- Seven periods are present in the modern periodic table.

Q.34 Besides gallium, which other elements have been since discovered to fill the gaps left by Mendeleev in his periodic table? (Any two)

Ans- Eka-boron (Scandium) and Eka-silicon (Germanium)

Q.35 What is the fundamental difference in the electronic configuration of the group I and group II elements?

Ans- Group I elements have 1 electron in their outermost shell whereas group II elements have 2 electrons in their outermost shell.

Q.36 How many groups are in the long form of the periodic table?

Ans- There are 18 vertical columns called groups in the long form of periodic table.

Q.37 State the modern periodic law.

Ans- According to the modern periodic law the properties of elements are periodic function of their atomic numbers.

Q.38 What was the criteria used by Mendeleev in creating his periodic table?

Ans- Mendeleev used atomic mass of the elements as the criteria to create his periodic table.

Q.39 What was the main limitation of Dobereiner’s triad?

Ans- Dobereiner’s classification failed to arrange all the known elements in the form of triads of elements having similar chemical properties.

Q.40 Why do you think the noble gases are placed in a separate group?

Ans- Noble gases are placed in a separate group because all of them have completely filled shells. Therefore, these gases are inert in nature and do not generally participate in any chemical reaction.

Q.41 An element ‘X’ belongs to second period and second group of the periodic table.
Find:
(i) the number of valence electrons.
(ii) the valency of the element.
(iii) whether element X forms acidic or basic oxide.

Ans-

(i) Group number of element = Number of valence electrons
Therefore, number of valence electrons will be 2.
(ii) The valency of elements will be 2.
(iii) Nature of its oxide will be basic because it is a metal.

Q.42 Write the names of
(a) two elements that have seven electrons in their outermost shells
(b) two elements that have two electrons in their outermost shells
(c) two elements which have completely filled outermost shells

Ans-

(a) Chlorine (Cl) and Bromine (Br)
(b) Magnesium (Mg) and Calcium (Ca)
(c) Neon (Ne) and Argon (Ar)

Q.43 How many elements were known when Mendeleev classified the elements?

Ans- At the time of Mendeleev, 63 elements were known.

Q.44 Why are the elements in the same group of the periodic table show close resemblance in chemical behaviour?

Ans- The elements in the same group of the periodic table show close resemblance in their chemical behaviour as they have the same number of valence electrons.