RBSE Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements

Rajasthan Board RBSE Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements Textbook Exercise Questions and Answers.

RBSE Class 10 Science Solutions Chapter 5 Periodic Classification of Elements

RBSE Class 10 Science Chapter 5 Periodic Classification of Elements InText Questions and Answers

Page 81.

Question 1.
Did Dobereiner’s triads also exist in the columns of Newland’s Octaves? Compare and find out.
Answer:
Yes, Dobereiner’s triads also exist in the columns of Newland’s octaves.
Example: lithium (Li), sodium, (Na) and potassium (K) which are present in the second column (‘re’ column) of Newland’s octave and this is also the example of Dobereiner’s triads. If we consider lithium as the first element, then the eighth element from it is sodium and if sodium is considered as first element then the eighth element from it is potassium.

Question 2.
What were the limitations of Dobereiner’s classification?
Answer:
Dobereiner’s classification of triads was applicable only for few elements. Dobereiner could identify only three triads from the elements known at that time so, this system of classification into triads was not found to be succesful.

Question 3.
What were the limitations of Newland’s Law of Octaves?
Answer:
Newlands Law of Octave’s had the following limitations.
(1) The 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) It was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future. But, later on, several new elements were discovered, whose properties did not fit into the Law of Octaves.

(3) In order to fit elements into his Table. Newlands put two elements in the same slot, but also put some unlike elements under the same note. For example cobalt and nickel are placed in the same slot and these are placed in the same column (Sa, column) with fluorine, chlorine and bromine which have very different properties than cobalt and nickel. Iron, which resembles cobalt and nickel in properties, has been placed far away from these elements.

(4) Newlands’Law of Octaves worked well with lighter elements only.

(5) With the discovery of noble gases, the Law of Octaves become irrelevant.

Page 85.

Question 1.
Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements : K, C, Al, Si, Ba.
Answer:
(i) K (Potassium) belongs to group IA of Mendeleev’s periodic table. Hence, its valency is 1. Similarly oxygen belongs to group VI A, Hence its valency is 2. Thus, the formula of its oxide is K₂O.

(ii) C (Carbon) belongs to group IV A of Mendeleev’s periodic table. Hence, its valency is 4. Thus, the formula of its oxide is CO₂

(iii) Al (Aluminium) belongs to group III A of Mendeleev’s periodic table. Hence, its valency is 3. Thus, the formula of its oxide is A₂O₃.

(iv) Si (Silicon) belongs to group IV A of Mendeleev’s periodic table. Hence, its valency is 4. Thus, the formula of its oxide is SiO₂.

(v) Ba (Barium) belongs to group II A of Mendeleev’s periodic table. Hence, its valency is 2. Thus, the formula of its oxide is BaO.

Question 2.
Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table (any two)?
Answer:
Besides gallium, scandium and germanium have since been discovered that were left by Mendeleev in his periodic table. The properties of scandium, gallium and germanium were similar to Ma – boron, Eka – aluminium and silicon respectively.

Question 3.
What were the criteria used by Mendeleev in creating his periodic table?
Answer:
The criteria were used by Mendeleev increating his periodic table was the relationship between the atomic masses of the elements and their physical properties and chemical properties. Among chemical properties he concentrated on the compounds formed by elements with oxygen and hydrogen.

Question 4.
Why do you think the noble gases are placed in a separate group?
Answer:
The noble gases like helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe) were placed in a separate group called zero group because these are most unreactive so they do not react with other elements. These gases were discovered very late because they are very inert and present in extremely low concentrations in our atmosphere.

Page 90.

Question 1.
How could the Modem Periodic Table remove various anomalies of Mendeleev’s Periodic Table?
Answer:
Atomic number was adopted as the basis of Modem Periodic Table. With the help of this table the following anomalies of Mendeleev’s Periodic Table were removed:

(i) Position of Hydrogen:
In the Modem Periodic Table, hydrogen is placed in the first group on the basis of atomic number, whereas it could not be assigned a fixed position in the Mendeleev’s Table.

(ii) Position of Isotopes:
In the modem Periodic Table same position was given to all the Isotopes of an element because the atomic number of these are same.

(iii) Putting Heavy Elements before Highter Elements:
The place of heavy elements which comes before lighter elements on the basis of atomic weight, is proper on the basis of atomic number. Like the atomic number of Argon (Ar) and Potassium are 18 and 19 while atomic weights are 39.84 and 39.1. Argon comes first when elements are arranged on the basis of increasing atomic numbers of elements while the atomic weights are just opposite to it. This anomaly is removed in the modem Periodic Table.

(iv) Position of Rare Earth Elements:
All the rare earth elements are placed on one place in the Periodic Table because the number of electrons in the outermost shell of these are same.

Question 2.
Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Answer:
Since magnesium has two electrons in the valence shell, thus, all the elements having two electrons in their valence shell will show similar properties Because according to Modem Perodic Table the elements which have same outermost electronic configuration have same property.

Element (GrOUp 2)	Atomic number	Electronic Configuration
		K	L	M	N
Beryllium (Be)	4	2	2
Magncsium(Mg)	12	2	8	2
Calicum (ca)	20	2	8	8	2

Hence, the two elements which show chemical reactions similar to magnesium are beryllium (Be) and Calcium (Ca) and the basis of our choice is electronic configuration.

Question 3.
Name (a) Three elements that have a single electron in their outermost shell.
(b) Two elements that have two electrons in their outermost shells.
(c) Three elements with filled outer-most shells.
Answer:
(a) The alkali metals (group I) such as lithium (Li), Sodium (Na), Potassium (K) etc. have one electron in their outermost shell as shown below:

Element (Group I)	Atomic number	Electronic Configuration
		K	L	M	N
Lithium (Li)	3	2	1
Sodium (Na)	11	2	8	1
Potassium (K)	19	2	8	8	1

(b) The alkaline earth metals (group 2) such as Magnesium and Calcium have two electrons in their outermost shell as shown below

Element Group II	Atomic number	Electronic Configuration
		K	L	M	N
Magnesium (Mg)	12	2	8	2
Calicium (Ca)	20	2	8	8	2

(c) The noble gases (group 18) such as Helium, Neon, Aigon etc., have completely filled outermost shells as shown below:

Element Group of 18	Atomic number	Electronic Configuration
		K	L	M
Helium (He)	2	2
Neon (Ne)	10	2	8
Agron (Ar)	18	2	8	8

Question 4.
(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) Atoms of lithium, sodium and potassium have the tendency to lose electron so these gives free H? gas by the reduction of hydrogen present in water.

The similarity in the atoms of these elements is that all those metals have one electron in their respective valence shell (outermost shell).

(b) Helium and neon are noble gases (Inert gases) and thus, have extremely low reactivity. The common thing in the atoms these gases is that they have completely filled outermost shell, (valence shells)

Question 5.
In the Modem Periodic Table, which are the metals among the first ten elements?
Answer:
The first ten elements in the Modem Periodic Table are : H, He, Li, Be, B, C, N, O, F and Ne. Among these elements i.e., Li and Be are metals and the rest are all non-metals.

Element	Atomic number	Electronic Configuration
		K	L	M
N	7	2	5
F	9	2	7
P	15	2	8	5
Ar	18	2	8	8

Question 6.
By considering their position in the Periodic Table, which one of the following elements would you expect to have the maximum metallic characteristic? Ga, Ge, As, Se, Be.
Answer:
Let us arrange the given elements in different groups and periods according to their increasing atomic numbers. Thus, we have

Periods	Group 2	Group 13	Group 14	Group 15	Group 16
Second	Be	–	–	–	–
Third	–	–	–	–	–
Fourth	–	Ga	Ge	As	Se

In the Periodic Table metallic character decreases on moving from left to right in a period and increases on moving down a group. Thus, out of the above elements, Ga is expected to have maximum metallic characteristic on the basis of their position in the Periodic Table.

RBSE Class 10 Science Chapter 5 Periodic Classification of Elements Textbook Questions and Answers

Question 1.
Which of the following statement is not a correct statement about the trends when 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:
(c) The atoms lose their electrons more easily.

Question 2.
Element X forms a chloride with the formula XC₁₂, 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.
Answer:
(b) Mg

Question 3.
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) All noble gases (group 18) have their respective outermost shells completely filled with electrons, thus, the given element should be a noble gas. As this element has two completed shells hence, it belongs to second period so, the element is neon (Ne) (2, 8)

(b) The element with electronic confi-guration 2, 8, 2 is magnesium (Mg) which have atomic number 12.

(c) As the given element has three shells with four electrons in the valence shell, thus, the electronic configuration of the element is 2, 8, 4. Hence, the element is silicon (Si) with atomic number 14.

(d) As the given element has two shells with three electrons in the outermost shell, thus, the electronic configuration of the element is 2, 3. Hence, the element must be boron (B) with atomic number 5.

(e) It is given that the element has two shells. But in the first shell, the maximum number of electrons is 2. Since the number of electrons in the second shell, is twice the electrons in the first shell, thus, the number of electrons in the second shell, is four so the electronic configuration of the element is 2, 4. Hence, the element must be carbon (C) with atomic number 6.

Question 4.
(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) (i) All the elements which lie in the same column as that of boron belong to group 13 and have same outermost electronic configuration. Thus they have three electrons in their outermost shells.

(ii) All the elements of Boron column shows +3 oxidation state (valence).

(b) The elements which lie in the same column as that of fluorine are known as halogens. They are of group 17 and have same outermost electronic configuratioa Thus, they have seven electrons in their outermost shell. Hence, their valency is one. All these elements (F, Cl, Br, I) are non-metals and forms diatomic molecules.

Question 5.
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?

Element	Atomic number	Electronic Configuration
		K	L	M
N	7	2	5
F	9	2	7
P	15	2	8	5
Ar	18	2	8	8

Out of the above elements, F with atomic number 9 has 7 electrons in its outermost shells just like that of the given element. Thus, the given element will be chemically similar to fluorine (F). Also, both the elements belong to same group i.e., group 17.

Question 6.
The position of three elements A, B and C in the Periodic Table are shown below

Group 16	Group 17
–	–
–	A
–	–
B	C

(a) State whether A is a metal or a 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?
(d) Which type of ion, cation or anion, will be formed by element A?
Answer:
On the basis of group 16 and group 17 given above it. Proves at:
(a) Element A is a non metal because metallic character decreases on moving left to right in a period and the elements of group 17 have 7 electrons in their outermost shells. Hence, they have a high tendency to gain one electron to complete their respective octets.

(b) Element C is less reactive than element A because these elements belongs to halogen family (non-metals) and reactivity. Decreases on moving top to bottom in the group of non metals.

(c) Elements B and C belongs to the same period and on moving left to right in a period atomic size decreases so the size of C is smaller than B. .

(d) Since the element A has 7 (group 17) electrons in its valence shell, it has a strong tendency to gain one electron to complete its octet. Hence, element A forms an anion. A + e → A

Question 7.
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:
The electronic configuration of nitrogen and phosphorus are:

Element	Atomic number	Electronic Configuration
		K	L	M
N	7	2	5
P	15	2	8	5

Nitrogen is more electronegative than phosphorous because electronegativity decreases in a group on moving top to bottom and nitrogen is present on the upper side than phosphorous in the group. Down the group, the effective nuclear charge experienced by valence electrons is decreasing because the outermost electrons are further away from the nucleus. Therefore, these can be lost easily. And thus thus the tendency to attract electrons decreases and hence decreasing the electronegativity.

Question 8.
How does the electronic configuration of an atom relate to its position in the Modem Periodic Table?
Answer:
Modem Periodic Table is based on atomic numbers and the electronic configuration of elements is written on the

Element	Atomic number	Electronic Configuration
		K	L	M	N	O
12	M	2	8	2
19	K	2	8	8	1
20	Ca	2	8	8	2
21	Sc	2	8	9	2
38	Sr	2	8	18	8	2

basis of atomic number. The chemical properties of elements depends on electronic configuration so the elements having same number of electrons in the outermost shell belongs to same group. Further each period starts with filling of a new shell and the number of outermost shell electrons increases by one unit as the atomic number increases by one unit on moving from left to right in a period. So the position of an element in the Modem Periodic Table depends on electronic configuration.

Question 9.
In the Modern Periodic Table, calcium (Atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?
Answer:
The electronic configuration of given elements are as under:
It is clear that the outermost shell electronic configuration of elements with atomic number 12 and 38 is similar to calcium (atomic number 20) and two electrons are present in the outermost shell of these elements, so physical and chemical properties of these are similar to calcium.

Question 10.
Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and the Modern Periodic Table.
Answer:
The comparision of arrangements of elements in Mendeleev’s Periodic Table Modern Periodic Table:
(i) The basis of classification of elements in the Mendeleev’s Periodic Table was atomic mass. While in Modem Periodic Table elements are classified on the basis of atomic number.

(ii) According to Mendeleev the property of elements are the periodic function of their atomic masses while according to Modem Periodic Law the property of elements are the periodic function of their atomic numbers.

(iii) There are only eight groups (vertical columns) in Mendeleev’s Periodic Table while. Eighteen columns (groups) are present in Modem Periodic Table.

(iv) According to Mendeleev, when elements are arranged in the order of their increasing atomic masses then the elements of similar physical and chemical properties comes again means they belongs to same group but in the Modem Periodic Table when elements are arranged in. the increasing order of their atomic numbers then elements of similar properties comes in the same group.

(v) Mendeleev’s left some gaps for the elements in his Periodic Table, which, were not known at that time but now in the Modem Periodic Table all the elements are arranged properly.

(vi) At the time of Mendeleev’s Periodic Table 63 elements were known but in the Modem Periodic Table 118 elements are known.

(vii) In the Mendeleev’s Periodic Table the position of cobalt and Nickel and Telurium and Iodine was wrong but in the Modem Periodic Table they are properly arranged according to rules. But the Modem Periodic Table is more suitable than’ Mendeleev’s Periodic Table because this is based on atomic number which is more basic property of an atom. On the basis of atomic number the change in the property of elements in groups and periods can easily be explained.