JAC Class 10 Science Solutions Chapter 5 Periodic Classification of Elements

Jharkhand Board JAC Class 10 Science Solutions Chapter 5 Periodic Classification of Elements Textbook Exercise Questions and Answers.

JAC Board Class 10 Science Solutions Chapter 5 Periodic Classification of Elements

Jharkhand Board Class 10 Science Periodic Classification of Elements Textbook Questions and Answers

Question 1.
Which of the following statements 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:
The atoms lose their electrons more easily.

Question 2.
Element X forms a chloride with the formula 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
Answer:
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) Neon (2, 8)
(b) Magnesium (2, 8, 2)
(c) Silicon (2, 8, 4)
(d) Boron (2, 3)
(e) Carbon (2, 4)

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) In the modern periodic table, Boron is an element of group 13. Its valency is 3. Thus, all other elements of this group have valency 3.

(b) In the modern periodic table, fluorine is an element of group 17. All the elements of this group have 7 electrons in their valence shells. Therefore, the valency of all elements of this group is 1.

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? (Atomic numbers are given in parentheses.)
N (7) F (9) P (15) Ar (18)
Answer:
(a) The atomic number of this element is 17(2 + 8 + 7).
(b) F (9) [ ∵ Electronic configuration of F is 2, 7.]

Question 6.
The position of three elements A, B and C in the periodic table are shown below:

Group	Group 17
–	–
–	A
–	–
B	C

(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?
(d) Which type of ion, cation or anion, will be formed by element A?
Answer:
(a) Element A is an element of group 17. There are 7 electrons in their valence shell and thus by gaining one more electron it acquire a complete octet. Thus, an element A is a non-metal.

(b) On going down in a group, the atomic s size increases. Therefore, the force of attraction of the nucleus on the incoming electron decreases. As a result, reactivity decreases down the group. Since element C has larger atomic size than A, the element C is less reactive than the element A. In reference of forming positive ion, element C is more reactive than the element A.

(c) Elements B and C belong to the same period. On moving left to right in a period, atomic size (volume) decreases. Thus, the atomic size of C is smaller than B.

(d) Since element A has 7 electrons in the valence shell, it has a tendency to gain one electron to complete its octet. Thus, element A forms an anion.
A + \(\overline{\mathrm{e}}\) → 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 as follows:

Element	Atomic number	Electronic configuration
		K	L	M
Nitrogen (N)	7	2	5	–
Phosphorus (P)	15	2	8	5

Nitrogen is more electronegative than phosphorus because electronegative character decreases on moving down a group.

Question 8.
How does the electronic configuration of an atom relate to its position in the modern periodic table?
Answer:
In the periodic table, position of an element depends on its electronic configuration. The position of an element can be determined by knowing the number of valence electron in its electronic configuration.
For example,

Possesses one electron in its valence shell. Hence, it belongs to group 1.
→ The number of shells in the electronic configuration of an element determines its position in a period.
For example,

has three shells (K, L and M). So, it belongs to 3rd period of the periodic table.

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:

Element	Atomic number	Electronic configuration
		k	L	M	N	O
Calcium	20	2	8	8	2	–
Magnesium	12	2	8	2	–	–
Potassium	19	2	8	8	1	–
Scandium	21	2	8	8	3	–
Strontium	38	2	8	18	8	–

Elements with atomic number 12 and 38 have 2 electrons in their last shell like calcium. So, they will resemble Ca in their chemical properties.

Question 10.
Compare and contrast the arrangement of elements in Mendeleev’s periodic table and the modern periodic table.
Answer:

Mendeleev’s periodic table	Modern periodic table
1. Mendeleev’s periodic table consists of seven periods and eight groups.	1. Modern periodic table consists of seven periods and eighteen groups.
2. Transition elements are not separated in the Mendeleev’s periodic table.	2. Transition elements are placed in a separate groups in the modern periodic table.
3. In Mendeleev’s periodic table, elements are arranged in increasing order of their atomic masses.	3. In the modern periodic table, elements are arranged in increasing order of their atomic numbers.
4. Period number and group number of an element cannot be predicted.	4. Period number and group number of an element can be determined easily.
5. Mendeleev’s periodic table has descripancies and limitations.	5. Modern periodic table is almost errorless.
6. Periodicity in the properties of elements cannot be explained.	6. Periodicity in the properties of elements can be explained.

Jharkhand Board Class 10 Science Periodic Classification of Elements InText Questions and Answers

Question 1.
Did Dobereiner’s triads also exist in the columns of Newlands’ octaves? Compare and find out.
Answer:
Dobereiner’s triads also exist in the columns of Newlands’ octaves.

• Lithium, sodium and potassium forms a Dobereiner’s triad.
• Lithium, the first element of this triad is considered as the first element as Newlands’ octave, then the eighth element from it is sodium. These elements possesses similar properties according to the law of triads and the law of octaves.
• Similarly, if sodium is considered as the first element then the eighth element from it is potassium.  Moreover, sodium and potassium possess similar properties according to both the laws.
• Apart from these, some other elements beryllium (Be), magnesium (Mg) and calcium (Ca) obeys law of triads and the law of octaves.
• Thus, Dobereiner’s triads also exist in the columns of Newlands’ octaves.

Question 2.
What were the limitations of Dobereiner’s classification?
Answer:
The limitations of Dobereiner’s classification are as follows:

• All the elements known at that time could not be arranged as Dobereiner’s triad. Hence this method of classification of elements into triads was not found to be successful.
• Three elements nitrogen (N), phosphorus (P) and arsenic (As) were then known elements, but these elements could not be classified as Dobereiner’s triad.

Question 3.
What were the limitations of Newlands’ law of octaves?
Answer:

• Newlands’ law of octaves was applicable only to lighter elements having atomic masses up to 40 u.
  The law of octaves was applicable only s upto calcium, because after calcium every eighth element did not possess properties similar to that of the first element.
• Newlands assumed that only 56 elements s existed in nature and no new 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.
• In order to fit elements into his table, Newlands adjusted two elements in the same slot, but also put some unlike elements under the same column.

For example, cobalt (Co) and nickel S (Ni) are placed in the same slot and these are placed in the same column as fluorine, chlorine and bromine which have very different properties S than these elements. Iron, which resembles cobalt and nickel in properties, has been placed far away from these elements.

Question 4.
Use Mendeleev’s periodic table to predict the formulae for the oxides of the following elements:
K, C, Al, Si, Ba
Answer:

Element	Group number (Valency)	Molecular formula of oxide
K	1	K<sub>2</sub>O
C	4	CO2
Al	3	Al2O3
Si	4	SiO2
Ba	2	BaO

Question 5.
Besides gallium, which other elements have since been discovered that were left by Mendeleev in his periodic table? (any two)
Answer:
Besides gallium, Mendeleev had left gaps for germanium and scandium in his periodic table.

Question 6.
What were the criteria used by Mendeleev in creating his periodic table?
Answer:
The following criteria was used by Mendeleev in creating his periodic table :

• The properties of elements are the periodic function of their atomic masses.
• Elements with similar properties are arranged in the same group.
• The formula of oxides and hydrides formed by an element.

Question 7.
Why do you think the noble gases are placed in a separate group?
Answer:
Noble gases like helium (He), neon (Ne) and argon (Ar) are chemically very inert and are present in extremely low concentrations in our atmosphere. Hence they are placed in a separate s group.

Question 8.
How could the modern periodic table remove various anomalies of Mendeleev’s periodic table?
Answer:
The modern periodic table removed three main anomalies of Mendeleev’s periodic table as discussed below:
(1) Position of isotopes : All the isotopes of an element have the same atomic number. Therefore, they are placed at one place in the same group of the periodic table.

(2) Anomalous position of some pairs of elements: In the Mendeleev’s periodic table, elements with similar properties are placed in the same group. For example, cobalt (atomic mass 58.9 u) placed first and nickel (atomic mass 58.7 u) placed later while in the modern periodic table elements are arranged in increasing order of their atomic numbers, therefore, cobalt with atomic number 27 placed first and nickel with atomic number 28 placed later.

(3) Uncertainty in discovery of new elements: Since atomic masses do not increase in a regular manner in going from one element to the next, therefore, in Mendeleev’s periodic table, it was not possible to predict as to how many new elements could be discovered between two known elements.

Since, modern periodic table is formed on the basis of atomic numbers of elements, discovery of new element become easy.

Question 9.
Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Answer:
In the modern periodic table, elements having same number of electrons in the valence shell show similar chemical properties.
Magnesium has two electrons in the valence shell, hence all the elements such as beryllium (Be), calcium (Ca) and strontium (Sr) having two electrons in the valence shell show similar chemical properties.

Elements of group 2
Element	Atomic number	Electronic configuration
		K	L	M	N	O
Beryllium (Be)	4	2	2
Magnesium (Mg)	12	2	8	2
Calcium (Ca)	20	2	8	8	2
Strontium (Sr)	38	2	8	18	8	2

Question 10.
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 (Li), sodium (Na) and potassium (K)
(b) Magnesium (Mg) and calcium (Ca)
(c) Neon (Ne), argon (Ar) and krypton (Kr)

Question 11.
(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 are alkali metals which react with water to form metal hydroxides with the release of hydrogen gas.
2M + 2H₂O → 2MOH + H₂
Where, M = Li, Na and K
All these metals have one electron in their respective valence shells.

(b) Helium and neon are noble gases. Both the elements have their outermost shells completely filled. Helium has only K shell which is complete with 2 electrons while neon has two shells, K and L. Both these shells are complete, i.e., K s shell has 2 electrons and L shell has 8 electrons.

Question 12.
In the modern periodic table, which are the metals among the first ten elements?
Answer:
The first ten elements of the modern periodic table are as follows :
₁H, ₂He, ₃Li, ₄Be, ₅B, ₆C, ₇N, ₈O, ₉F and ₁₀Ne.
Among these elements Li and Be are metals.

Question 13.
By considering their position in the periodic table, which one of the following elements would you expect to have maximum metallic characteristics?
Ga Ge As Se Be
Answer:
Among the given elements, Be and Ga will show maximum metallic characteristics.
The arrangement of given elements in different groups and periods is as follows :

Activity 5.1 [T. B. Pg. 84]

Looking at its resemblance to alkali metals and the halogen family, try to assign hydrogen a : correct position in Mendeleev’s periodic table.
To which group and period should hydrogen be assigned ?

Discussion:

• Hydrogen is an element having lowest atomic number (Z = 1) and lowest atomic mass (1.008 u).
• The electronic configuration of hydrogen resembles with alkali metals.
• Like alkali metals, hydrogen combines with halogens, oxygen and sulphur to form compounds having similar molecular formulae. Hence, hydrogen can be placed along with alkali metals of group (IA).
• Hydrogen exists as a diatomic molecule like halogens and it combines with alkali metals to form ionic compounds and with non-metals to form covalent compounds.
• Thus, hydrogen should be placed along with halogens in group (VII).

Conclusion :
The position of hydrogen in the periodic table is controversial, however, it would be more appropriate to place it in group I and period I.

Activity 5.2 [T. B. Pg. 85]

• Consider the isotopes of chlorine, Cl-35 and Cl-37.
• There are two known isotopes of chlorine : Cl-35 and Cl-37.
• The atomic number of these two isotopes is 17, hence they have similar electronic configuration and chemical properties, but their atomic masses are different.

Questions :

Question 1.
Would you place them in different slots because their atomic masses are different?
Answer:
According to Mendeleev’s periodic table, “The properties of elements are the periodic function of their atomic masses.”

• According to Mendeleev, if these two isotopes are arranged in the order of increasing atomic masses, then their position should be before K(39.1u), but there is no vacant position available for Cl-37 in between Cl-35 and K(39.1u). Therefore they cannot be placed at the different position.

Question 2.
Would you place them in the same position because their chemical properties are the same?
Answer:
As they have similar chemical properties, they should be placed at the same position in group 17.

Activity 5.3 [T. B. Pg. 85]

Questions:

Question 1.
How were the positions of cobalt and nickel resolved in the modern periodic table?
Answer:

• In modern periodic table, elements are arranged in increasing order of their atomic numbers.
• The atomic number of cobalt and nickel are 27 and 28 respectively. Hence, on the basis of increasing order of their atomic numbers, cobalt is placed s in group 9 and nickel is placed in group 10.

Question 2.
How were the positions of isotopes of various elements decided in the modern periodic table?
Answer:
In the modern periodic table, positions of isotopes of different elements are not fixed separately. Since the various isotopes of an element have the same atomic number, they are assigned the same position in the modern periodic table.

Question 3.
Is it possible to have an element with atomic number 1.5 placed between hydrogen and helium?
Answer:

• The atomic number of an element is always definite and whole number.
• In the modern periodic table, elements are arranged in increasing order of their atomic numbers.
• The atomic number cannot be represented in fraction number. Thus, an element with atomic number 1.5 cannot be placed between hydrogen S and helium.

Question 4.
Where do you think should hydrogen be placed in the modern periodic table?
Answer:
Hydrogen should be placed in period and group 1 in the modern periodic table, $ since it has atomic number one.

Activity 5.4 [T. B. Pg. 87]

Questions:

Question 1.
Look at the group 1 of the modern periodic table and name the elements present in it.
Answer:
The names of the elements present in group 1 are: hydrogen (H), lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr).

Question 2.
Write down the electronic configuration of the first three elements of group 1.
Answer:
The electronic configuration of the first three elements are as follows :

Element	H	Li	Na
Shell	K	K, L	K, L, M
Electronic configuration	1	2, 1	2, 8, 1

Question 3.
What similarity do you find in their electronic configurations?
Answer:
These elements possesses same number of electrons in their valence shell.

Question 4.
How many valence electrons are present in these three elements?
Answer:
These three elements have one electron in their respective valence shell.

Activity 5.5 [T. B. Pg. 87]

Questions:

Question 1.
If you look at the modern periodic table, you will find that the elements Li, Be, B, C, N, O, F and Ne are present in the second period. Write down their electronic configurations.
Answer:

Element	3Li	4Be	5B	6C	7NN	8O	9F	10Ne
Shell	K, L	K, L	K, L	K, L	K, L	K, L	K, L	K, L
Electronic configuration	2, 1	2, 2	2, 3	2, 4	2, 5	2, 6	2, 7	2, 8

Question 2.
Do these elements also contain the same number of valence electrons?
Answer:
These elements do not contain the same number of valence electrons.

Question 3.
Do they contain the same number of shells?
Answer:
These elements contain the same number of shells (2, K and L).

Activity 5.6 [T. B. Pg. 88]

Questions :

Question 1.
How do you calculate the valency of an element from its electronic configuration?
Answer:
The valency of an element is determined by the number of valence electrons present in the outermost shell of the atom.
For elements of group 1, 2, 13 and 14, the valency is equal to the number of valence electrons and for elements of group 15, 16, 17 and 18, the valency is equal to 8 minus number of valence electrons.

Question 2.
What is the valency of magnesium with atomic number 12 and sulphur with atomic number 16?
Answer:
Electronic configuration of Mg with atomic number 12 is K L M 2 8 2
∴ Valency of Mg = 2
Electronic configuration of S with atomic number 16 is K L M 2 8 6
∴ Valency of S = 8 – 6 = 2

Question 3.
Find the valency of the first twenty elements.
Answer:

Element	Atomic number	Group number	Electronic configuration				Number of valence electrons	Valency of the element
			K	L	M	N
H	1	1	1				1	1
He	2	18	2				2	2-2 = 0
Li	3	1	2	1			1	1
Be	4	2	2	2			2	2
B	5	13	2	3			3	3
C	6	14	2	4			4	4
N	7	15	2	5			5	8-5 = 3
O	8	16	2	6			6	8-6 = 2
F	9	17	2	7			7	8-7=1
Ne	10	18	2	8			8	8-8 = 0
Na	11	1	2	8	1		1	1
Mg	12	2	2	8	2		2	2
Al	13	13	2	8	3		3	3
Si	14	14	2	8	4		4	4
P	15	15	2	8	5		5	8-5 = 3
S	16	16	2	8	6		6	8-6 = 2
Cl	17	17	2	8	7		7	8-7=1
Ar	18	18	2	8	8		8	8-8 = 0
K	19	1	2	8	8	1	1	1
Ca	20	2	2	8	8	2	2	2

Question 4.
How does the valency vary in a period on going from left to right?
Answer:
In a period, the valency first increases and then decreases. In a period, the valency first increases from 1 to 4 and then decreases from 4 to 0.

Question 5.
How does the valency vary in going down a group?
Answer:
On moving down the group, the valency does not change. It remains constant.

Activity 5.7 [T. B. Pg. 88]

Questions:

Question 1.
Atomic radii of the elements of the second period are given below:

Period II elements:	B	Be	O	N	Li	C
Atomic radius (pm):	88	111	66	74	152	77

Arrange them in decreasing order of their atomic radii.
Answer:
Decreasing order of the atomic radii:
Period II elements : Li > Be > B > C > N > O
Atomic radius (pm) : 152  111 88 77 74 66

Question 2.
Are the elements now arranged in the pattern of a period in the periodic table?
Answer:
Now, the above elements are arranged in the pattern of a period in the periodic table.

Question 3.
Which elements have the largest and the smallest atoms?
Answer:
Lithium (Li) has the largest and oxygen (O) has the smallest atoms.

Question 4.
How does the atomic radius change as you go from left to right in a period?
Answer:
The atomic radii decreases as we move from left to right in a period.

Activity 5.8 [T. B. Pg. 89]

Questions:

Question 1.
Study the variation in the atomic radii of ? first group elements given below and arrange them in an increasing order.

Group I elements :	Na	Li	Rb	Cs	K
Atomic radius (pm):	186	152	244	262	231

Answer:
Increasing order of the atomic radii:
Group I elements : Li < Na < K < Rb < Cs
Atomic radius (pm): 152 186 231 244 262

Question 2.
Name the elements which have the smallest and the largest atoms.
Answer:
Lithium (Li) has the smallest atoms while Cesium (Cs) has the largest atoms.

Question 3.
How does the atomic size vary as you go down a group ?
Answer:
As we go down a group, the atomic size increases gradually.

Activity 5.9 [T. B. Pg. 89 ]

Questions:

Question 1.
Examine elements of the third period and classify them as metals and non-metals.
Answer:
The elements of the third period :

Question 2.
On which side of the periodic table do you find the metals?
Answer:
Metals are present on the left side of the periodic table.

Question 3.
On which side of the periodic table do you find the non-metals?
Answer:
Non-metals are present on the right side of the periodic table.

Activity 5.10 [T. B. Pg. 89]

Questions:

Question 1.
How do you think the tendency to lose electrons changes in a group?
Answer:
As we move from top to bottom in a group, the tendency to lose electrons increases.

Question 2.
How will this tendency change in a period?
Answer:
As we move from left to right in a period, the tendency to lose electrons decreases.

Activity 5.11 [T.B.Pg. 90]

Questions:

Question 1.
How would the tendency to gain electrons change as you go from left to right across a period?
Answer:
As we go from left to right across a period, the tendency to gain electrons increases.

Question 2.
How would the tendency to gain electrons change as you go down a group?
Answer:
As we go down a group, the tendency to gain electrons decreases.