Science and Technology Part I Class 10 Chapter 2 Question Answers Maharashtra Board

Periodic Classification of Element

Solutions

1. Rearrange the columns 2 and 3 so as to match with the column 1.

Answer:

Question 2.
Choose the correct option and rewrite the statement:

(a) The number of electrons in the outermost shell of alkali metals is…….

(a) 1
(b) 2
(c) 3
(d) 7

Answer:
(a) 1

(b) Alkaline earth metals have valency 2. This means that their position in the modern periodic table is in…….

(a) Group 2
(b) Group 16
(c) Period 2
(d) d-block

Answer:
(a) Group 2

(c) Molecular formula of the chloride of an element X is XCl. This compound is a solid having high melting point. which of the following elements be present in the same group as X.

(a) Na
(b) Mg
(c) Al
(d) Si

Answer:
(a) Na

(d) In which block of the modem periodic table are the nonmetals found?

(a) s-block
(b) p-block
(c) d-block
(d) f-block

Answer:
(b) p-block

Question 3.
An element has its electron configuration as 2, 8, 2. Now answer the following questions.

a. What is the atomic number of this element?

Answer: The atomic number of this element is 12.

b. What is the group of this element?

Answer: The group of this element is 2.

c. To which period does this element belong?

Answer: This element belongs to period 3.

d. With which of the following elements would this element resemble? (Atomic numbers are given in the brackets)
N(7), Be(4), Ar(18), Cl(17)

Answer: This element resembles Be(4).

Question 4.
Write down the electronic configuration of the following elements from the given atomic numbers. Answer the following question with explanation.

a. 

₃Li, ₁₄Si, ₂He, ₁₁Na, ₁₅P which of these elements belong to be period 3?

Answer:

Elements belong to the 3rd period: ₁₄Si, ₁₁Na and ₁₅P.

b. 

₁H, ₇N, ₂₀Ca, ₁₆S, ₄Be, ₁₈Ar. Which of these elements belong to the second group?

Answer:

Elements belongs to the 2nd group: ₄Be and ₂₀Ca.

c. 

₇N, ₆C, ₈O, ₅B, ₁₃Al Which is the most electronegative element among these?

Answer:

Among these, ₈O is the most electronegative element.

d. 

₄Be, ₆C, ₈O, ₅B, ₁₃Al Which is the most electropositive element among these?

Answer:

Among these, ₁₃Al is the most electropositive element.

e. 

₁₁Na, ₁₅P, ₁₇Cl, ₁₄Si, ₁₂Mg which of these has largest atoms?

Answer:

₁₁Na has the largest atomic size.

f.

₁₉K, ₃li, ₁₁Na, ₄Be Which of these atoms has smallest atomic radius?

Answer:

₄Be has smallest atomic radius.

g. 

₁₃Al, ₁₄Si, ₁₁Na, ₁₂Mg, ₁₆S Which of the above elements has the highest metallic character?

Answer:

₁₁Na has the highest metallic character.

h. 

₆C, ₃Li, ₉F, ₇N, ₈O Which of the above elements has the highest nonmetallic character?

Answer:

₉F has the highest non metallic character.

Question 5.
write the name and symbol of the element from the description.

a. The atom having the smallest size.

Answer: Helium(He).

b. The atom having the smallest atomic mass.
Answer: Hydrogen(H₂).

c. The most electronegative atom.
Answer: Fluorine(F₂).

d. The noble gas with the smallest atomic radius.
Answer: Helium(He).

e. The most reactive nonmetal.
Answer: Fluorine(F₂).

Question 6. Write short notes.

a. Mendeleev’s periodic law.

Answer:

when the elements are arranged in the order of their increasing atomic masses, Mendeleev found that the elements with similar physical and chemical properties repeat after a definite interval. On the basis of these finding Mendeleev stated the periodic law. The physical and chemical properties of elements are a periodic function of their atomic masses.

b. Structure of the modern periodic table.

Answer:

(1) In the modern periodic table, the elements are arranged in the order of their increasing atomic number. In the modern periodic table there are seven horizontal rows called periods and eighteen vertical columns (1 to 18) called groups. The arrangement or the periods and groups results into formation of boxes. Atomic numbers are serially indicated in the upper part of these boxes.

(2) Each box represents the place for one element. Apart from these seven rows, there are two rows of elements placed separately at the bottom of the periodic table. They are lanthanides and actinides series. There are 118 boxes in the periodic table including the two series that means there are 118 places for elements in the modern periodic table.

The formation of a few elements was established experimentally very recently and thereby the modern periodic table is now completely filled with 118 elements.

(3) On the basis of the electronic configuration, the elements in the modern periodic table are divided into four blocks, viz. s-block, p-block, d-block and f-block. The s-block constitute groups 1 and 2. The groups 13 to 18 constitute the p-block. Groups 3 to 12 constitute the d-block, while the lanthanide and actinide series at the bottom form the f-block. The d-bloclçelements are called transition elements.

A zig-zag line shown in the p-block of the periodic table. This zig-zag line shows the three traditional types of elements, i.e. metals, nonmetals and metalloids. The metalloid elements lie along the border of zig-zag line. All the metals lie on the left side of the zig-zag line while all the nonmetals lie on the right side.

c. Position of isotopes in the Mendeleev’s and the modern periodic table.

Answer:

Isotopes were discovered long time after Mendeleev put forth the periodic table. A challenge was posed in placing isotopes in Mendeleev’s periodic table, as isotopes have the same chemical properties but different atomic masses. Isotopes do not find separate places in this table.

Moseley found out that atomic number is a fundamental property of an element rather than its atomic mass. The atomic number of any element is increased by one unit (number) from the atomic number or subsequent element. In the modern periodic table, the elements are arranged in the order of their increasing atomic numbers, that time the problem or discrepancy in the pairs or isotopes of elements observed in Mendeleev’s periodic table was solved. The isotopes or ₁₇Cl³⁵ and ₁₇Cl³⁷ were placed in the same group as both have the same atomic number.

Question 7. Write scientific reasons.

a. Atomic radius goes on decreasing while going from left to right in a period.

Answer:

(1) In a period while going from left to right, atomic radius goes on decreasing and the atomic number increases one by one, that means positive charge on the nucleus increases by one unit at a time.

(2) However, the additional electron is added to the same outermost shell. Due to the increased nuclear charge the electrons are pulled towards the nucleus to a greater extent, as a result the size of atom decreases i.e., atomic radius decreases.

b. Metallic character goes on decreasing while going from left to right in a period.

Answer:

(1) Metals have a tendency to lose the valence electrons to form cations. This tendency of an element is called the metallic character of the element.

(2) while going from left to right within a period the outermost shell remains the same and electrons are added to the same shell. However, the positive charge on the nucleus goes on Increasing while the atomic radius goes on decreasing and thus the effective nuclear charge goes on increasing. As a result of this the tendency of atom to lose electrons decreases, i.e., electropositivity decreases. Thus, metallic character goes on decreasing within a period from left to right.

c. Atomic radius goes on increasing down a group.

Answer:

The size of an atom is indicated by its radius. while going down a group a new shell is added. Therefore, the distance between the outermost electron and the nucleus goes on increasing. These electrons experience lesser pull from the nucleus. Thus, atomic radius goes on increasing down a group.

d. Elements belonging to the same group have the same valency.

Answer:

(1) The valency of an element is determined by the number of valence electron in the outermost shell of an atom of an element.

(2) All the elements in a group have the same number of valence electrons. Therefore, elements in the same group should have the same valency. For example, the elements of group I contain only one valence electron: the valency of elements of group I is one. Similarly for group II, the valency is two.

e. The third period contains only eight elements even through the electron capacity of the third shell is 18.

Answer:

(1) In the modern periodic table, there are seven horizontal rows called periods. In a periods elements are arranged in an increasing order of their atomic numbers. The third row contains 8 elements and the electron capacity of the third shell is 18.

(2) In the third period, while moving from left to right, atomic number increases, number of electrons increases in the shell. The number of elements present in 3^rd period is decided on the basis of electronic configuration and octet rule.

Argon (Ar) is the last element of the third period and has a capacity of maximum 18 electrons. Its octet of electrons is completed and as argon belongs to zero group, the third shell contains 18 electrons.

Question 8. Write the names from the description.

a. The period with electrons in the shells, K, L and M.

Answer: Third period.

b. The group with valency zero.

Answer: Group 18.

c. The family of nonmetals having valency one.

Answer: Halogen family.

d. The family of metals having valency one.

Answer: Group 1.

e. The family of metals having valency two.

Answer: Group 2.

f. The metalloids in the second and third periods.

Answer: Boron, Silicon.

g. Nonmetals in the third period.

Answer: Phosphorous, sulfur and chlorine and argon.

h. Two elements having valency 4.

Answer: Carbon, silicon.