Science Chapter 2 - Periodic Classification Of Elements SSC, SCIENCE PART I, NEW SYLLABUS, FOR BOARD EXAM 2020,

Question 1:

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

Column 1	Column 2	Column 3
i. Triad ii. Octave iii. Atomic number iv. Period v. Nucleus vi. Electron	a. Lightest and negatively charged particle in all the atoms b. Concentrated mass and positive charge c. Average of the first and the third atomic mass d. Properties of the eighth element similar to the first e. Positive charge on the nucleus f. Sequential change in molecular formulae	1.Mendeleev 2. Thomson 3. Newlands 4. Rutherford 5. Dobereiner 6. Moseley

ANSWER:

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

Column 1	Column 2	Column 3
i. Triad ii. Octave iii.Atomic number iv. Period v. Nucleus vi. Electron	a. Average of the first and the third atomic mass b. Properties of the eighth element similar to the first c. Positive charge on the nucleus d. Sequential change in molecular formulae  e. Concentrated mass and positive charge f.  Lightest and negatively charged particle in all the atoms	1.  Dobereiner 2.  Newlands 3.  Mendeleev 4.  Moseley 5.  Rutherford 6.  Thomson

Question 2:

Choose the correct option and rewrite the statement.

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

(i) 1 (ii) 2 (iii) 3 (iv) 7

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

(i) Group 2  (ii) Group 16

(iii) Period 2 (iv) d-block

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.

(i) Na  (ii) Mg (iii) Al  (iv) Si

d. In which block of the modern periodic table are the nonmetals found?

(i) s-block   (ii) p-block

(iii) d-block (iv) f-block

ANSWER:

a. The number of electrons in the outermost shell of alkali metals is 1.

b. Alkaline earth metals have valency 2. This means that their position in the modern periodic table is in group2.

c. Molecular formula of the chloride of an element X is XCl. This compound is a solid having high melting point. An element to be present in the same group as X is Na.

d. In p-block of the modern periodic table are the nonmetals found.

Question 3:

An element has its electron configuration as 2, 8, 2. Now answer the following question.

a. What is the atomic number of this element?

b. What is the group of this element?

c. To which period does this element belong?

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:

An element has its electron configuration as 2, 8, 2.

a. The atomic number of this element is 12.

b. The group  number of this element is second.

c. This element belongs to third period.

d. This element resembles with Be(2).

Question 4:

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

a. 3Li, 14He, 11Na, 15P Which of these elements belong to be period 3?

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

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

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

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

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

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

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

ANSWER:

a. 3Li, 14He, 11Na, 15P

Electronic configuration of the following elements is:

3Li = 2,1

14He =2,8,4

11Na = 2,8,1

15P = 2,8,5

14He, 11Na, 15P belong to the third period because according to their electronic configuration, each element contains three shell i.e. K,L,M.

b. 1H, 7N, 20Ca, 16S, 4Be, 18Ar

Electronic configuration of the following elements is:

1H = 1

7N = 2,5

20Ca = 2,8,8,2

16S = 2,8,6

4Be = 2,2

18Ar = 2,8,8

20Ca, 4Be belong to second group because these elements have 2 electrons in its outermost shell.

c. 7N, 6C, 8O, 5B, 13Al

Electronic configuration of the following elements is:

7N =2,5

6C = 2,4

8O = 2,6

5B = 2,3

13Al = 2,8,3

8O is the most electronegative element among these because electronegativity increases as we move from left to right in a period of the periodic table.

d. 4Be, 6C, 8O, 5B, 13Al

Electronic configuration of the following elements is:

4Be = 2,2 

6C = 2,4 

8O = 2,6

5B = 2,3

13Al = 2,8,3

13Al is the most electropositive element among these because 4Be, 6C, 8O, 5B belong to same period, but 13Al belong to next period. According to the trend, electropositive character of an elements increases as we move from top to bottom in a group of the periodic table. This happens as the tendency of an atom to lose electrons increases due to decrease in nuclear charge and increase in numbers of shell.

e. 11Na, 15P, 17Cl, 14Si, 12Mg

Electronic configuration of the following elements is:

11Na = 2,8,1

15P = 2,8,5

17Cl = 2,8,7

14Si = 2,8,4

12Mg = 2,8,2

11Na has largest size among these because according to the trend, atomic radius decreases as we move from left to right in a period of the periodic table. The atomic number of elements increases which means the number of protons and electrons in  the atoms increases. Due to large positive charge on the nucleus, the electrons are pulled closer to the nucleus and the size of atom decreases.

f. 19K, 3Li, 11Na, 4Be

Electronic configuration of the following elements is:

19K = 2,8,8,1 

3Li = 2,1

11Na = 2,8,1

4Be = 2,2

4Be has the smallest atomic radius because 19K, 3Li, 11Na are present in the same group 1 but Be is present in group 2. According to the trend, as we move from left to right atomic size of an atoms decreases. Due to large positive charge on the nucleus, the electrons are pulled closer to the nucleus and the size of atom decreases.

g. 13Al, 14Si, 11Na, 12Mg, 16S

Electronic configuration of the following elements is:

3Al = 2,8,3

14Si = 2,8,4

11Na = 2,8,1

12Mg = 2,8,2

16S = 2,8,6

11Na has the highest metallic character because metallic character of an elements decreases as we move from left to right in a modern periodic table. This happens as the tendency of an atom to lose electrons decreases due to gradual increase in the number of protons and nuclear charge.

h. 6C, 3Li, 9F, 7N, 8O

Electronic configuration of the following elements is:

6C = 2,4

3Li = 2,1

9F = 2,7

7N =  2,5

8O = 2,6

9F has the highest nonmetallic character because  non-metallic character of an elements increases as we move from left to right in a period of the periodic table. This happens as the tendency of an atom to gain electrons increases due to increase in nuclear charge, the valence electrons are pulled in strongly by the nucleus and it becomes easier for an atom to gain electrons.

Question 5:

Write the name and symbol of the element from the description.

a. The atom having the smallest size.

b. The atom having the smallest atomic mass.

c. The most electronegative atom.

d. The noble gas with the smallest atomic radius.

e. The most reactive nonmetal.

ANSWER:

a. The atom having the smallest size = Hydrogen (H)

b. The atom having the smallest atomic mass = Hydrogen (H)

c. The most electronegative atom = Fluorine (F)

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

e. The most reactive nonmetal = Fluorine (F)

Question 6:

Write short notes.

a.  Mendeleev’s periodic law.

ANSWER:

According to Mendeleev’s Periodic Law, “Physical and chemical properties of elements are a periodic function of their atomic masses”.

Mendeleev classified elements according to their atomic masses and arranged these elements in increasing order of  their atomic masses.

Mendeleev classified periodic table into horizontal rows and vertical columns. The horizontal rows are called periods and vertical columns are called groups. Mendeleev’s Periodic Table contains seven horizontal rows and nine vertical columns.

The elements with similar properties comes into same group.

Mendeleev also left gaps in his periodic table for undiscovered elements like aluminum, silicon and Boron in his periodic table and named them Eka-Aluminium, Eka-silicon and Eka-Boron.

Mendeleev not only predicted the existence of Eka-Aluminium, Eka-silicon and Eka-Boron but also described the general physical properties of these elements.

These elements discovered later and named as Gallium, Germanium and Scandium.

Mendeleev's periodic table could predict the properties of several elements on the basis of their position in the periodic table.

Mendeleev's periodic table could accommodate noble gases when they were discovered.

Demerits of Mendleev's periodic table:

The position of isotopes could not explained.

Wrong order of atomic masses of some elements could not be explained.

Position of Hydrogen could not be assigned in a periodic table.

b.Structure of the modern periodic table:

ANSWER:

Periodic Table: It is the table of chemical elements arranged in order of atomic number such that elements with similar atomic structure appear in the vertical columns.

The Modern periodic law states that The chemical and physical properties of elements are periodic functions of their atomic numbers. Modern periodic table is based on modern periodic law. 

Main features:

Groups - There are 18 vertical columns in the periodic table. Each column is called a group. All elements in a group have similar chemical and physical properties because they have the same number of valence electrons.

Periods - In periodic table elements are arranged in a series of rows. Elements of the same period have the same number of electron shells.

Classification of elements:

Group 1 contains alkali metals (Li, Na, K, Rb, Cs and Fr).

The alkaline earth metals are metallic elements found in the group 2 of the periodic table. 

Elements present in groups 3 to 12 in the middle of the periodic table are called transition elements. In the transition elements, valence electrons are present in more than one shell.

Group 18 on extreme right side position contains noble gases ( He, Ne, Ar, Kr, Xe and Rn ). Their outermost shell contains 8 electrons except He as its outermost shell is K shell and it can hold only 2 electrons. 

Inner transition elements:

1. 14 elements with atomic numbers 58 to 71 (Ce to Lu) are called lanthanides  and they are placed along with the element lanthanum (La), atomic number 57 in the same position (group 3 in period 6) because of very close resemblance between them.

2. 14 elements with atomic numbers 90 to 103 (Th to Lr) are called actinides and they are placed along with the element actinium (Ac), atomic number 89 in the same position (group 3 in period 7) because of very close resemblance between them.

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

ANSWER:

Isotopes: Isotopes are atoms having the same atomic number but different atomic masses.

Therefore, according to Mendeleev’s classification these should be placed at different places depending upon their atomic masses.

For example, hydrogen isotopes with atomic masses 1,2 and 3 should be placed at three places. However, isotopes have not been given separate places in the periodic table because of their similar properties. So this was a drawback of Mendeleev's periodic table as he could not explain the position of isotopes.

Modern periodic table is based upon arrangement of the elements on the basis of their atomic number. So that, all the isotopes of hydrogen should be placed at the same place depending upon their atomic number.

Question 7:

Write scientific reasons.

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

ANSWER:

Atomic radius goes on decreasing while going from left to right in a period because the atomic number of the elements increases which means the number of protons and electrons in the atoms increases(the extra electrons being added to the same shell). Due to large positive charge on the nucleus, the electrons are pulled closer to the nucleus and the size of an atom decreases.

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

ANSWER:

Metallic character goes on decreasing while going from left to right in a period because the tendency of atoms of the elements to lose electrons(or gain  electrons) changes in a period. As we move from left to right in a period, the nuclear charge increases due to gradual increase in the number of protons. Due to the increase in nuclear charge, the valence electrons are pulled strongly by the nucleus and it becomes difficult for the atoms to lose electrons. Hence, metallic character decreases.

c.  Atomic radius goes on increasing down a group.

ANSWER:

Atomic radius increases as we move from top to bottom in a group of the periodic table because a new shell of electrons is added to the atoms at every step. As the number of shells in the atoms increases gradually due to which the size of atoms also increases. As the size of the atoms increases which leads to increase in atomic radius of an atom.

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

ANSWER:

Elements belong to the same group have the same valency because the number of valence electrons in a group is same due to which the tendency of an atom to lose or gain electrons in order to attain nearest noble gas configuration is also same.

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

ANSWER:

The third period contains only eight elements even through the electron capacity of the third shell is 18 because when the other shells get filled and the resultant no of electrons becomes eighteen, it gets added up and settles in the third electron shell and three shells is acquired by fourth period.

Question 8:

Write the names from the description.

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

b. The group with valency zero.

c. The family of nonmetals having valency one.

d. The family of metals having valency one.

e. The family of metals having valency two.

f. The metalloids in the second and third periods.

g. Nonmetals in the third period.

h. Two elements having valency 4.

ANSWER:

a. The period with electrons in the shells K, L and M = 3 period

b. The group with valency zero = 18 group

c. The family of nonmetals having valency one = Halogens

d. The family of metals having valency one = Alkali metals

e. The family of metals having valency two = Alkaline earth metals

f. The metalloids in the second and third periods = Boron( second period), Silicon (third period) 

g. Nonmetals in the third period = Sulphur, Chlorine

h. Two elements having valency 4 = Carbon, Silicon