1. Rearrange the columns 2 and 3 so as to match with the column 1.
Ans. 
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
Ans. 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 ………….
(i) Group 2 (ii) Group 16 (iii) Period 2 (iv) d-block
Ans. Alkaline earth metals have valency 2. This means that their position in the modern periodic table is in 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.
(i) Na (ii) Mg (iii) Al (iv) Si
Ans. Na
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
Ans. p-block
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?
Ans. The atomic number of this element is 12.
b. What is the group of this element?
Ans. As it has 2 valence electrons this element belongs to group 2.
c. To which period does this element belong?
Ans. As it has 3 shells, it 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)
Ans. Number of valence electrons of this element is 2 and also number of valence electron of Be is 2. So this element resembles to Be
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?
Ans.
Electronic configuration of Li is (2, 1)
Electronic configuration of Si is (2, 8, 4)
Electronic configuration of He is (2)
Electronic configuration of Na is (2, 8, 1)
Electronic configuration of P is (2, 8, 5)
As the elements Silicon (Si), Sodium (Na) and Phosphorous (P) have electrons in the three shells; K, L and M, therefore these elements belong to period 3 as the number of shells indicates the period number.
b. 1H, 7N, 20Ca, 16S, 4Be, 18Ar Which of these elements belong to the second group?
Ans.
Electronic configuration of H is (1)
Electronic configuration of N is (2, 5)
Electronic configuration of Ca is (2,8,8,2)
Electronic configuration of S is (2, 8, 6)
Electronic configuration of Be is (2, 2)
Electronic configuration of Ar is (2, 8, 8)
As the elements Calcium (Ca) and Beryllium (Be) have same number of valence electrons, i.e. 2, therefore
Calcium and Beryllium belong to the second group.
c. 7N, 6C, 8O, 5B, 13A1 Which is the most electronegative element among these?
Ans.
Electronic configuration of N is (2, 5)
Electronic configuration of C is (2, 4)
Electronic configuration of O is (2, 6)
Electronic configuration of B is (2, 3)
Electronic configuration of Al is (2, 8, 3)
Oxygen is the most electronegative element, since it is present at the extreme right amongst the given elements of period 2. Electronegativity increases moving across a period from left to right.
| d. 4Be, 6C, 8O, 5B, 13A1 Which is the most electropositive element among these? |
Ans.
Electronic configuration of Be is (2, 2)
Electronic configuration of C is (2, 4)
Electronic configuration of O is (2, 6)
Electronic configuration of B is (2, 3)
Electronic configuration of Al is (2, 8, 3)
Electropositivity decreases moving across a period from left to right and increases as we move from top to bottom in a group. Hence Al is the most electropositive element among these.
e. 11Na, 15P, 17C1, 14Si, 12Mg Which of these has largest atoms?
Ans.
Electronic configuration of Na is (2, 8, 1)
Electronic configuration of P is (2, 8, 5)
Electronic configuration of Cl is (2, 8, 7)
Electronic configuration of Si is (2, 8, 4)
Electronic configuration of Mg is (2, 8, 2)
Atomic radius goes on decreasing along a period. Hence Na has largest atom.
f. 19K, 3Li, 11Na, 4Be Which of these atoms has smallest atomic radius?
Ans.
Electronic configuration of K is (2, 8, 8, 1)
Electronic configuration of Li is (2, 1)
Electronic configuration of Na is (2, 8, 1)
Electronic configuration of Be is (2, 2)
Atomic radius goes on decreasing along a period and it goes on increasing along a group. Hence Be has smallest radius.
g. 13A1, 14Si, 11Na, 12Mg, 16S Which of the above elements has the highest metallic character?
Ans.
Electronic configuration of Al is (2, 8, 3)
Electronic configuration of Si is (2, 8, 4)
Electronic configuration of Na is (2, 8, 1)
Electronic configuration of Mg is (2, 8, 2)
Electronic configuration of S is (2, 8, 6)
Metallic character goes on decreasing as we move along a period from left to right. Hence Na has the highest tendency to loose electrons.
h. 6C, 3Li, 9F, 7N, 8O Which of the above elements has the highest nonmetallic character?
Ans.
Electronic configuration of C is (2, 4)
Electronic configuration of Li is (2, 1)
Electronic configuration of F is (2, 7)
Electronic configuration of N is (2, 5)
Electronic configuration of O is (2, 6)
Nonmetallic character goes on increasing as we move along a period from left to right. Hence F has the highest non metallic character.
5. Write the name and symbol of the element from the description.
a. The atom having the smallest size.
Ans.
Helium (He)
b. The atom having the smallest atomic mass.
Ans.
Hydrogen (H)
c. The most electronegative atom.
Ans.
Fluorine (F)
d. The noble gas with the smallest atomic radius.
Ans.
Helium (He)
e. The most reactive nonmetal.
Ans.
Fluorine (F)
6. Write short notes.
a. Mendeleev’s periodic law.
Ans.
Mendeleev, stated the following periodic law “Properties of elements are periodic function of their atomic masses”. He arranged the elements in an increasing order of their atomic masses. He organized the periodic table on the basis of the chemical and physical properties of the elements. He found that the elements with similar physical and chemical properties repeat after a definite interval. In the Mendeleev’s periodic table, vertical columns are called groups and horizontal rows are called periods.
b. Structure of the modern periodic table.
Ans.
(i) The Modern Periodic table contains seven horizontal rows called the periods 1 to 7. (ii) Similarly, the eighteen vertical columns are the groups 1 to 18. (iii) The arrangement of the periods and groups results into formation of boxes. (iv) Atomic numbers are serially indicated in the upper part of these boxes. (v) Apart from these seven rows, two rows are shown separately at the bottom of the periodic table.(vi) These are called Lanthanide series and Actinide series respectively (vii) There are 118 boxes in the periodic table including the two series. (viii) The entire periodic table is divided into four blocks, viz, s-block, p-block, d-block and f-block. (ix) A zig-zag line can be drawn in the p-block of the periodic table. (x) The metalloid elements lie along the border of this zig-zag line. (xi) All the metals lie on the left side of the zig-zag line, while all the non-metals lie on the right side.
c. Position of isotopes in the Mendeleev’s and the modern periodic table.
Ans.
(i) Isotopes are the atoms with the same atomic number, but differ in atomic mass. (ii) Mendeleev arranged the elements in the order of increasing atomic mass. (iii) As isotopes have the same chemical properties but different atomic masses, a challenge was posed in placing them in Mendeleev’s periodic table. (iv) Based on chemical properties, they were to be placed in same group, while based on atomic masses they were to be placed in different groups. Hence, there was no definite place for isotopes in Mendeleev’s periodic table. (v) However in modern periodic table elements are arranged in the order of increasing atomic number. (vi) As a result isotopes in the modern periodic table are placed in the same box as they have same atomic number.
7. Write scientific reasons.
a. Atomic radius goes on decreasing while going from left to right in a period.
Ans.
(i) While going from left to right within a period, the atomic number increases one by one, meaning the positive charge on the nucleus increases by one unit at a time. (ii) However, the additional electron gets added to the same outermost shell. (iii) Due to the increased nuclear charge the electrons are pulled towards the nucleus to a greater extent and thereby the size of the atom decreases. (iv) Hence, atomic radius goes on decreasing while going from left to right in a period.
b. Metallic character goes on decreasing while going from left to right in a period.
Ans.
(i) The metallic character of an atom is its tendency to lose electrons. (ii) While going from left to right within a period the outermost shell remains the same. 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. (iii) As a result of this the tendency of atom to lose valence electrons decreases within a period from left to right. (iv) So, metallic character goes on decreasing while going from left to right in a period.
c. Atomic radius goes on increasing down a group.
Ans.
(i) While going down a group a new shell is added. (ii) Therefore the distance between the outermost electron and the nucleus goes on increasing. (iii) As a result of this the atomic size increases in spite of the increased nuclear charge. (iv) And so, the atomic radius goes on increasing down a group.
d. Elements belonging to the same group have the same valency.
Ans.
(i) Valency of an element is determined by the number of electrons present in the outermost shell of its atoms, that is, the valence electrons. (ii) Elements in the same group have same number of valence electrons, irrespective of the number of orbits. (iii) Hence, elements in the same group have the same valency.
e. The third period contains only eight elements even through the electron capacity of the third shell is 18 .
Ans.
(i) Electron capacity of K, L, M shells are: 2, 8, 18. (ii) But it has been observed that the third shell contains only 8 electrons due to the law of electron octet in which every atom aims to have eight electrons in its outer shell in order to get the nearest noble gas configuration, any extra electrons that come go to the next shell. (iii) Thus, The third period contains only eight elements even though the electron capacity of the third shell is 18.
8. Write the names from the description.
a. The period with electrons in the shells K, L and M.
Ans.
Period 3
b. The group with valency zero.
Ans.
Group 18
c. The family of nonmetals having valency one.
Ans.
Halogens – Fluorine, Chlorine, Bromine, Iodine.
d. The family of metals having valency one.
Ans.
Alkali metals – Lithium, Sodium, Potassium, Rubidium, Cesium, Francium
e. The family of metals having valency two.
Ans.
Alkaline Earth metals – Beryllium, magnesium, Calcium, Strontium, Barium, Radon.
f. The metalloids in the second and third periods.
Ans.
Boron and Silicon
g. Nonmetals in the third period.
Ans.
Phosphorus, Sulphur, Chlorine and Argon.
h. Two elements having valency 4.
Ans.
Ans. Carbon, Silicon.