CBSE Board Questions Ch- 5 Periodic Classification of Elements

Question.  Which physical and chemical properties of the elements were used by Mendeleev in creating his periodic table? List two observations which posed a challenge of Mendeleev’s periodic law. (C.B.S.E 2009)

Ans. The creation of Mendeleev’s periodic table was based upon certain physical and chemical properties.

Physical properties:
The atomic mass of the elements was taken into account and the elements were
arranged in order of increasing atomic masses. The influences of their physical properties such as melting
points, boiling points, density etc.

Chemical properties: 
The distribution of the elements into different groups was linked with formation of hydrides by combining with hydrogen and formation of oxides by combining with oxygen. This is linked with
the valency of the elements.

The two main observations which posed challenge to Mendeleev’s periodic table are as follows.

(i) Position of isotopes: Since the isotopes of an element differ in their atomic masses, they must be assigned
separate slots or positions in the periodic table.
(ii) Anomalous positions of some elements: In the Mendeleev’s periodic table, certain elements with higher
atomic masses precede or placed before the elements with lower atomic masses. For example, the element Ar (Atomic mass = 39.9) is placed before the element K (Atomic mass = 39.1)

Question.  Using the part of the periodic table given below, answer the questions that follow.

Group   Period	I	II	III	IV	V	VI	VII	Zero
1	H							He
2	Li	Be	B	C	N	O	F	Ne
3	Na	Mg	Al	Si	P	S	Cl	Ar
4	K	Ca

(i) Na has physical properties similar to which elements and why?

(ii) Write the electronic configuration of N and P

(iii) State one property common to fluorine and chlorine. (C.B.S.E. All India 2008)

Ans. (i) Na has physical properties similar to Li and K. All the three elements have one electron each in the valence of their atoms. These are known as alkali metals.

(ii) Electronic configuration of N(z = 7) = 2,5

Electronic configuration of P (z = 15) = 2,8,5

(iii) Both the elements have seven electrons in the valence shells as their atoms

Fluorine (z = 19) = 2, 7

Chlorine (z = 17) = 2,8,7

Question. .How and why does the atomic size vary as you go :

(i) from left to right across a period? (ii) down a group? [2009, 2011 (T-II)]

Ans. (i) Atomic size decreases on moving from left to right across a period. This is due to the increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.

(ii) Atomic size increases on moving down a group. This is due to addition of new shells which increases the distance between the outermost electrons and the nucleus even though nuclear charge increases

Question. How will the tendency to gain electrons change as we go from left to right across a period?

Why? [2009, 2011 (T-II)]

Ans. On moving from left to right across a period, metallic character decreases and non-metallic

character increases.Since metals tend to lose electrons and non-metals tend to gain electrons, the tendency to gain electrons increases as we move from left to right across a period.

Question. How does electronic configuration of atoms change in a period with increase in atomic

number? [2009]

Ans. On moving across a period from left to right, the atomic number of the elements increases, therefore, the number of electrons in the valence shell increases from 1 to 8, i.e, the first element in the given period will have one electron in its valence shell and the last element in the same period will have eight electrons.

Question.  Lithium, sodium and potassium form a Dobereiner’s triad. The atomic masses of lithium and potassium are 7 and 39 respectively. Predict the atomic mass of sodium. [2009]

Ans. Atomic mass of sodium = (At. mass of lithium + At. mass of potassium)/2

                                            =(7+39)/2 =23

State the first limitation of Mendeleev’s Periodic Table. [2009]

Ans. Hydrogen was not given a fixed position in Mendeleev’s Periodic Table.

Question.  Why was the system of classification of elements into triads not found suitable? [2009]

Ans. (i) Only a few similar elements could be grouped into triads and quite a large number of elements were left out.

(ii) It was also possible to group quite dissimilar elements into triads.

Question. . Why could no fixed position be given to hydrogen in Mendeleev’s Periodic Table? [2009]

Ans. Hydrogen resembled both the alkali metals and the halogens. So, it was placed above both the groups and could not be given a fixed position in Mendeleev’s Periodic Table. 

Question. What are ‘groups’ and ‘periods’ in the periodic table? [2009]

Ans. The vertical columns in the periodic table are known as groups while the horizontal rows are

known as periods.  

Question. Three elements A,B and C with similar properties have atomic masses X,Y and Z respectively.  The mass of Y is approximately equal to the average mass of X and Z. What is such an arrangement of elements called? Give one example of such a set of elements. [HOTS]

Ans. The arrangement of these elements is known as Dobereiner’s triad. Example, lithium, sodium

and potassium.

Question.  Elements have been arranged in the following sequence on the basis of their increasing atomic masses. [HOTS] F, Na, Mg, Al, Si, P, S, Cl, Ar, K

(a) Pick two sets of elements which have similar properties. 

(b) The given sequence represents which law of classification of elements?

Ans. (a) (i) F and Cl (ii) Na and K.

(b) Newland’s Law of Octaves.

Question. Two elements M and N belong to groups I and II respectively and are in the same period of

the periodic table. How do the following properties of M and N vary? [2009, 2011 (T-II)]

(i) Sizes of their atoms                         (ii) Their metallic characters

(iii) Their valencies in forming oxides  (iv) Molecular formulae of their chlorides

Ans. (i) The atomic radii of M is greater than N.

(ii) M is more metallic than N.

(iii) M has a valency of 1 and N has a valency of 2.

(iv) MCl, MCl2

Question. An element X belongs to group 17 and third period of the periodic table.

(a) Write electronic configuration of the element. What is its valency?

(b) Predict its nature, whether it is a metal or non-metal.

(c) Give the formula of the compound formed when it combines with an element Y having valency three. [2011 (T-II)]

Ans. (a) Electronic configuration — 2, 8, 7. Its valency is one.

(b) It is a non-metal (c) YX3

Question. How do atom exists ?

Answer: Atoms of most of the elements are chemically very reactive and do not exist in the free state (as single atoms ). Atoms usually exist in two ways :
(a) In the form of molecules
(b)In the form of ions

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