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CHAPTER-5 PERIODIC CLASSIFICATION OF ELEMENTS
1.What was Dobereiner’s basis of classifying elements?What was its limitation?
Doberiner arranged three elements having similar propertied in to groups called
triads.
When these three elements are arranged in the increasing order of their atomic
masses the atomic mass of the middle element was average of atomic mass of
other two elements.
Examples of Doberieners triads
Element
atomic mass
Li
6.9
Na
23
K
39
average atomic mass
6.9  39
2
= 22.95
Doberiener could identify only four triads from elements known at that time.
2.State Newland’s law of octaves?What was its limitation?
When elements are arranged in order of their increasing atomic masses , the
properties of every eighth element has properties similar to that of the first
element,like the eighth note of the musical scale.
Limitations
1.This law was applicable only to lighter elements having atomic masses up to 40
That is calcium.
2.He assumed that only 56 elements existed in nature and no new elements would
be discovered in future.
3.State Mendeleev’s Periodic law.
Mendeleev;s periodic law states that The properties of elements are the periodic
functions of their atomic masses.
4.What are the characteristic features of Mendeleev’s periodic table?
1.Mendeleev’s periodic table consists of seven horizontal rows called periods.
2.There are eight vertical columns called groups.
3.Except group VIII each group is divided in to subgroups designated as A and B.
Subdivision is made on the basis of difference in their properties.
4.Group VIII contains nine transition elements in three sets each containing three
elements.
5.He arranged the elements in groups and periods by following two principles
a)increasing atomic mass b)grouping of similar elements together.
6.Noble gases were not known at the time of Mendeleev.These were added later
after their discovery as a new group called zero group.
5.What are the achievements (merits) of Mendeleev’s periodic table?
1.Systematic study of elements.:- He was the first to arrange elements in to groups
and periods which made the study of elements simple.
2.Correction of doubtful atomic masses.:-His periodic table helped in correcting
the doubtful atomic masses of some elements.Eg Beryllium.
3.Prediction of new elements and their properties:-He left some blank spaces in
periodic table for the elements which were not know at that time.He predicted
their properties also.
Eg scandium,gallium and germanium.Mendeleev named these elements as ekaboron,eka-aluminium and eka-silicon respectively.
6.What are the limitations of Mendeleevs periodic table?
1.Anomalous position of hydrogen:-The positon of hydrogen in the periodic table
is controvertial.
2.Position of isotopes:-Since elements are arranged in increasing order of atomic
masses isotopes should be placed at different positions.
The isotopes of an element have same chemical properties, therefore they should
be placed in same position in periodic table.Thus position of isotopes is
anomalous.
3.Anomalous position of some pairs of elements:- Although the elements in the
periodic table are arranged in increasing order of their atomic mass, in some cases
elements with higher atomic mass is kept before the atom with lower atomic
mass.Eg:- Co ( At.Mass 58.93 is placed before Ni(At Mass58.71)
Te(At Mass 127.6) is placed before I ( At Mass 126.9)
4.Similar elements placed separately:-Li , Na K,Rb and Cs present in IA are quite
different from coinage metals Cu ,Ag Au present in IB.
5.Dissimilar elements placed together:-Cu and Hg with many similar properties
but Cu is placed in IB while Hg is placed in Group II B.
6.Uncertianty in prediction of new elements:- It was not possible to predict how
many new elements could be predicted between two elements especially among
heavier elements where difference in atomic mass is high.
7.Define modern periodic law. Name the scientist who gave modern periodic table.
Modern periodic law states that the properties of elements are the periodic
functions of their atomic number.
Henry Moseley.
8.Atomic number is the more fundamental property of an element. Explain.
Atomic number represents the number of protons in the nucleus, which is equal to
the number of electrons in shells.
Since the properties of elements depend on the number of electrons atomic
number is more fundamental property of an element.
Moreover atomic number is always a whole number never a fraction,
It increases by 1 from one element to the next,
Atomic number of every element is fixed and
no two elements can have same atomic number.
9. What is meant by periodicity of properties.What is the cause of periodicity in
modern periodic table?
The repetition of properties of elements after certain regular intervals when they
are arranged in increasing atomic number(atomic mass in Mendeleev’s table) is
called periodicity of properties.
The cause of periodicity in the properties of the elements is the repetition of
similar outer electronic configuration after certain regular intervals.
10..How could modern periodic table remove the anomalies of Mendeleevs periodic
table?
1.Position of isotopes:-All the isotopes of an element have the same atomic
number. So they can be put at one place in same group of periodic table.
2.Wrong order of atomic masses of some elements:- Since in modern periodic
table the elements are arranged in the increasing order of atomic number
placement of Co (At No 27 ) before Ni (28) is justified.
Similarly Te (AT No 52) is placed before I (At No-53). And Ar (18) is placed
before K (19).
3.Uncertainty in the prediction of new elements:- Since atomic number increases
by 1 in going from one element to next. Thus the number of elements to be
discovered in between any known elements is equal to the difference in their
atomic numbers.
11.What are the advantages of modern periodic table?
1.Easy to remember.
2.Position of isotopes has been justified.
3.Easy to predict the properties of elements from the position in periodic table.
4.Uncertanty in the position of new element has been removed.
12.How many groups and periods are there in modern periodic table?Which is the
longest period?
There are 18 groups and 7 periods in modern periodic table.
Sixth period is the longest period in modern periodic table.
13.Where would you expect to find the element with atomic number 11?
Electronic configuration of atom with atomic number 11 is 2,8,1.
Since there is a single electron in the last orbit it will be in the first group.
Since there are three shells it will be in third period.
14.Why do elements in the same group have similar properties?
The chemical properties of an element depends on the number of valence
electrons.The number of valence electrons in atoms of same group are same.
Therefore they have similar properties.
15.What is meant by valency?How would you calculate the valency of an atom from
electronic configuration?
The combining capacity of an element with hydrogen or any univalent element is
called valency.
Eg in HCl,H2O,NH3,CH4 valency of Cl is 1,valency of O is 2 , valency of N is 3
and valency of C is 4.
The number of electrons lost or gained by an atom to achieve electronic
configuration of nearest noble gases gives valency of an atom.
16.How does size of atom changes down the group? Why?
Size of atom increases down the group.
The number of shells increases down the group.Even if charge of the nucleus
increases down the group the effect of additional shell outweigh the effect of
increased nuclear charge thus the size increases.
17.How does the size of atom changes along the period ? Why?
Size of atom decreases along the period.
In moving along the period from left to right the charge on the nucleus increases
by one unit due to increase in atomic number but the additional electron goes to
the same shell. As a result outer electron are pulled closer to the nucleus.This
results in decrease in size.
18.What happens to the metallic character down the group and along the period?
Why?
Metallic character increased down the group and decreases along the period.
Down the group the size of atom increases. As a result valence electron can be
easily given out. This results in increase in metallic character.
Along the period the size decreases. As a result the valence electrons cannot be
easily given out instead the atoms show more tendency to accept electrons. This
results in increase in non-metallic character.
19.How does the acid – base character of oxides/hydroxides vary along a period?
As we move along the period the metallic character decreases while non metallic
character increases.
Since oxides of metals are basic and those of non-metals are acidic ,therefore
basic character of their oxides/hydroxides decreases while their acidic character
increases.
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