PERIODIC CLASSIFICATION OF ELEMENTS
Important terms & condition:
Classification of elements: the arrangement of elements in such a manner that elements with similar
properties are grouped together while elements with dissimilar properties are separated.
EARLY ATTEMPTS OF CLASSIFICATION OF ELEMENTS
1. Lavoisier’s classification of elements: In 1789, Lavoisier first attempted to classify the elements into
two divisions namely Metals and Non-metals. However this classification was not satisfactory as there
were many exceptions in each category.
2. Dobereiner’s classification of elements: In 1817, Johann Wolfgang Dobereiner grouped three
elements into what he termed triads.
In each case, the middle element has an atomic mass almost equal to the average atomic masses of
the other two elements in the triad.
For example, elements like lithium, sodium and potassium have atomic masses 7, 23 and 39
respectively. They are grouped together into a triad as,
Li (7) Na (23) K (39)
Here the atomic mass of sodium is the average of atomic masses of lithium and potassium.
Limitation of Dobereiner‟s law
Only three triads were identified from the element known at that time. Hence, this classification was
not useful.
Newland‟s law of octaves:
In 1863, John Newland suggested another classification of elements . He arranged the elements in
the order of increasing atomic masses starting with hydrogen (least atomic mass) and ended with
thorium having atomic mass 56. According to him, the properties of every eighth element are similar
to the first element. It was compared to music notations Sa, Re, Ga, Ma, Pa, Da, Ni, Sa, and thus the
name Newland‘s law of octaves (note of music). He then arranged the 49 elements known at that
time into seven groups of seven each. Newland referred to his arrangement as the Law of octaves.
The Law of octaves: If elements be arranged in ascending order of their atomic masses then every
eighth element was a kind of repetition of the first one either succeeding or preceding it like eighth
note in octave of music.
Periodicity : Periodicity is the recurrence of similar physical and chemical properties of
elements when arranged in a particular order.
For example, Sodium is similar to Lithium. Similarly Magnesium is similar to Beryllium
Limitations:
1. It was applicable only for lighter elements having atomic mass up to 40 amu, i.e.upto calcium.
2. He believed that only 56 elements existed in nature but later on more elements were discovered
whose properties did not fit into Newland’s law of octaves.
3. Some elements having different properties were grouped together like cobalt and nickel have been
placed with halogens. Due to above limitations, Newlands law of octaves was rejected.
Lothar Meyer‟s classification of elements:
In 1864, Lothar Meyer plotted atomic weight against atomic volume of various elements. He found
that elements with similar properties and valency fell under one another. However, this also could not
give the better understanding.
Mendeleev‟s periodic table:
Mendeleev (1834-1907), a Russian chemist arranged the elements in order of increasing atomic
masses, similarity in physical and chemical properties of elements. Properties of hydrides and oxides
of different elements were studied and elements with similar properties were grouped together
He classified the elements in table Consisted of vertical columns called groups and horizontal rows
called periods. There were 7 groups in table and group is subdivided into subgroups A and B except
group 7 which has three sets of elements in 4th, 5th and 6th period. (
Mendeleev„s periodic table is mainly based on two facts-
(i)Atomic mass
(ii)Similarity of chemical properties
Mendeleev‟s periodic law : “The physical and chemical properties of elements are the periodic
functions of their atomic masses”.
Characteristics of Mendeleev‟s Periodic table:
1. Mendeleev felt that similar properties occurred after periods (horizontal rows) of varying length.
2. Mendeleev made an eight-column table of elements.
3. He had to leave some blank spaces in order to group all the elements with similar properties in
the same column.
4. Mendeleev suggested that there must be other elements that had not been discovered.
5. He predicted the properties and atomic masses of several elements that were known at that time.
Later on, when these elements were discovered their properties remarkably agreed with the predicted
one.
For example, He left a gap below silicon in group IV A, and called the yet undiscovered
element as „Eka silicon‟. Discovery of „Germanium‟ during his life time proved him correct.
6. Similarly Scandium for ‘eka-boron’ and Gallium for ‘eka-aluminium’ vacancies were later
discovered during his life time.
7. Eight out of ten vacant spaces left by Mendeleev were filled by the discovery of new elements.
8. Incorrect atomic masses of some arranged elements were corrected. For example, atomic mass
of Beryllium as corrected from 13 to 9.
Limitations of Mendeleev, periodic table:
1. Few elements having a higher atomic mass were placed before elements having a lower atomic
mass. Example: Argon (39.9) was placed before Potassium (39.1) Cobalt (58.9) was placed before
Nickel (58.6) Tellurium (127.9) was placed before Iodine (126.9)
2. No separate position has been given to isotopes of an element.
3. Position of hydrogen in the periodic table was not certain about keeping it with either in group IA or
in group VII A.
4. Chemically dissimilar elements were placed in the same group.
The difficulty in the Mendeleev’s periodic table is overcome by introduction of Modern periodic table.
It is also known as Long form of periodic table. In this table, properties of elements are dependent on
their electronic configurations (distributions). Hence, modern periodic law is defined as the properties
of elements are the periodic function of their atomic numbers.
Modern periodic table:
In 1913 Moseley found that frequency of X-ray emitted by different elements is directly proportional to
atomic number. These studies show that properties of elements depend upon atomic number but not
atomic mass.
So atomic number is the basis of classification of element. Moseley gave modern periodic law which
stated as
“The physical and chemical properties of the elements are periodic function of their atomic
number”
In this table, elements have been arranged in order of increasing atomic number. This table also
consists of vertical rows called groups and horizontal rows called periods which are
discussed as:
(i) There are 18 groups designated as 1 to 18.
(ii)All the elements in a group have same no. of electrons in outermost shell.
(iii)The elements of each group have similar chemical properties due to same valence electrons .
(iv)There are in all seven periods.
(v)First three periods with 2,8,8 element called short periods. The next three periods with 18,18,32
element called long period while seventh period is an incomplete period. (vi)The number of elements
in these periods are based on filling of electron into various shell on the formula 2n2
(n-number of
shell)
For Ex:-K-shell (n=1)-(2.1)2=2 so first period has two elements
L-shell (n=2)-(2.2)2=8. So 2nd period has eight elements.
M-shell (n=3)-(2.3)2=18, but the outer most shell can have only 8-electron so third period also has
eight elements.
Advantages of long form of periodic table:-
1. The position of the elements are linked with their electronic configuration.
2. Position of isotopes of an element is justified since they have same atomic number.
3. Similar properties of element in a group is also justified due to same valence electrons.
4. Each group is an independent group and idea of sub-group discarded.
Groups:
Elements in group 1 are called alkali metals.
Elements in group 2 are called alkaline earth metals.
Elements in group 17 are called halogens.
Elements in group 18 elements are called inert gases or noble gases.
Significance of group in the periodic table is that an element in a group has same no. of valence
electrons, valence and thus identical chemical properties.
Periods :
1st period – 2 elements and is called very short period.
2nd period – 8 elements and is called short period.
3rd period - 8 elements and is called short period.
4th period – 18 elements and is called long period.
5th period – 18 elements and is called long period.
6th period – 32 elements and is called very long period.
7th period – incomplete period.
The number of shells present in the element indicates the period to which it belongs.
Valency: it is defined as the combing capacity of an atom of an element to acquire noble gas
configuration. It is equal to the number of electrons lost, gained or shared during the formation of a
chemical compound.
(i) In groups the elements present have same valence.
(ii) In periods: valence increases 1 to 4 and then decreases to zero.
Atomic size / Atomic radii: It is defined as the distance from the Centre of nucleus to the outermost
shell of the atom. It is generally expressed in Pico meter (Pm).
On moving down the group the group the atomic radii increases.
Because on moving down the group a new energy shell is added which increases the distance
between the outermost electrons and the nucleus. Although the nuclear charge also increases, but it
is compensated by the additional shell being added thus, increasing the size of the atom.
Hence, In periods atomic radius decreases in moving from left to right due to increase in nuclear
charge which have tendency to attract electron closer to the nucleus and reduces the size of atom.
Across the period the atomic radii decrease.
Due to the increased nuclear charge, the pull on the electrons increases and hence, they are pulled
Closer to the nucleus thus, decreasing the atomic size.
Hence, in groups: Atomic size increases down the group because new shells are added down the
group which increases distance between nucleus and outer most electrons.
Oxides and its nature: Metals react with oxygen to form oxides by loss of electrons. These oxides
on dissolution in water form bases.
Reactivity of elements:
Down the group reactivity of metals increases as the tendency to lose electrons increases due to
increased atomic size.
Reactivity of non- metals decreases down the group
Because of the increased atomic size and the tendency to gain electrons decreases.
On moving across the period, the reactivity first increases due to the decrease in the metallic
character and increase in non metallic character Metallic and Non-metallic Properties
The metallic character of an element is expressed in terms of its electron releasing tendency while
non-metallic character in term of electron accepting tendency.
In group: Metallic character of the element increases down the group due to increasing atomic size or
because outermost electrons are farther away from the nucleus .So they can be easily lost.
In periods:- Metallic character of the element decreases along a period due to decrease in atomic
size along a period or outermost electrons are closer to nucleus. So they can not be easily lost.
NOTE:- Non-metallic elements follow the same reverse trend. In modern periodic table a zig - zag
line separates metals from Non-metals. The border line elements, B, Si, Ge, As, Sb, Te and Po are
called metalloids or semi-metal.
2 Marks Questions
Important terms & condition:
Classification of elements: the arrangement of elements in such a manner that elements with similar
properties are grouped together while elements with dissimilar properties are separated.
EARLY ATTEMPTS OF CLASSIFICATION OF ELEMENTS
1. Lavoisier’s classification of elements: In 1789, Lavoisier first attempted to classify the elements into
two divisions namely Metals and Non-metals. However this classification was not satisfactory as there
were many exceptions in each category.
2. Dobereiner’s classification of elements: In 1817, Johann Wolfgang Dobereiner grouped three
elements into what he termed triads.
In each case, the middle element has an atomic mass almost equal to the average atomic masses of
the other two elements in the triad.
For example, elements like lithium, sodium and potassium have atomic masses 7, 23 and 39
respectively. They are grouped together into a triad as,
Li (7) Na (23) K (39)
Here the atomic mass of sodium is the average of atomic masses of lithium and potassium.
Limitation of Dobereiner‟s law
Only three triads were identified from the element known at that time. Hence, this classification was
not useful.
Newland‟s law of octaves:
In 1863, John Newland suggested another classification of elements . He arranged the elements in
the order of increasing atomic masses starting with hydrogen (least atomic mass) and ended with
thorium having atomic mass 56. According to him, the properties of every eighth element are similar
to the first element. It was compared to music notations Sa, Re, Ga, Ma, Pa, Da, Ni, Sa, and thus the
name Newland‘s law of octaves (note of music). He then arranged the 49 elements known at that
time into seven groups of seven each. Newland referred to his arrangement as the Law of octaves.
The Law of octaves: If elements be arranged in ascending order of their atomic masses then every
eighth element was a kind of repetition of the first one either succeeding or preceding it like eighth
note in octave of music.
Periodicity : Periodicity is the recurrence of similar physical and chemical properties of
elements when arranged in a particular order.
For example, Sodium is similar to Lithium. Similarly Magnesium is similar to Beryllium
Limitations:
1. It was applicable only for lighter elements having atomic mass up to 40 amu, i.e.upto calcium.
2. He believed that only 56 elements existed in nature but later on more elements were discovered
whose properties did not fit into Newland’s law of octaves.
3. Some elements having different properties were grouped together like cobalt and nickel have been
placed with halogens. Due to above limitations, Newlands law of octaves was rejected.
Lothar Meyer‟s classification of elements:
In 1864, Lothar Meyer plotted atomic weight against atomic volume of various elements. He found
that elements with similar properties and valency fell under one another. However, this also could not
give the better understanding.
Mendeleev‟s periodic table:
Mendeleev (1834-1907), a Russian chemist arranged the elements in order of increasing atomic
masses, similarity in physical and chemical properties of elements. Properties of hydrides and oxides
of different elements were studied and elements with similar properties were grouped together
He classified the elements in table Consisted of vertical columns called groups and horizontal rows
called periods. There were 7 groups in table and group is subdivided into subgroups A and B except
group 7 which has three sets of elements in 4th, 5th and 6th period. (
Mendeleev„s periodic table is mainly based on two facts-
(i)Atomic mass
(ii)Similarity of chemical properties
Mendeleev‟s periodic law : “The physical and chemical properties of elements are the periodic
functions of their atomic masses”.
Characteristics of Mendeleev‟s Periodic table:
1. Mendeleev felt that similar properties occurred after periods (horizontal rows) of varying length.
2. Mendeleev made an eight-column table of elements.
3. He had to leave some blank spaces in order to group all the elements with similar properties in
the same column.
4. Mendeleev suggested that there must be other elements that had not been discovered.
5. He predicted the properties and atomic masses of several elements that were known at that time.
Later on, when these elements were discovered their properties remarkably agreed with the predicted
one.
For example, He left a gap below silicon in group IV A, and called the yet undiscovered
element as „Eka silicon‟. Discovery of „Germanium‟ during his life time proved him correct.
6. Similarly Scandium for ‘eka-boron’ and Gallium for ‘eka-aluminium’ vacancies were later
discovered during his life time.
7. Eight out of ten vacant spaces left by Mendeleev were filled by the discovery of new elements.
8. Incorrect atomic masses of some arranged elements were corrected. For example, atomic mass
of Beryllium as corrected from 13 to 9.
Limitations of Mendeleev, periodic table:
1. Few elements having a higher atomic mass were placed before elements having a lower atomic
mass. Example: Argon (39.9) was placed before Potassium (39.1) Cobalt (58.9) was placed before
Nickel (58.6) Tellurium (127.9) was placed before Iodine (126.9)
2. No separate position has been given to isotopes of an element.
3. Position of hydrogen in the periodic table was not certain about keeping it with either in group IA or
in group VII A.
4. Chemically dissimilar elements were placed in the same group.
The difficulty in the Mendeleev’s periodic table is overcome by introduction of Modern periodic table.
It is also known as Long form of periodic table. In this table, properties of elements are dependent on
their electronic configurations (distributions). Hence, modern periodic law is defined as the properties
of elements are the periodic function of their atomic numbers.
Modern periodic table:
In 1913 Moseley found that frequency of X-ray emitted by different elements is directly proportional to
atomic number. These studies show that properties of elements depend upon atomic number but not
atomic mass.
So atomic number is the basis of classification of element. Moseley gave modern periodic law which
stated as
“The physical and chemical properties of the elements are periodic function of their atomic
number”
In this table, elements have been arranged in order of increasing atomic number. This table also
consists of vertical rows called groups and horizontal rows called periods which are
discussed as:
(i) There are 18 groups designated as 1 to 18.
(ii)All the elements in a group have same no. of electrons in outermost shell.
(iii)The elements of each group have similar chemical properties due to same valence electrons .
(iv)There are in all seven periods.
(v)First three periods with 2,8,8 element called short periods. The next three periods with 18,18,32
element called long period while seventh period is an incomplete period. (vi)The number of elements
in these periods are based on filling of electron into various shell on the formula 2n2
(n-number of
shell)
For Ex:-K-shell (n=1)-(2.1)2=2 so first period has two elements
L-shell (n=2)-(2.2)2=8. So 2nd period has eight elements.
M-shell (n=3)-(2.3)2=18, but the outer most shell can have only 8-electron so third period also has
eight elements.
Advantages of long form of periodic table:-
1. The position of the elements are linked with their electronic configuration.
2. Position of isotopes of an element is justified since they have same atomic number.
3. Similar properties of element in a group is also justified due to same valence electrons.
4. Each group is an independent group and idea of sub-group discarded.
Groups:
Elements in group 1 are called alkali metals.
Elements in group 2 are called alkaline earth metals.
Elements in group 17 are called halogens.
Elements in group 18 elements are called inert gases or noble gases.
Significance of group in the periodic table is that an element in a group has same no. of valence
electrons, valence and thus identical chemical properties.
Periods :
1st period – 2 elements and is called very short period.
2nd period – 8 elements and is called short period.
3rd period - 8 elements and is called short period.
4th period – 18 elements and is called long period.
5th period – 18 elements and is called long period.
6th period – 32 elements and is called very long period.
7th period – incomplete period.
The number of shells present in the element indicates the period to which it belongs.
Valency: it is defined as the combing capacity of an atom of an element to acquire noble gas
configuration. It is equal to the number of electrons lost, gained or shared during the formation of a
chemical compound.
(i) In groups the elements present have same valence.
(ii) In periods: valence increases 1 to 4 and then decreases to zero.
Atomic size / Atomic radii: It is defined as the distance from the Centre of nucleus to the outermost
shell of the atom. It is generally expressed in Pico meter (Pm).
On moving down the group the group the atomic radii increases.
Because on moving down the group a new energy shell is added which increases the distance
between the outermost electrons and the nucleus. Although the nuclear charge also increases, but it
is compensated by the additional shell being added thus, increasing the size of the atom.
Hence, In periods atomic radius decreases in moving from left to right due to increase in nuclear
charge which have tendency to attract electron closer to the nucleus and reduces the size of atom.
Across the period the atomic radii decrease.
Due to the increased nuclear charge, the pull on the electrons increases and hence, they are pulled
Closer to the nucleus thus, decreasing the atomic size.
Hence, in groups: Atomic size increases down the group because new shells are added down the
group which increases distance between nucleus and outer most electrons.
Oxides and its nature: Metals react with oxygen to form oxides by loss of electrons. These oxides
on dissolution in water form bases.
Reactivity of elements:
Down the group reactivity of metals increases as the tendency to lose electrons increases due to
increased atomic size.
Reactivity of non- metals decreases down the group
Because of the increased atomic size and the tendency to gain electrons decreases.
On moving across the period, the reactivity first increases due to the decrease in the metallic
character and increase in non metallic character Metallic and Non-metallic Properties
The metallic character of an element is expressed in terms of its electron releasing tendency while
non-metallic character in term of electron accepting tendency.
In group: Metallic character of the element increases down the group due to increasing atomic size or
because outermost electrons are farther away from the nucleus .So they can be easily lost.
In periods:- Metallic character of the element decreases along a period due to decrease in atomic
size along a period or outermost electrons are closer to nucleus. So they can not be easily lost.
NOTE:- Non-metallic elements follow the same reverse trend. In modern periodic table a zig - zag
line separates metals from Non-metals. The border line elements, B, Si, Ge, As, Sb, Te and Po are
called metalloids or semi-metal.
Chapter wise test-Periodic Classification of elements
2 Marks Questions [2 x 5 = 10 ]
1. An element has electronic configuration 2, 8, 3. What is the atomic number of this element? To which (i) group and (ii) period this element belong?
2. State the positions of (i) isotopes of the same element and (ii) hydrogen in the Modern Periodic Table.
3. How does the tendency to gain electrons change as we go down the 16th group of periodic table? Why?
4. Give reasons for the following : (a) Lithium atom is smaller than sodium atom. (b) Chlorine (atomic number 17) is more electronegative than sulphur (atomic number 16)
5. (a) State the Modern Periodic Law. (b) Name the element which has twice as many electrons in its second shell as in its first shell. Write its electronic configuration also.
3 Marks Questions [ 3 x 5 = 15 ]
1. 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.
2. Three elements A, B and C have atomic number 7, 8 and 9 respectively. (a) What would be their positions in the Modern Periodic Table (Mention group and period both)? (b) Arrange A, B and C in the decreasing order of their size. (c) Which one of the three elements is most reactive and why?
3. (a) The elements of the second period along with their atomic number in parenthesis are given below : B (5), Be (4), O (8), N (7), Li (3), C (6), F (9) (i) Arrange them in the same order as they are in the periodic table. (ii) Which element has the (i) largest (ii) smallest atom? (b) Why does the atomic radius change as we move from left to right in a period?
4. (a) How is the valency of an element determined from its position in the periodic table? (b) Magnesium has atomic number 12. To which (i) group (ii) period of the periodic table does it belong? (c) The valency of all the elements in a group is same. Why?
5. A part of the periodic table has been shown below :
Group →
|
1
|
2
|
13
|
14
|
15
|
16
|
17
|
18
|
Period ↓
| ||||||||
1
| ||||||||
2
|
A
|
C
|
E
|
G
| ||||
3
|
B
|
D
|
F
|
Answer the following questions on the basis of position of elements in the above table.
(a) Which element is a noble gas? Give reason.
(b) Which element is most electronegative? Give reason.
(c) Write the electronic configuration of (i) B and (ii) E.
5 Marks Questions [ 5 x 1 = 5]
1. What are electron affinity and ionization energy? How they are vary in group and periods in modern periodic table? OR, Two elements ‘A’ and ‘B’ belong to group 1 and 2 respectively in the same period. Compare them with respect to :– (a) Number of valence electrons (b) Valency (c) Metallic character (d) Size of atom (e) Formulae of their oxides.
Answer:
1. Atomic number is 13. (i) Group – 13 (ii) 3rd Period
2. (i) All isotopes have same atomic number. So they have same place for a particular element.
(ii) Hydrogen is not given a definite position. It was placed in group I A and group VII B in the Modern Periodic Table.
3. As we go down the 16th group of periodic table, tendency to gain electrons decreases because the electronegative character (non metallic character) of elements gradually decreases.
4. As we move down in a group, an electron shell is added after every change of period. Sodium has one more shell than lithium.
Li (3) = 2, 1
Na (11) = 2, 8, 1
Due to less number of shells, lithium atom is smaller than sodium atom.
(b) The nucleus of chlorine has more tendency to attract an extra electron than the nucleus of sulphur because chlorine needs only one 1 electron to complete its shell. Hence, chlorine is more electronegative than sulphur.
5. (a) Modern Periodic Law : Properties of elements are the periodic function of their atomic numbers.
(b) Carbon (atomic number 6) has twice electrons in its second shell as compared to first shell because its electronic configuration is 2, 4.
3 Marks Questions
1. (a) Electronic configuration — 2, 8, 7. Its valency is one. (b) It is a non-metal (c) YX3
2. (a) A, B and C occupy 2nd period of periodic table and 15, 16 and 17 groups respectively.
(b) A > B > C because atomic size decreases as we move from left to right across a period.
(c) C has electronic configuration 2, 7. It needs only one electron to complete its outermost shell. So it is more reactive.
3(a) (i) Li (3), Be (4), B (5), C (6), N (7), O (8), F (9).
(ii) Li is largest and F is smallest atom.
(b) As one moves from left to right across a period, there is a consecutive addition of a proton
in the nucleus that increases the positive charge, which consequently pulls the extra nuclear electrons in the outermost shell inward. Thus, atomic size decreases with the increase in the number of protons as one moves from left to right in a period.
4. (a) The valency of an element with respect to hydrogen increases from 1 to 4 and then falls to 1 as in 2nd period elements.
The valency of elements with respect to oxygen increases from 1 to 7 for 3rd period elements.
(b) (i) 2nd group (ii) 3rd period
(c) In a group all elements have same number of electrons in their outermost shell. So they have same valency.
No comments:
Post a Comment