Download Chapter 5

CHAPTER 5: PERIODIC CLASSIFICATION OF ELEMENTS
EARLY ATTEMPTS OF CLASSIFICATION
*Elements were classified as metals and nonmetals based on their physical and
chemical properties.
*In 1808, Dalton stated in his atomic theory that atom of one element can be
distinguished from the atom of another element based on it’s atomic mass.
*So, all earlier classifications were based on atomic mass.
DOBERENIER’S LAW OF TRIADS
“When the elements are arranged in groups of 3 in increasing order of atomic masses,
the middle element of a group has the atomic mass and properties roughly the average
of the other 2 elements.” These elements show similarity in their properties. Eg
Elements Li
Atomic
7
Mass
Na
23
K
39
Cl
35.5
Br
80
I
127
Average atomic mass of first and second
(7+39)/2 = 23
(35.5+127)/2 = 81.2
LIMITATION
All the elements could not be classified into triads discovered at that time. Only a limited
number of elements could be arranged in such triads.
NEWLAND’S LAW OF OCTAVES
Newland classified elements on the basis of “Law of Octaves” stated by him. According
to this law, “when the elements are arranged in the order of increasing atomic mass, the
properties of the 8th element are repetition of the properties of the first element.”
LIMITATIONS
All the elements discovered @ that time could not be classified into octaves.
MENDELEEV’S CLASSIFICATION
Mendeleev’s arranged 63 elements known at that time on the basis of similarity in
properties that is on the basis of similarities in the formula of their oxides, hydrides, etc
formed by these elements. He observed that most of the elements were placed in the
increasing order of their atomic masses.
Mendeleev’s periodic law states that “the physical and chemical properties of elements
are periodic function of their atomic mass”.
In Mendeleev’s Periodic table:
1. There are 8 vertical columns called groups. The groups from one to seven are
subdivided into 2 groups, A and B. Thus, on the whole, 15 vertical columns.
2. The horizontal rows in the periodic table are called periods.
3. In a period, the properties of elements gradually changes from metallic to nonmetallic
characters.
ADVANTAGES OF MENDELEEV’S PERIODIC TABLE
1. He could classify all the 63 elements discovered at that time on the basis of
similarities in properties.
2. He left gaps for yet to be discovered elements.
3. He predicted the properties of undiscovered elements and thus helped in the
discovery of these elements later on.
4. He named them by prefixing a Sanskrit numeral Eka to the name of the preceding
element in the same group.
5. It helped in correction of atomic weight of cirtain elements on the basis of their
position.
LIMITATIONS OF MENDELEEV’S PERIODIC TABLE
1. Although most of the elements were placed in the order of increasing atomic masses,
increasing order could not be maintained in all cases. Eg cobalt (atomic mass 58.9
preseded nickel 58.7) but he could maintain similarity in properties.
2. Mendeleev’s periodic table did not provide place for noble gasses which were
discovered later.
3. There were no place for isotopes in Mendeleev’s periodic table although they differ in
atomic mass.
MODERN PERIODIC TABLE
Henry Maseley, an English physicist found that the atomic number (Z) was the
fundamental property of an element and not the atomic mass of classification of
elements.
MODERN PERIODIC LAW
Properties of elements are periodic functions of their atomic number.
LONG FORM OF PERIODIC TABLE
In long form of periodic table, the elements are arranged in groups and periods on the
basis of electronic configuration of elements.
MAIN FEATURES OF THE LONG FORM OF THE PERIODIC TABLE
1. There are 18 vertical columns known as groups.
2. The group 1 is on the extreme left-hand side and group 18 on the extreme right-hand
side of the table.
3. The group 1, 2, and then 13 to 17 contain normal elements. The normal elements are
also called representative elements.
4. The elements in group 3 to group 12 are called transition elements.
5. The elements in the group 18 are known as noble gasses or inert gasses.
6. Group 1 elements are called alkate metals. Group 2 elements are called alkaline
earth metals.
Group 13 elements belong to boron family.
Group 14 elements belong to carbon family.
Group 15 elements belong to nitrogen family.
Group 16 elements belong to oxygen family
Group 17 elements are called halogens.
7. The horizontal rows in the periodic table are called periods.
8. There are 7 periods in the long form of periodic table.
9. The first period contains 2 elements hydrogen and Helium. They have only one shell
(K).
10. The 2nd period contains 8 elements.
Lithium, Baryllium, Borbon, carbon, nitrogen, oxygen, fluorine and neon.
The second period has 2 shells. (K and L).
11. The third period contains 8 elements. It has 3 shells (KLM).
12. Fourth period has 18 elements. Fifth period has 18 elements. Sixth period has 32
elements. Seventh period has 32 elements.
13. The number of period to which an element belongs is given by the number of
outermost shell.
14. Elements with atomic number 57 to 71 are called lanthanide series and elements
with atomic number 89 to 103 are called actinide.
15. Elements having 1 valence electron are placed in group one. Elements having 2
valence electrons are placed in group 2.
16. Elements having 3 valence electrons are placed in group 13.
Elements having 4 valence electrons are placed in group 14.
Elements having 5 valence electrons are placed in group 15.
Elements having 6 valence electrons are placed in group 16.
Elements having 7 valence electrons are placed in group 17.
Elements having 8 valence electrons are placed in group 18 except helium which has 2
electrons in the valence shell.
17. First period is called very short period.
Second and third periods are called short period.
Fourth and fifth periods are called long period.
Sixth period is called very long period.
Seventh period is incomplete.
CHARACTERISTICS OF PERIODS
1. Valence electrons: on moving from left to right in a period, the number of valence
electrons in elements increases from one to eight.
11
12
13
Na
Mg
Al
2, 8, 1
2, 8, 2
2, 8, 3
1
2
3
(number of valence electrons)
14
Si
2, 8, 4
4
15
P
2, 8, 5
5
16
S
2, 8, 6
6
17
Cl
2, 8, 7
7
18
Ar
2, 8, 8
8
2. Valency: On moving from left to right in each short period, the valency of elements
increases from one to 4 and then decreases to 0.
Na
Mg
2, 8, 1
2, 8, 2
1
2
(valency)
Al
2, 8, 3
3
Si
2, 8, 4
4
P
2, 8, 5
3
S
2, 8, 6
2
Cl
2, 8, 7
1
Ar
2, 8, 8
0
3. Size of atoms (or atomic size): The size of an atom is the distance between the
center of the neucleous and the outermost shell of an atom. The size of an atom is
indicated by writing it’s radius called atomic radius. It is expressed in Picometre whose
symbol is “pm”.
On moving from left to right, in a period of the periodic table, the size of atoms
decreases.
That is, as we move from left to right the atomic number increases which means the
number of protons and electrons in the atom increases. Due to large positive charge on
the neucleous, the electrons are pulled in more close to the nucleous and the size of the
atom decreases.
Na
186
Mg
160
AL
143
Si
118
P
110
S
104
Cl
99
Ar
4. Metallic character: On moving from left to right in a period, the metallic character of
elements decreases but the nonmetallic character increases.
Na
Metals
Mg
Al
Si
P
S
Metalloids Nonmetals
Cl
Ar
Noble
gas
(metallic character decrease, nonmetallic character increases).
So the greatest metallic character is found in the element on the extreme left side of a
period and the greatest nonmetallic character is found in the elements on the right side.
So, in this period, sodium is the most metallic element and clorin is the most nonmetallic
element.
On moving from left to right in a period, the electropositive character of elements
decreases, but the electronegative character increases.
Metals loose electrons and form positive ion. So metals are electropositive elements.
Nonmetals accept electrons and form negative ions, so nonmetals are called
electronegative elements. So, on moving from left to right in a period, the tendency of
atoms to loose electrons decreases and the tendency of atoms to gain electrons
increases.
5. Chemical reactivity: On moving from left to right in a period, the chemical reactivity of
elements first decreases then increases. The elements in the last group is chemically
inert.
Na
Very
reactive
MG
Al
Si
Least
reactive
P
S
Cl
Very
high
reactive
Ar
Noble
gas
6. Nature oxides: On moving from left to right in a period, the basic nature of oxides
decreases and the acidic nature of oxides increases.
Na
Highly
basic
Mg
Al
Si
P
Amphoteric Amphoteric
S
Cl
Highly
acidic
Ar
CHARACTERISTICS OF GROUPS
1. Valence electrons: All the elements of a group of the periodic table has the same
number of electrons.
Group 1
H
Li
Na
K
Rb
Cs
Fr
1
2, 1
2, 8, 1
2, 8, 8 1
2, 8, 18 8 1
2, 8, 18, 18, 8 1
2, 8, 18, 32, 18, 8 1
2. Valency: All the elements in a group have the same valency.
Valency of group 1 elements is 1.
Valency of group 13 elements is 3.
Valency of group 17 elements is 20.
3. Size of atoms: On moving down in a group of the periodic table, the size of atoms
increases.
Li
Na
K
Rb
Cs
Fr
Atomic Size
152
186
231
244
262
270
Smallest atom
Biggest atom
The number of electron shell in the atoms increases gradually due to which size of
atoms also increases.
4. Metallic character: On going down in a group of the periodic table, the metallic
character of elements increases. The greatest metallic character is found in the
elements in the lowest part of a group.
Li
Na
K
Rb
Cs
Fr
Least metallic character
Most metallic character
Metallic character or electropositive character increases on going down in a group.
On moving down a group, the tendency of an atom to loose electrons (electropositive
character) increases.
Group 17
F
Cl
Br
I
Most electronegative (nonmetallic)
Least electronegative
On moving down a group, the tendency of an atom to gain electrons (electronagetive
character) decreases.
5. Chemical reactivity: The chemical reactivity of metals increases on going down in a
group of the periodic table.
Due to an increase in size of atom, valence electrons of metal atoms (which take part in
chemical reactions) become more and more far away from the neucleous and hence
can be removed easily. So, as we go down in a group of metals, the tendency of their
atoms to loose electrons increases and hence their chemical reactivity increases.
The chemical reactivity of nonmetals decreases on going down in a group.
Due to increase in size of atom, the neucleous of atom gos more and more deep and
hence it’s attraction for the incoming electrons decreases. So the tendency of their
atoms to gain electrons decreases, due to which their reactivity also decreases.
Group 1 (metals)
Li
Na
Least reactive
K
Rb
Cs
Fr
Most reactive
Most reactive
Chemical reactivity of metals increases on going down in a group.
Group 17 (nonmetals)
Group 17 (nonmetals)
F
Cl
Br
I
Most reactive
Least reactive
Chemical reactivity of nonmetals decreases on going down in a group.
6. Nature of oxides: On going down in a group of the periodic table, there is no change
in the nature of oxides of elements. Eg group 1 will be basic, and group 17 will be
acidic.
MERITS OF MODERN PERIODIC TABLE
1. The modern periodic table is based on atomic number of element which is the most
fundamental property of elements.
2. It helps us understand why element shows similar properties but elements in different
groups show different properties.
3. It explains the reasons for the periodicity in properties
4. It tells us why elements are repeted after 2, 8, 18 and 32 elements.
ADVANTAGES OF THE PERIODIC TABLE
1. The periodic table has made the study of chemistry systematic and easy.
2. It is easier to remember the properties element if it’s position in the periodic table is
known.
3. The type of compounds formed by an element can b predicted by knowing the
position in the periodic table.
4. A periodic table chart is used as a teaching aid in chemistry.