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
intotwo 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. (“R” is used to represent any
of
the elements in a group)
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 number
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
atomicnumber”
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.
Read more topics
acids-and-alkalis
carbon-and-its-compound-notes
periodic-classification-of-elements
unsaturated-hydrocarbon
metal-and-non-metals-notes
chemical-reaction-and-equations-notes-2
periodic-classification-of-elements
EARLY tries OF CLASSIFICATION OF components
1. Lavoisier’s classification of elements: In 1789, chemist 1st tried to classify the weather intotwo divisions specifically Metals and Non-metals. but this classification wasn't satisfactory as there have been several exceptions in every class.
2. Dobereiner’s classification of elements: In 1817, Johann Wolfgang Dobereiner classified 3 elements into what he termed triads. In every case, the center part has associate degree mass virtually up to the typical atomic plenty ofthe opposite 2 components within the triad.For example, components like metallic element, Na and metallic element have atomic plenty seven, 23 and 39 respectively. they're classified along into a triad as, Li (7) Na (23) K (39)
Here the mass of Na is that the average of atomic plenty of metallic element and metallic element. Limitation of Dobereiner‟s law
Only 3 triads were known from the part best-known at that point. Hence, this classification wasn'thelpful.
Newland‟s law of octaves:In 1863, John Newland instructed another classification of components . He organized the weatherwithin the order of skyrocketing masses beginning with atomic number 1 (least atomic mass) andfinished with metallic element having atomic mass fifty six. consistent with him, the properties of each eighth part ar the same as the primary part. it absolutely was compared to music notationsStorm Troops, Re, Ga, Ma, Pa, Da, Ni, Sa, and therefore the name Newland‘s law of octaves (note of music). He then organized the forty nine components best-known at that point into seven teams of seven every. Newland said his arrangement because the Law of octaves. The Law of octaves: If components be organized in ascending order of their atomic plenty then eacheighth part was a form of repetition of the primary one either succeeding or preceding it like tone in octave of music. Periodicity : regularity is that the return of comparable physical and chemical properties of elements once organized in a very explicit order. For example, Na is comparable to metallic element. equally Mg is comparable to metal Limitations:
1. it absolutely was applicable just for lighter components having mass up to forty amu, i.e.uptometallic element.
2. He believed that solely fifty six components existed in nature however in a while a lot ofcomponents were discovered whose properties failed to match into Newland’s law of octaves.3. Some components having totally different properties were classified along like atomic number 27and nickel are placed with halogens. thanks to on top of limitations, Newlands law of octaves was rejected
Read more topics
acids-and-alkalis
carbon-and-its-compound-notes
periodic-classification-of-elements
unsaturated-hydrocarbon
metal-and-non-metals-notes
chemical-reaction-and-equations-notes-2
periodic-classification-of-elements
EARLY tries OF CLASSIFICATION OF components
1. Lavoisier’s classification of elements: In 1789, chemist 1st tried to classify the weather intotwo divisions specifically Metals and Non-metals. but this classification wasn't satisfactory as there have been several exceptions in every class.
2. Dobereiner’s classification of elements: In 1817, Johann Wolfgang Dobereiner classified 3 elements into what he termed triads. In every case, the center part has associate degree mass virtually up to the typical atomic plenty ofthe opposite 2 components within the triad.For example, components like metallic element, Na and metallic element have atomic plenty seven, 23 and 39 respectively. they're classified along into a triad as, Li (7) Na (23) K (39)
Here the mass of Na is that the average of atomic plenty of metallic element and metallic element. Limitation of Dobereiner‟s law
Only 3 triads were known from the part best-known at that point. Hence, this classification wasn'thelpful.
Newland‟s law of octaves:In 1863, John Newland instructed another classification of components . He organized the weatherwithin the order of skyrocketing masses beginning with atomic number 1 (least atomic mass) andfinished with metallic element having atomic mass fifty six. consistent with him, the properties of each eighth part ar the same as the primary part. it absolutely was compared to music notationsStorm Troops, Re, Ga, Ma, Pa, Da, Ni, Sa, and therefore the name Newland‘s law of octaves (note of music). He then organized the forty nine components best-known at that point into seven teams of seven every. Newland said his arrangement because the Law of octaves. The Law of octaves: If components be organized in ascending order of their atomic plenty then eacheighth part was a form of repetition of the primary one either succeeding or preceding it like tone in octave of music. Periodicity : regularity is that the return of comparable physical and chemical properties of elements once organized in a very explicit order. For example, Na is comparable to metallic element. equally Mg is comparable to metal Limitations:
1. it absolutely was applicable just for lighter components having mass up to forty amu, i.e.uptometallic element.
2. He believed that solely fifty six components existed in nature however in a while a lot ofcomponents were discovered whose properties failed to match into Newland’s law of octaves.3. Some components having totally different properties were classified along like atomic number 27and nickel are placed with halogens. thanks to on top of limitations, Newlands law of octaves was rejected
0 comments:
Post a Comment