Periodic Table of Elements
More than 115 elements have been
discovered which need to be arranged systematically, first elements were
classified as metals and non metals.
=> 9th 10th Class Science - Periodic Table of Elements Podcast-
History of classification of elements
1.
Prout’s
Hypothesis – 1815
2.
Dobereiner’s
Triads – 1829
3.
Newland’s
Law of Octaves – 1863
4.
Lother
Meyer Curves – 1869
5.
Mendeleev’s
Periodic Table – 1869
6.
Modern
Periodic Laws of Moseley -1913
7.
Modern
Periodic Table ( of 18 columns )
Prout’s Hypothesis
According
to this hypothesis atomic mass of elements is multiple of mass of hydrogen
atom. So elements are related to each other on the basis of their atomic
masses. It fails due to the reason that some elements do not have atomic mass
in whole no. (Like chlorine have atomic mass of 35.5).
Dobereiner’s Triads
According
to Dobereiner’s triads elements arranged in the group of three elements so that
atomic mass of central element is appropriate mean of 1st and 3rd
element. These groups of three elements are known as Dobereiner’s triads.
Example-
Element
Li Na K
Li Na K
Atomic Mass
7 23 39
7 23 39
Mean of Atomic Masses of 1st
and 3rd elements is (7+39)/2 =
46/2 = 23
Drawback of Dobereiner’s triads : All
elements are not arranged in triads.
Newland’s Law of Octaves
According to Newland’s Law of Octaves
elements arranged in the group of 8 elements and every 8th element
is similar to the 1st element.
Li
Be B C N
O F
Na Mg Al Si P S Cl
Drawback of Newland’s Law of Octaves :
Heavy elements do not follow this
law, inert gases disturb the order of octave.
Lother Meyer Curves
In 1869, Lother Meyer derived following
conclusion by obtaining information from graph between the atomic weight at
x-axis and atomic volume at y-axis.
1.
Elements
which have similar properties found on the same position on the graph curves.
2.
All
alkali metals like Li, Na, K, Rb, Cs, and Fr found at highest peak of curve.
3.
Halogens
like F, Cl, Br and I are found on ascending portion of curve.
4.
Alkaline
earth metals like Mg, Ca, Sr, Ba and Ra are found on descending portion of
curve.
5.
Elements
which do not melt easily like Be, B, C, Al, Si, Cu etc. are found at lowest
point of curve.
On the basis of above conclusions Lother Meyer give periodic Law:
According to Lother Meyer
periodic law – “Atomic volume of elements are periodic function of their
atomic weights”.
Periodic Table
To
classify elements a chart is prepared in which elements are arranged in rows
and columns. These elements are arranged in table in such a way that the
elements with similar properties are repeat after some intervals; also elements
with similar properties are put in same column below one another. In periodic
table vertical columns are called group while horizontal rows are called
periods. In periodic table this repetition of properties in regular interval is
known as periodicity. Read below chemistry notes of periodic table like
Mendeleev periodic table, Mosley periodic table, Modern periodic table etc.
Mendeleev’s Periodic Table
In
Mendeleev’s Periodic Table, Mendeleev classify elements according to their
atomic masses and arranged these elements in table according to their
increasing order of atomic masses.
Mendeleev’s Periodic Law
According
to Mendeleev’s Periodic Law – “Physical and chemical properties of elements are
periodic function of their atomic masses”.
Mendeleev’s Periodic Table contains
seven horizontal rows known as periods and nine vertical columns known as
groups.
Groups of Mendeleev’s Periodic Table
1.
In
original Mendeleev’s Periodic Table only 8 groups present because Zero group
contains noble gases is added later after discovery of noble gases.
2.
So,
total vertical column is 9 including group 1 to 8 and Zero group.
3.
Group
1 to 7 are divided into two subgroups (A and B) each.
4.
In
eighth group, three elements found together in each period (from fourth period
onward) known as transition triplet.
Periods of Mendeleev’s Periodic Table
1.
Mendeleev’s
Periodic Table has seven periods.
2.
First
period is known as very short period because contain only two elements.
3.
Second
and Third period is known as short period because contain only 8 elements each.
4.
Fourth
and Fifth period is known as long period because contain 18 elements each.
5.
Sixth
period is known as very long period because contain 32 elements each, 18
elements in table and remaining 14 elements called Lanthanides (from atomic no.
58 to 71)
6.
Seventh
period is known as incomplete period because it contains only 24 elements and
some places are left vacant for elements as they are not known yet. 10 elements
in the table and remaining 14 elements called Actinides (from atomic no. 90 to
103) placed outside at bottom of periodic table.
Demerits of Mendeleev’s Periodic Table
These below
points are some Demerits of Mendeleev’s Periodic Table
1.
Position
of Hydrogen: Hydrogen is placed in 1st as well as in 7th
group because its properties are similar to both groups, which is not explained
well.
2.
Elements
which have similar properties are placed apart in different groups.
Example: Copper
and Mercury with similar properties placed in group first and second.
3.
Elements
with different properties are placed in one group.
Example: Coin
metals like Li, Na, K, Rb, Cs.
4.
Atomic
weights are not kept in serial order.
Example: Argon
with higher atomic weight 39.948 is placed before potassium, which have lower
atomic weight of 39.102
5.
Position
of isotope is not proper because all isotopes of elements are placed in one
group though the atomic weights of isotopes are different.
6.
Eighth group elements position is not
appropriate because elements put in triplet form.
7.
Rare
earth elements (Lanthanides and Actinides) position is not proper so they placed
outside at bottom of periodic table.
Modern Periodic Law and Modern Periodic Table
Moseley
in 1913, after doing many experiments comes to a conclusion that the elements
should be arranged as per their atomic no. and not according to atomic masses.
As atomic no. is fundamental property of all elements of periodic table. So Mosley
gives a new law which is called is Modern Periodic Law.
Modern Periodic Law
According to Modern Periodic Law- “Physical
and Chemical properties of elements are periodic function of their atomic
numbers”.
Long Form of Periodic Table
Bohr Bury invented new periodic table,
which is called as Long Form of Periodic Table. This Long Form of Periodic
Table is also known as Modern Periodic Table.
Special features of Modern or Long Form of Periodic Table
These below are Special features of
Modern or Long Form of Periodic Table
1.
18
vertical columns known as groups.
2.
Horizontal
rows known as periods.
3.
Light
metals – These are elements of periodic table of group 1 and 2.
4.
Heavy
metals or Transition metals - These are elements of periodic table of group 3,
4, 5, 6, 7, 8, 9, 10, 11 and 12.
5.
Non-Metals
– These are elements of periodic table of group 13, 14, 15, 16 and 17.
6.
Zero
group – These are elements of periodic table of group 18.
Properties of periods
1.
All
periods start with alkali metal and end with noble gases.
2.
If
we see table, it is very clear that from 1st to 7th
period there is an addition of one shell in the each period like n=1,2,3,4,5………………
Properties of groups
1.
In
any group, outermost shell electron are known as valance electrons and these
electrons are same so main properties of elements of group is similar.
2.
Elements
are divided into four blocks, which is s, p, d, f according to valance electrons.
3.
s-block
elements – elements of 1 and 2 group.
4.
p-block
elements – elements of 13 to 18 group.
5.
d-block
elements – elements of 3 to 12 group.
6.
f-block
elements – elements of the Lanthanide and Actinide series.
7.
Representative
Elements – elements of s-block and p-block collectively called as Representative
elements also known as Normal elements or Typical elements.
8.
Transition
Elements – elements of d-block.
9.
Inner
Transition Elements – elements of f-block, also known as Rare Earth Elements.
10.
Alkali Metals – elements of 1st
group.
11.
Alkaline Earth Metals – elements of 2nd
group.
Solution for the demerits of Mendeleev’s Periodic Table in the Long Form of Periodic Table (or Modern Periodic Table)
1.
Mendeleev’s
Periodic Table contains some heavy elements placed before lighter elements,
this problem is solved automatically in Modern Periodic Table
2.
All
isotopes of an element have same atomic number so no need to place them
separate.
3.
All
inert gases (or noble gases) got their appropriate place in zero group (or 18th
group).
4.
Mendeleev’s
Periodic Table have problem with 8th group but it is resolved in Modern
Periodic Table as it is divided into three groups.
Drawback of Modern Periodic Table
1.
Position
of Hydrogen is still not clear in Modern Periodic Table.
2.
Position
of f-block elements (Lanthanides and Actinides) is not proper as these placed
at bottom, separately from periodic table that is impractical.
Periodic Properties of the Elements
Periodic properties are the
properties of the elements which depend on their electronic configuration and
these properties changes on moving down in the group and on moving left to
right in the period of the periodic table. Some periodic properties are atomic
size, electron affinity, ionization energy, metallic and non metallic
properties etc. are the main periodic properties.
Atomic Size
Size
of any atom is explained with the help of atomic radius.
Atomic Radius
The
distance between outermost shell electrons and centre of nucleus of isolated
atom is called atomic radius.
There are two hypotheses for the
calculation of atomic radius i.e.
1. Van der Waals Radii
It is the one half of the distance
between the two adjacent atoms nuclei centres, is known as Van der Waals radii.
Van der Waals radii is calculated as follows, first
calculate the distance from the centre of nuclei of one atom to centre of
nuclei of another adjacent atom in solid state of same substance. Then divide this
distance by two. Van der Waals distance is represented by Angstrom A.
Where, 1 A = 10-8 cm.
2. Covalent Radii
It is the one half of the distance
between the diatomic molecule nuclei centres, is known as Covalent Radii.
Covalent radii is calculated as follows, first
calculate the distance from the centre of nuclei of one atom to centre of
nuclei of another atom in diatomic molecule containing covalent single bond.
Then divide this distance by two.
Isolated Atom
Isolated
atom is an absolutely alone single atom.
Periodicity in Atomic Size
In Period: On moving left to right in any
period atomic size decreases because atomic radii decreases, As electrons are
attracted towards nucleus due to increasing nuclear charge.
In Group: On moving down in any group atomic
size increases because atomic radii increase, as more electrons shells added.
An ionic radius of cation is smaller
than its normal atom while that of anion is larger than its normal atom.
Cations are formed by loss of
electron or electrons and carry positive charge.
Anions are formed by gain of electron
or electrons and carry negative charge.
Ionization Energy
The
energy required for the removal of one electron from the isolated atom in
gaseous state is called ionization energy.
Ionization Energy is represented by
I.E.
Ionization Energy is expressed in Electron
Volts per Atom or Kilo Joules per Mole or Kilo Calorie per Mole.
Ionization Energy is also known as Ionization Potential.
Equation for I.E.
Atom (g) + Ionization Energy (I.E.)
---ร Cation + Electron (e-)
Removal of further electron from
cation is difficult so value of Ionization Energy kept on increasing.
I.E.1 < I.E.2
< I.E.3 < I.E.4 …………………………………..
Periodicity in I.E. (Ionisation Energy)
In Period: On moving left to right in any
period ionization energy increases because nuclear charge increases with the
increasing atomic number, so more energy required to remove electron. Therefore
value of I.E. increases on moving left to right in any period.
In Group: On moving down in any group ionisation
energy decreases because atomic size increase, as more electrons shells added.
Electron Affinity
The
amount of energy release as a result of addition of electron in any atom in
gaseous state to form anion is known as Electron Affinity.
Electron Affinity is represented by
E.A.
Electron Affinity is expressed in Electron
Volts per Mole or Kilo Joule per Mole
Values of all electron affinity are
negative except first Electron Affinity which is positive.
Periodicity in E.A. (Electron Affinity)
In Period: On moving left to right in any
period the value of Electron Affinity increases because atomic size decreases
so amount of energy released is more.
In Group: On moving down in any group the
value of Electron Affinity decreases with some irregularities.
Metallic and Non-Metallic characters
In Period: On moving left to
right in any period Metallic character decreases and Non Metallic character
increases.
In Group: On moving down in any group Metallic
character increases and Non Metallic character decreases.
In Periodic Table: Metallic elements are found on left
hand side and Non Metallic elements are found on right hand side.
Metalloids
The elements which are
neither metals nor non metals are known as metalloids.
Example of Metalloids: B, Si, As, Te etc.
) particle scattering experiment. Rutherford bombard alpha particle on gold foil of thickness 0.0004cm and found that-
rays, mass of alpha particles is 4 a.m.u. and charge is 2 units of positive charge.
rays, mass and charge of beta particle is equal to mass and charge of electron.
rays, gamma rays are electromagnetic rays and not made from matter particle.