Periodic Classification of Elements
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• Doberiner’s Law of Triads : According to this law, “in certain
triads (group of three elements) the atomic mass of the
central element was the arithmetic mean of the atomic masses of the other two elements.”
E.g., atomic masses of Li, Na and K are respectively 7, 23 and
39, thus the mean of atomic masses of 1st and 3rd element is
7 +39/2=23
Limitations of Doberiner’s Triads: He could identify only a
few such triads and so the law could not gain importance. In the triad of Fe, Co, Ni, all the three elements have a nearly
equal atomic mass and thus does not follow the above law.
• Newland’s Law of Octaves : According to this law “the
elements are arranged in such a way that the eighth element starting from a given one has properties which are a repetition of those of the first if arranged in order of increasing atomic weight like the eight note of musical scale.”
Drawback of newland’s law of octaves:
(i) This law was applicable for lighter elements e.g., H, F and
Cl, but fails in the case of heavier elements.
(ii) According to Newland only 56 elements exists in nature.
(iii) No space was left for the elements which were unknown
at that time.
(iv) In order to fit new elements into his table Newland adjust
two elements in the same column, but put some unlike elements under the same column.
• Mendeleev’s Periodic Table : According to Mendeleev “the
properties of the elements are a periodic function of their
atomic masses.”
• Merits of Mendeleev’s Periodic Table: Mendeleev left some
gap for new elements which were not discovered at that time e.g., gallium and germanium were not known at that time.One of the strengths of Mendeleev’s periodic table was that, when inert gases were discovered they could be placed in a new group without disturbing the existing order.
• Characteristics of the periodic table :
(i) In the periodic table, the elements are arranged in vertical
rows called groups and horizontal rows called periods.
(ii) There are eight groups indicated by Roman Numerals I,
II, III, IV, V, VI, VII, VIII. The elements belonging to first
seven groups have been divided into sub-groups
designated as A and B on the basis of similarities. Group
VIII consists of nine elements arranged in three triads.
(iii) There are six periods (numbered 1, 2, 3, 4, 5 and 6). In
order to accomodate more elements, the periods 4, 5, 6 are
divided into two halves.
• Achievements of mendeleev’s periodic table
(i) The arrangement of elements in groups and periods made the study of elements quite systematic.
(ii) Many gaps were left in this table for undiscovered elements. However, properties of these elements could
be predicted in advance from their expected position.
(iii) Mendeleev corrected the atomic masses of certain elements with the help of their expected positions and
properties.
• Limitations of mendeleev’s classification :
(i) He could not assign a correct position of hydrogen in his
periodic table, as the properties of hydrogen resembles
both with alkali metals as well as with halogens.
(ii) The isotopes of the same element will be given different
position if atomic number is taken as basis, which will disturb the symmetry of the periodic table.
(iii) The atomic masses do not increases in a regular manner
in going from one elements to the next.
• Modern Periodic Law : This law was given by Henry
Moseley in 1913. It states, “Properties of the elements are
the periodic function of their atomic numbers”.
• The cause of periodicity is the resemblance in properties of
the elements is the repetition of the same valence shell
electronic configuration.
• Modern Periodic Table
(i) The modern periodic table has 18 vertical columns called
“groups” and seven horizontal rows called “periods”.
(ii) The elements belonging to a particular group make a family
and usually named after the first member. In a group all
the elements contain the same number of valence
electrons. e.g., in halogen family all the elements i.e., F, Cl,
Br, I have 7 electrons in their valence shell.
(iii) In a period all the elements contain the same number of
shells, but as we move from left to right the number of
valence shell electrons increases by one unit. The
maximum number of electrons that can be accommodated
in a shell can be calculated by the formula 2n^2
where n is
the number of the given shell from the nucleus e.g.
K shell = 2 × (1)^2
= 2
L shell = 2 × (2)^2
= 8
M shell = 2 × (3)^2
= 18
N shell = 2 × (4)^2
= 32
• Important characteristics of groups in a modern periodic
table :
(i) The elements present in a group are separated by definite
gaps of atomic numbers (8, 8, 18, 18, 32)
(ii) There are 18 groups in long form of periodic table.
(iii) The elements in a group have the same valency.
(iv) The elements present in a group have identical chemical
properties.
• Characteristics of periods : We know that there are seven
periods in the modern periodic table.
(i) In all the elements present in a period, the electrons are
filled in the valence shell.
(ii) As the number of electrons in the valence shell change,
there also occurs a change in the chemical properties of
the elements present in a period.
• Merits of modern periodic table over Mendeleev’s periodic
table :
(i) Position of isotopes : All isotopes of the same elements
have different atomic masses but same atomic number. Therefore, they occupy the same position in the modern
periodic table which they should have because all of them
are chemically similar.
(ii) Anomalous pairs of elements : When elements are
arranged in the periodic table according to their atomic numbers the anomaly regarding certain pairs of elements
in Mendeleev’s periodic table disappears.
(iii) It explains the periodicity of the properties of the elements
and relates them to their electronic configurations.
(iv) The table is simple, systematic and easy way for
remembering the properties of various elements and
moreover lanthanides and actinides are placed separately.
• Classification based on differentiating electron :
s-block elements : Those elements of the periodic table in
which the last electron enters in s–orbital, are called
s-block elements. s-orbital can accommodate a maximum of
two electrons. Their general formulae are ns1
and ns2 respectively, where n = (1 to 7).
p-block elements : Those elements of the periodic table in
which the last electron gets filled up in the p-orbital, called p- block elements. A p-orbital can accommodate a maximum of
six electrons.
d-block elements : Those elements of the periodic table in
which the last electron gets filled up in the d-orbital, called d block elements.
f-block elements : Those elements of the periodic table in
which the last electron gets filled up in the f orbital, called f-block elements. There are 28
f-block elements in the periodic
table. The elements from atomic number 58 to 71 are called
lanthanides because they come after lanthanum (57). The
elements from 90 to 103 are called actinides because they come after actinium (89).
• Trends in the modern periodic table :
(i) Valency : The combining capacity of an atom or radical is
known as its valency. Valency of an element is defined as the number of hydrogen, chlorine and double the number of
oxygen atom with which atom of an element can combine.
Valency in a period : The number of valence electrons increases
in a period from 1 to 8 from left to right. It reaches 8 in group 18 elements (noble gases) which show practically no chemical
activity under ordinary conditions and their valency is taken
as zero.
Valency in a group : All the elements of a group have the
same number of valence electrons. Therefore, they all have the same valency.
(ii) Atomic Radii : For an isolated atom atomic radius may be
taken as the distance between the centre of nucleus of atom and the outermost shell of electrons.
Variation of atomic radii in a period : In a period there is a
gradual increase in the nuclear charge with increase in atomic number. Since valence electrons are added in the same shell since, the electrons in the same shell do not screen each other
from the nucleus, the increase in nuclear charge is not neutralised by the extra valence electron. As a result effective nuclear charge increases therefore valence electrons are more
and more strongly attracted towards nucleus. This gradually decreases atomic radii across a period.
Variation of atomic radii in a group : In moving down the
group the nuclear charge increases with increase in atomic number. However, while going down in a group from one atom
to another the number of inner shells also increases, although
the number of electrons in the outermost shell remains the same. The effect of increase in the size of the electron cloud (due to increase in number of shells) is more pronounced than
the effect of increased nuclear charge. Thus the distance of outermost electron from the nucleus increases as we move down a group. This gradually increases atomic radii along a
group.
(iii)Ionization Energy : The minimum amount of energy required
to remove an electron from a gaseous atom in its ground state to form a gaseous ion is called ionization energy. It is measured in unit of kJ mol–1 .
Variation of ionization energy in a group : Force of attraction
between valence electrons and nucleus decreases in a group from top to bottom because of increase in atomic size due to addition of inner shells. As a result, the electron becomes less
and less firmly held to nucleus as we move down the group.
Ionization energy decreases in a group from top to bottom.
Variation of ionization energy in a period : We know that the force of attraction between valence electron and nucleus increases in a period from left to right due to increase in nuclear
charge. As a consequence of this, the ionization energy
generally increases in a period from left to right.
(iv) Electron Affinity : It is the energy change when an electron is
accepted by an atom in the gaseous state. It corresponds to the process :
X (g) + e ➡X‾(g) + E
Variation of electron affinity in a group : In a group, the
electron affinity decreases on moving from top to bottom.
Variation of electron affinity along a period : On moving across a period, the size of atom decreases and nuclear charge increases. Both of these factors result into greater attraction
for incoming electron. Thus electron affinity increases in a
period from left to right.
(v) Electronegativity : Electronegativity is relative tendency of a
bonded atom to attract the bond-electrons towards itself. Electronegativity generally decreases in a group from top to bottom. Electronegativity generally increases in a period from left to
right.
(vi) Metallic and Non-Metallic Character : Characteristic
properties of a metal. They are electropositive in nature (the tendency to lose electrons), have luster, ductility, malleability
and electrical conductance.
Variation of metallic character in a group : Metallic character of elements increases from top to bottom. As we move down in a group atomic size increases therefore distance between
valence electrons and nucleus also increase. Thus electrostatic force of attraction on valence electrons decreases and they can be easily removed.
Variation of metallic character in a period : Metallic character of elements decreases in a period from left to right. As we move from left to right in a period atomic size decreases. Thus
electrostatic force of attraction increases for valence electrons
thereby decreasing electropositive character.
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