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Chapter: Periodic Classification of Elements
Subject: Science (Chemistry)
Class: X
Chapter-5: Periodic Classification of Elements
(Notes)
Introduction
• Matter around us is present in the form of elements, compounds and mixtures.
• Elements are substances containing atoms of only one type. E.g., Na, Mg, Au, etc.
• There are 118 elements known to us. All these have different properties.
• To make the study of these elements easy, these elements have been divided into few groups in
such a way that elements in the same group have similar properties.
Dobereiner’s Triads
• 30 elements had been discovered by 1829.
• Dobereiner, a German scientist, divided some of these elements into a group of
3 and named them as Triads.
• Elements in his triads have similar chemical properties.
• The atomic mass of the middle element of the triad was roughly the average of
the atomic masses of the other two elements.
Triad Elements Atomic Arithmetic Mean
Mass
Lithium 7 !"#$
Triad-1 Sodium 23 % = 23
Potassium 39
Chlorine 35.5 #&.& " )%*.&
Triad-2 Bromine 80 % = 81
Iodine 126.5
Calcium 40 +, ")#!
Triad-3 Strontium 87 % = 88
Barium 137
Limitations of Dobereiner’s Triads
Only three triads were recognized from the elements known at that time. Only 9 elements (as mentioned
above) out of 30 could be arranged in the triad form.
Newland’s Law of Octaves
• By 1866, 56 elements had been discovered.
• Newland, an English scientist, arranged the then known elements in the order of
th
increasing atomic masses and found that the property of every 8 element repeats.
th st th
(i.e. property of 8 element = property of 1 element; property of 9 element =
nd
property of 2 element and so on)
• He compared this to the octaves found in music (sa, re, ga, ma….) and called it
the ‘Law of Octaves’. For example, the properties of lithium (Li) and sodium (Na)
were found to be the same.
Chapter: Periodic Classification of Elements
Subject: Science (Chemistry)
Class: X
Limitations of Newland’s Law of Octaves
• It was applicable for 17 elements only up to calcium (for lighter elements only).
• Properties of new discovered elements did not fit into the Law of Octaves.
• At certain places, two elements had been put together in one slot and that too in the column of
unlike elements having very different properties.
• Placing of iron (Fe) far away from nickel (Ni) and cobalt (Co), having properties similar to that of
iron, could not be explained.
Mendeleev’s Periodic Table
• Dmitry Mendeleev, a Russian chemist, in 1869 gave Mendeleev’s Periodic Table.
• 63 elements were discovered by then.
• Mendeleev arranged elements in the increasing order of their atomic mass and found that element
with similar properties occur at regular intervals.
• He tried to put elements with similar properties in a group. Due to this, certain empty boxes are
found in his periodic table.
• According to Mendeleev “the properties of the elements are a periodic function of their atomic
masses.”
• The horizontal rows present in the periodic table are called periods.
• The vertical columns present in it are called groups. There were total eight groups in Mendeleev’s
periodic table, I to VIII.
• Properties of elements in a particular period show regular gradation (i.e. increase or decrease) from
left to right.
• Groups I to VII are subdivided into A and B subgroups. Groups VIII don’t have any subgroups.
• All the elements in a particular group have similar properties. They show regular gradation in their
physical properties and chemical reactivities.
Merits of Mendeleev’s Periodic Table
(i) Mendeleev predicted properties of elements on the basis of their positions in the periodic table.
(ii) 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.
(iii) The arrangement of elements in groups and periods made the study of elements quite systematic
in the sense that if properties of one element in a particular group are known, those of the others
can be easily predicted.
Chapter: Periodic Classification of Elements
Subject: Science (Chemistry)
Class: X
(iv) Prediction of new elements and their properties: Many gaps were left in this table for
undiscovered elements. However, properties of these elements could be predicted in advance
from their expected position. This helped in the discovery of these elements. The elements
Gallium (Ga), Scandium (Sc) and Germanium (Ge) were discovered in this manner.
(v) Correction of doubtful atomic masses: Mendeleev corrected the atomic masses of certain
elements with the help of their expected positions and properties.
Limitations of Mendeleev’s Classification
(i) Isotopes are atoms of same element having different atomic masses but have similar chemical
properties. Isotopes are placed together by Mendeleev as they have similar properties. But then
this violated the arrangement scheme of increasing atomic masses. Mendeleev could not explain
that problem.
(ii) Properties of hydrogen (H) are similar to Group-I as well as Group-VII. But Mendeleev placed
it in Group-I without any proper explanation.
(iii) Wrong order (decreasing order) of atomic masses of some elements could not be explained.
Example – Co, Ni and Te, I.
Terms/ Definitions to Remember (for Modern Periodic Table)
Atomic Number
• Atomic number is defined as the total number of protons present in the nucleus of an atom. It is
denoted by ‘Z’.
• Atoms of two different elements will always have different number of protons.
• Atoms of same element have same number of protons and thus they have same atomic number ‘Z’.
In fact, elements are defined by the number of protons they possess. For hydrogen, Z = 1, because in
hydrogen atom, only one proton is present in the nucleus.
Electron Distribution in Orbits
• It is arrangement of electrons in atomic orbitals.
Rules for Electron Distribution are as follows:
2
(i) An orbit can have a maximum of 2n e.
No. of Electron Shell Maximum Capacity
electrons
2
1 K Shell 2(1) = 2 electrons
2
2 L Shell 2(2) = 8 electrons
2
3 M Shell 2(3) = 18 electrons
2
4 N Shell 2(4) = 32 electrons
(ii) Orbits are filled from inside to outside. First, n = 1 shell is filled, then n = 2 shell, and so on.
(iii) The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has a
capacity to accommodate more electrons. It is very important rule and is also called the Octet
Rule. The presence of 8 electrons in the outermost shell makes the atom very stable.
Chapter: Periodic Classification of Elements
Subject: Science (Chemistry)
Class: X
Electronic configuration of some elements:
Valency
• Valence Electrons: Valence electrons are the electrons in the outermost orbit of an atom.
Outermost orbit is also called valence shell. On moving from left to right in each period, the
valency of elements increases from 1 to 4 and then decreases to 0. Valency remains the same down
in a group.
Third period elements Na Mg Al Si P S Cl Ar
Valency 1 2 3 4 3 2 1 0
Valency remains the same down in a group.
• Stable and Unstable Electronic Configuration: If K shell is outermost shell of an atom and if
-
the atom has 2e in outermost shell OR if K shell is not the outermost shell of an atom and if the
-
atom has 8e in outermost shell, the arrangement of electrons is called stable electronic
configuration.
§ Atoms do chemical reactions with each other to achieve stable electronic configuration.
§ Noble gases (He, Ne and Ar) are inert as they already have stable electronic configuration.
§ Valency of an element is the number of electrons that its atom should give away or take to
attain stable electronic configuration.
§ The number of electrons present in the valence shell/outermost shell determines the valency.
§ Silver has 1 electron in its outermost shell. Silver donates one electron to complete its octet
so valency of silver is 1.
Modern Periodic Table
• In 1913, Moseley showed or proved that atomic number is a very important property of an element.
• After that, Neil Bohr made the Modern Periodic Table using atomic number.
Basic concept of Modern Periodic Table:
• Most of the properties of an element depend on number of valence electrons.
• Elements having same number of valance electrons are grouped together.
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