3. CLASSIFICATION
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Placing similar groups and species together is known
as Classification. Classification is needed to easily
understand the properties of different elements in a
periodic table.
4. DOBEREINER’S TRIAD
Dobereiner’s triad consider three elements,
in which atomic mass of central element is
the arithmetic mean to two other elements.
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5. LIMITATIONS
• Dobereneir’s idea of
classification of elements
into triads did not receive
wide acceptance as he
could arrange only 9
elements in triadform.
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6. NEWLAND’S
LAW OF
OCTAVES
• Newland an English chemist gave Law of Octaves.
• Till then 56 elements were known.
• Law of Octaves says that “If elements are arranged
by the increasing order of their atomic masses,
property of every eighth element (starting from
first element) repeats”.
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7. CHARACTERISTICS OF LAW OFOCTAVES
• It contains the elements
starting from hydrogen and
ends at thorium.
• Properties of every eighth
element follow of that of first
element.
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8. LIMITATIONSOF NEWLANDS LAW OF OCTAVES
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• Similarity in properties of elements as per the law was seen up to
calcium only.
• Only 56 elements known that time were talked about.
• At many places, 2 elements were placed in a single slot (Co and Ni).
• Placing of iron far away from cobalt and nickel, which have similar
properties as iron, could also not be explained.
9. MENDELEEV’S PERIODICTABLE
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• Dmitry Mendeleev a Russian chemist in 1869 gave Mendeleev’s
Periodic Table.
• Till then 63 elements were known.
• Mendeleev arranged elements in increasing order of their atomic
mass.
• He tried to put elements with similar properties in a group.
• Due to this we find empty boxes in histable.
11. PROPERTIES OF GROUPS STUDIED BY MENDELEEV
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(a) Formation of Oxides:
Oxides are compoundsof elements with oxygen.
For example:
Li2O , Na2O and K2O resembles toR2
• MgO, CaO, ZnO resembles to RO.
12. (b) Formation ofHydrides:
• Hydrides are compounds of elements withhydrogen.
• The horizontal rows present in the periodic table are calledperiods.
• 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 toright.
• Groups I to VII are subdivided into A and B subgroups. Groups VIII don’thave
any subgroups.
• All the elements in a particular group have similar properties. Theyshow
regular gradation in their physical properties and chemical reactivities.
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13. LIMITATIONS OFMENDELEEV’S
PERIODICTABLE
1. Position of Isotopes
• 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 atomicmasses.
• Mendeleev could not explain that problem.
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14. 2. Anomalous pairsof elements
At some locations,
elements were put in
order of decreasing
atomic mass.
For example; Co, Ni
and Te, I.
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15. 3. Position ofhydrogen
Properties of H are similar
to group 1 as well as group
7. But Mendeleev placed it
in group 1 without any
proper explanation.
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16. MERITS OF MENDELEEV’S PERIODIC CLASSIFICATION
Earlier 63 elements were known.
Mendeleev discovered Prediction of new elements.
Mendeleev’s periodic table had some blank spaces in it.
For example, he proposed the existence of some unknown elements
Eka – boron → Scandium, Eka – aluminium → Gallium, Eka – silicon →
Germanium 16
17. MODERN PERIODICLAW
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• The physical and chemical properties of elements are the periodic
function of their atomic numbers.
Cause of periodicity – It is due to the repetition of same outer shell
electronic configuration at a certain regular interval.
Modern Periodic Table -
D. Mendeleev discovered the modern periodic table in the year 1869.
According to modern periodic law, “properties of an element are the
periodic function of their increasing atomicnumber”.
19. PROPERTIES OF MODERN PERIODIC TABLE
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• Isotopes are placed at one place in the same group.
• There is no element between hydrogen and helium as atomic
masses always comes in whole numbers.
• Atomic number is represented by Z, and it is equal to the number
of protons in the nucleus of the atom.
• It also consists of 18 vertical columns known as Groups and 7
horizontal rows known as Periods.
• Elements having same number of valence electrons are placed in
the same group.
20. PROPERTIES OF MODERN PERIODIC TABLE
As we go down in a
group, number ofshell
increases.
Elements having same
number of occupied
shells are placed in
same period.
Each period has a new
electronic shell getting
filled.
Number of elements
placed in a particular
period depends on the
point how electronsare
filled into various shell.
To find out the number
of electrons in a shell,
2n2 formula can be
used, where is n is shell
number.
K Shell n = 1 or 2n2 =
2(1)2 =2
L shell n = 2 or 2n2 =
2(2)2 =8
Valence electron also
determines the number
of bonds which is
formed by an element.
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21. NOBLEGASES
• The elements in the eighteenth group, helium (He), neon (Ne), argon
(Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn) are called
noble gases.
• They are all odourless, colourless and monatomic gases with very low
chemical reactivity.
• Since their valence shell is considered to be “full”, they have little
tendency to participate in chemical reactions.
• When discovered and identified, scientists thought they are exceedingly
rare, as well as chemically inert, and therefore these gases were also
given the names ‘rare’ or ‘inert’ gases.
• Number of valence electrons = 8
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22. POSITION OF ELEMENTS IN THE MODERN
PERIODICTABLE
The modern periodic table is also classified into
metals, nonmetals and metalloids.
METALS -
• Metals are electropositive as they form bonds by losing
electrons.
• In general cases, oxides of metals are basic in nature. 22
24. METALLOIDS
The elements which show
the properties of both
metals and nonmetals are
called metalloids or
semimetals.
For example – Boron, silicon,
germanium, arsenic,
antimony, tellurium, and
polonium.
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25. TRENDS IN MODERN PERIODICTABLE
• Valency and Valence Electrons - On
moving left to right in a period,
valency increases and then it
decreases. But it remains same
down in a group. As we move from
left to right in a period, valence
electron increases and remainsame
as we go down thegroup.
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26. •Atomic Size - It decreases left to right in a period as the
nuclear charge increases due to large positive charges on
the nucleus. Atomic size increases down in a group due
to decrease in nuclear charges and addition of new shell.
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27. • Metallic Character - Ability of atom to lose the electron is known
as Metallic Character. Metallic character decreases from left to right in a
period. This is due to increase in nuclear charge. But non-metallic
character increases left to right in a period. And metallic character
increases down the group as the size increases it can easily lose electron.
• Ionization Energy is the energy required to remove an electron from an
isolated gaseous atom. Ionization energy increases as we move left to
right in a period. This is due to increase in nuclear charge as we move left
to right in a period. But down in a group ionization energy decreases due
to decrease in nuclear charge but there are some exceptional cases.
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28. • Electropositive Character decreases from left to right in a
periodic table and increases down the group. This is due to
decrease in metallic character from left to right in aperiod.
• Basic Character of Oxides increases down the group as atomic
radius increases and ionization energy decreases. This is due to
increase in metallic character or electro positivity of elements.
Acidic character of oxides decreases as non-metallic character of
elements decreases from top to bottom.
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