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Structure of atom
Introduction
Matters are made of tiny particles called atom
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These particles are
called fundamental particles of an atom or sub atomic particles
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Further experiments were performed which show that atom is
made up of smaller particles called subatomic particles or
fundamental particles, which are electron , proton and neutron
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That is why we consider the mass of electron negligible
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Structure of an atom
 Thomson’s model –
This model is also known as plum
pudding model or water melon model
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of electrons embedded to neutralize the positive
charge
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9%) pass through the foil without undergoing any deflection
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 Very few (only 1 in 20,000)were deflected
Bach at more than 90 degree
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 Some alpha particles were deflected
through small angles , these could be deflected only by some
positively charged body present within the atom
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 Drawbacks of Rutherford’s Model –
 It does not obey the Maxwell theory of
electrodynamics, according to it “A small charged particle moving
around an oppositely charged centre continously loses its
energy”
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But We know that atoms are quite stable
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;

 Bohr’s model –
In 1930 Neil’s Bohr proposed a new model that
electron revolve only in certain fixed orbit (discrete orbit)
around the nucleus without losing energy
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 These orbits are also known as energy
levels or shells
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These shells are represented by “K,
L,M,N,……
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The atom is stable and do not
collapse
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 Energy is lost or gain by an electron only
when it jumps from one orbit to the another
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 Neutron –
In 1932, James Chadwick confirmed the existence of
neutron in the nucleus
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He found that
some neutral particles were ejected from the nucleus which
carried no charge but had a mass nearly equal to that of
proton (1
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11x10-31
Mass of proton = 1
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675x10-27

 Distribution of Electron in Shells –
The distribution or
arrangement of the electrons in the different shells of the atom
is called the electronic confrigation of the element
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ofd
shells is equal to 2n2
K, n=1
2n2
=2(1)2
=2x1
=2
L, n=2
2n2
=2(2)2
=2x4
=8

M, n=3
2n2
=2(3)2
=2x9
=18
N, n=4
2n2
=2(4)2
=2x16
=32

 Valency –
The no
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Valency is the combining capacity of the atom
and their tendency to react and form molecule with same or
different element
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Hence its valency is 1
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of an element is equal to no
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 Atomic no
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 No two elements have the same atomic no
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of element does not change during a chemical
reaction
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= no
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of an element is the sum of the no
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Mass no
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of Proton +No
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of proton = Atomic no
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of electron
No
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– Atomic no
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but different
mass no
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Isotopes of an element differ only in the no
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 Isotopes of Hydrogen –

 Isotopes of Carbon –

 Isotopes of Oxygen –

 Isotopes of chlorine –

 Isotopes of Uranium –

 Characteristics of Isotopes –
 Same atomic no
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 Same chemical properties
 Different physical properties
 Different nuclear properties
 Average Atomic Mass –
Atoms with different mass no
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I-131 is used in the diagnosis and
goiter disease
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 In geological dating –
U 238 is used to find out the age of
rocks or earth
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 Isobars –
Elements which have same mass no
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THANK YOU FOR READING



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