Chapter - 4 "Structure of Atom" Concept Details Key Concepts
Chapter - 4 "Structure of Atom" Concept Details Key Concepts
Chapter - 4 "Structure of Atom" Concept Details Key Concepts
CONCEPT DETAILS
Pre requisites:
Difference between an atom & molecule.
Gram atomic mass & Molar mass.
Dalton’s Atomic theory.
SURVEY ANALYSIS
Conceptual levels of comprehension on the basis of feedback taken from the students
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Contribution to the Atomic theory Timeline
Democritus
Greek philosopher
(400 B.C)
More than 2400 years ago, he named the smallest piece of matter “ATOMOS ,”
meaning “not to be cut.”
To Democritus,
Atoms were small, hard particles that were all made of the same material but were
different shapes and sizes.
Atoms were infinite in number, always moving and capable of joining together
John Dalton
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First recorded evidence that atoms existed.
Using his theory, Dalton rationalized the various laws of chemical combination
Dalton's theory was based on the premise that the atoms of different elements could be
distinguished by differences in their weights.
Limitations
o The indivisibility of an atom was proved wrong , for, an atom can be further
subdivided into protons, neutrons and electrons.
o The atoms of same element are similar in all respects , but isotopes of same
element have different mass.
o Dalton's theory was based on the premise that the atoms of different
elements could be distinguished by differences in their weights.
2. J J Thomson Experiments:
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An electron is a negatively charged component of an atom which exists outside the
nucleus. Each electron carries one unit of negative charge and has a very small
mass as compared with that of a neutron or proton.
JJ Thomson used cathode ray tubes to demonstrate that the cathode ray responds to both
magnetic and electric fields.
Since the ray was attracted to a positive electric plate placed over the cathode ray tube
(beam deflected toward the positive plate) he determined that the ray must be composed of
negatively charged particles.
He called these negative particles "electrons."
Limitation: Model failed to explain how protons and electrons were arranged in atom so
close to each other.
Eugene Goldstein:
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E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and
called them canal rays. These rays were positively charged radiations which ultimately
led to the discovery of another sub-atomic particle.
Used a Cathode Ray Tube to study "canal rays" which had electrical and magnetic
properties opposite of an electron
Canal Rays: The positively charged radiation produced in the discharge tube at low
pressure and high voltage are called canal rays.
Protons:
The canal rays have positively charged sub-atomic, particles known as protons (p).
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Experiment: Rutherford took a thin gold foil and made alpha particles , [ He 2+ ] positively
charged Helium fall on it.
Alpha particle
Limitation: In Rutherford’s atomic model , Nucleus & electrons are held together by
electrostatic force of attraction which would lead to the fusion between them. This
does not happen in the atom.
Atomic radius ~ 100 pm = 1 x 10-10 m
Nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m
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SCIENTIST PROPOSED ATOMIC MODEL
James Chadwick
In 1932, James Chadwick proved that the atomic nucleus contained a neutral particle
which had been proposed more than a decade earlier by Ernest Rutherford officially
discovered the neutron in 1932,
Chadwick received the Nobel Prize in 1935.
A neutron is a subatomic particle contained in the atomic nucleus. It has no net electric
charge, unlike the proton's positive electric charge.
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4. Sub Atomic Particles:
Name Symbol Location in the atom Charge Relative Mass Actual Mass (g)
Electron E Around the nucleus 1- 1/1840 9.11 x 10 -28
Proton P In the nucleus 1+ 1 1.67 x 10 -24
Neutron n In the nucleus 0 1 1.67 x 10 -24
“Atomic number of an element is defined as the number of unit positive charges on the
nucleus (nuclear charge) of the atom of that element or as the number of protons present
in the nucleus.”
Mass number[ A] : It is defined as the sum of the number of protons & neutrons present in
the nucleus of an atom.
Mass Number = Mass of protons + Mass of neutrons
Eg :- Carbon – Mass number = 12 (6 protons + 6 neutrons) Mass = 12u
Aluminium – Mass number = 27 (13 protons + 14 neutrons) Mass = 27u
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mass number
atomic number Symbol
Q.1 The mass number of an element is 18. It contains 7 electrons .What is the number of
protons and neutrons in it ?
Q.2 An atom contains 3 protons , 3 electrons and 4 neutrons .
i) What is its atomic number ? ii) What is its mass number?
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Q.3 An element is represented by 15 P . Give the number of electrons and neutrons in it.
Danish physicist
Main Postulates of the Bohr Model [refer NCERT Text Book article 4.3 ,page number-49]
Q.1 What happens when an electron jumps from lower to higher energy level?
Q.2 Which energy shell is nearest to the nucleus of an atom?
Q.3 Which energy shell has higher energy L or N ?
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The outermost shell of an atom cannot accommodate more than 8 electrons, even if it
has a capacity to accommodate more electrons. This is a very important rule and is also
called the OCTET RULE. The presence of 8 electrons in the outermost shell makes the
atom very stable.
Q.1 An atoms has atomic number 13. What would be its configuration.
Q.2 What is octet rule?
Q.3 How many electrons M shell can accommodate?
Q.4 If an atom has complete K and L shell, what would be its atomic number?
ISOTOPES ISOBARS
Chemically same , physically different Chemically different , physically same
Number of electrons is same Number of electrons is different .
Cannot be separated by chemical means Can be separated by chemical means
[ for application of isotopes refer NCERT Text Book article 4.6 ,page number-53]
Q.1 Why atoms of isotopes are chemically same?
Q.2 Give the representation of three isotopes of carbon which are C-12 , C-13 and C-14.
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