2. Important Highlights
• INTRODUCTION
• RUTHERFORD’S ATOMIC MODEL
• BOHR’S ATOMIC MODEL
• DISTRIBUTION OF EXTRA-NUCLEAR
ELECTRONS IN SHELLS & SUB-SHELLS
• ELECTRONIC CONFIGURATION OF ATOM
• WAVE-NATURE OF AN ATOM
• ATOMIC NUMBER
• MASS NUMBER BESU SHIBPUR,HOWRAH
3. Introduction
• ATOM : An atom is the smallest particle of an element, which
Can not be subdivided.
Does not exist in free state. But,
Participate in chemical reaction.
The structure of an atom can be broadly divided into two parts
Nuclear Parts
Extra-Nuclear Parts
Nuclear part basically consist of 2 major particles– protons &
neutrons. Apart from this there are some more tiny particles
such as deuteron, neutrino, positron etc.
Extra-Nuclear parts consists of mainly electrons.
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4. Rutherford’s Atomic Model
• Also known as “Planetary Model” of Atom.
K e y F e a tu re s :
• An atom consists of a central nucleus. This nucleus is
composed of positively charged protons, and electrically
uncharged (neutral) neutrons.
• Negatively charged electrons revolve around the nucleus
in definite orbits.
• The orbits themselves can be at any distance from the
nucleus.
• In any atom, the number of protons is equal to the number
of electrons, and hence it is electrically neutral.
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5. Drawbacks of Rutherford’s Model
• It could not explain the stability of the atom. A/c to
classical theory of mechanics, during uniform revolution, any
body accelerates, & an accelerating charged particle must
emit radiation, and lose energy. Due to the fact, the electron
must emit radiation and lose energy. As a result, the
electron will follow a spiral path, and ultimately fall into
nucleus .So, Rutherford has given concept of unstable atom.
• The Rutherford’s model of atom does not say anything about
the arrangement of electrons in an atom.
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7. Bohr’s Atomic Model
The main points of Bohr’s atomic model are….
• Electrons revolve around the nucleus in orbits that have
fixed radius and energy. So, such orbits are also known
as stationary orbit ,Energy shells etc.
• The energy of the orbit is related to its size. The lowest
energy is found in the smallest orbit and its vice-versa.
• Radiation is absorbed or emitted when an electron jumps
from one orbit to another.
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8. Bohr postulated that the angular momentum of the electron is
quantized as shown…
Where, h=6.626*10^(-34) Js
Then by using simple physical equation he deduced the
expression for energy,velocity,radius,wavelength,wave-no. etc.
L=nh/2ᴨ= mvr
F= Ze*e/r*r=mv*v/r
Where,L=Angular accn., Z=Atomic number, v=Orbital velocity
m=mass of electron, n=Principal Quantum Number
r=Radius of Orbit, h=Planck’s constant=6.626*10^34Js
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9. Deduced Formulae:
• Radius, r= (n^2/Z)*0.529 Å
• Energy, E= -(Z^2/n^2)* 13.6 eV
• Wave No.=1/wave-length=R(1/nf2-1/ni2)
• Velocity, v=(Z/n)*21.88*10^5 m/s
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10. e.g. Emission of a Quantum:1st Balmer Transition
-3.4eV
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11. Limitation of Bohr’s Atomic Model
• The Bohr atomic model is applicable only for one-electron
system. e.g. H, He+, Li++, Be+++ etc.
• The Bohr Model provides an incorrect value for the ground
state orbital angular momentum.
• It does not predict the relative intensities of spectral lines.
• The Bohr Model does not explain fine structure and hyperfine
structure in spectral lines.
• It violates the Heisenberg Uncertainty Principle because it
considers electrons to have both a known radius and orbit.
• It does not explain the Stark’s & Zeeman’s Effect.
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12. Concept of shells and subshells
• Shells, n, represents the distance of an electron from
Nucleus. It is represented by K,L,M,N,O,…for n=1,2,3,4,..
• Shell no., n, also represents the maximum no. of electrons
accommodated by a particular shell, which is 2n(2). i.e. for
n=1, First Shell, maximum no. of electrons=2.
Similarly, for n=2, it’s 8.
n=3, it’s 18 and so on.
• Subshell represents the traversal path of any electron in
3D view or, shape of the Orbital. For any Orbit ,n, it’s
value ranges from 0 to(n-1). It is represented by s,p,d,f,…
for l=0,1,2,3,…
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13. Quantum Numbers of Wave funtions
Quantum # Symbol Values Description
Principal n 1,2,3,4,… Size & Energy of orbital
Angular l 0,1,2,…(n-1) Shape of orbital
Momentum for each n
Magnetic ml -l…,0,…+l Relative orientation of orbitals within
for each l same l
Spin ms +1/2 or –1/2 Spin up or Spin down
Angular Momentum Quantum # l Name of Orbital
0 s (sharp)
1 p (principal)
2 d (diffuse)
3 f (fundamental)
4 g
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14. Electronic Configuration
The electrons gather around the nucleus in quantum orbitals
following four basic rules, called the Aufbau principle.
• no two electrons in the atom will share the same four quantum
numbers n, l, m, and s. (Pauli’s Exclusion Principle)
• electrons will first occupy orbitals of the lowest energy level.
• orbitals will be filled with the same spin number until all the
orbitals are filled, before it will begin to fill in the opposite spin.
(Hund’s Rule of maximum multiplicity)
• (n+l) Rule: electrons will fill orbitals by the sum of the
quantum numbers n and l. In case equal values of (n+l), orbitals
with the lower value of n, will be filled first.
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15. Electronic Configuration contd.
Hence, Aufbau Principle Gives order of filling of electrons in
different subshells. It can be shown as follows….
• 1s2
• 2s22p6
• 3s23p63d10
• 4s24p64d104f14
• 5s25p65d105f14(5g18)
• 6s26p66d106f14(6g18)(6h22)
• 7s27p67d106f14(6g18)(6h22)(7i26)
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16. Electronic Configuration contd.
• The electronic configuration of elements generally follows
Aufbau principle.
• except some cases viz. Cu,Cr type d-block elements(due
to some stability factor).
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17. Wave Nature of an Atom
• Wave–particle duality postulates that, all particles exhibits
both particle as well as wave characteristics.
• Louis-Victor de Broglie describes the atom considering
"Dual Properties of Matter”.
• He formulated a hypothesis, claiming that all matter, not just
light, has a wave-like nature; he related wavelength (denoted
as λ), and momentum (denoted as p):
λ =h/p
This is the generalization of Einstein’s and Planck’s Equation.
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18. Atomic Number
• Atomic Number, Z: The number of protons in an atom.
• As atoms are neutral,
So,
N(p)=N(e)=Z
Where,
N(p)= number of protons
N(e)=number of electrons
Z=Atomic Number
. BESU SHIBPUR,HOWRAH
19. Mass Number
Mass Number: The total numbers of neutrons and
protons in an atom.
So, if we know mass no. and atomic number, we can
find out the number of neutrons in an atom .
Mass Number - Atomic Number=Number of neutrons
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20. Notation of an Atom
Mass Number
65
Cu
29
Atomic Number
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