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Points To Remember
Class: XI
Ch 2: Structure O Atom
Top Concepts
1. Atomic theory of matter was proposed by John Dalton
2. Electrons were discovered by Michael Faraday.
3. Electrons were discovered using cathode ray discharge tube experiment.
4. Cathode ray discharge tube experiment: A cathode ray discharge tube
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made of glass is taken with two electrodes. At very low pressure and high
voltage, current starts flowing through a stream of particles moving in the
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tube from cathode to anode. These rays were called cathode rays. When a
perforated anode was taken, the cathode rays struck the other end of the
glass tube at the fluorescent coating and a bright spot on the coating was
developed
Results: ing
a. Cathode rays consist of negatively charged electrons.
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b. Cathode rays themselves are not visible but their behavior can be
observed with help of fluorescent or phosphorescent materials.
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c. In absence of electrical or magnetic field cathode rays travel in
straight lines
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d. In presence of electrical or magnetic field, behaviour of cathode rays
is similar to that shown by electrons
e. The characteristics of the cathode rays do not depend upon the
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material of the electrodes and the nature of the gas present in the
cathode ray tube.
pic
5. Charge to mass ratio of an electron was determined by Thomson. The
charge to mass ratio of an electron as 1.758820 x 10 11 C kg-1
w.
6. Charge on an electron was determined by R A Millikan by using an oil drop
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experiment. The value of the charge on an electron is -1.6 x 10-19 C.
7. The mass on an electron was determined by combining the results of
Thomsons experiment and Millikans oil drop experiment. The mass of an
electron was determined to be 9.1094 x 10-31 kg.
8. Discovery of protons and canal rays: Modified cathode ray tube experiment
was carried out which led to the discovery of protons.
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9. Canal rays are positively charged particles called protons
10.Characteristics of positively charged particles:
a. Charge to mass ratio of particles depends on gas from which these
originate
b. The positively charged particles depend upon the nature of gas present in
the cathode ray discharge tube
c. Some of the positively charged particles carry a multiple of fundamental of
electrical charge.
d. Behaviour of positively charged particles in electrical or magnetic field is
opposite to that observed for cathode rays
11.Neutrons were discovered by James Chadwick by bombarding a thin sheet
m
of beryllium by - particles. They are electrically neutral particles having a
mass slightly greater than that of the protons.
.co
ing
12.Thomson model of an atom: This model proposed that atom is considered
as a uniform positively charged sphere and electrons are embedded in it.
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13. An important feature of Thomson model of an atom was that mass of atom
is considered to be evenly spread over the atom.
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14.Thomson model of atom is also called as Plum pudding, raisin pudding or
watermelon model
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15.Thomson model of atom was discarded because it could not explain certain
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experimental results like the scattering of - particles by thin metal foils
16.Observations from - particles scattering experiment by Rutherford:
pic
a. Most of the - particles (nearly 99 %) passed through gold foil undeflected
b. A small fraction of - particles got deflected through small angles
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c. Very few - particles did not pass through foil but suffered large deflection
nearly 180 o
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17.Observations Rutherford drew from - particles scattering experiment:
a. Since most of the -particles passed through foil undeflected, it means
most of the space in atom is empty
b. Since some of the -particles are deflected to certain angles, it means that
there is positively mass present in atom
c. Since only some of the -particles suffered large deflections, the positively
charged mass must be occupying very small space
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d. Strong deflections or even bouncing back of -particles from metal foil
were due to direct collision with positively charged mass in atom
18.Rutherfords model of atom: This model explained that atom consists of
nucleus which is concentrated in a very small volume. The nucleus
comprises of protons and neutrons. The electrons revolve around the
nucleus in fixed orbits. Electrons and nucleus are held together by
electrostatic forces of attraction.
19.Drawbacks of Rutherfords model of atom:
a. According to Rutherfords model of atom, electrons which are negatively
charged particles revolve around the nucleus in fixed orbits. Thus, the
electrons undergo acceleration. According to electromagnetic theory of
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Maxwell, a charged particle undergoing acceleration should emit
electromagnetic radiation. Thus, an electron in an orbit should emit radiation.
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Thus, the orbit should shrink. But this does not happen.
b. The model does not give any information about how electrons are
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distributed around nucleus and what are energies of these electrons
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20. Atomic number (Z): It is equal to the number of protons in an atom. It is
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also equal to the number of electrons in a neutral atom.
21.Mass number (A): It is equal to the sum of protons and neutrons.
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22.Isotopes: These are the atoms of the same element having the same
atomic number but different mass number.
pic
23. Isobars: Isobars are the atoms of different elements having the same
mass number but different atomic number.
w.
24. Isoelectronic species: These are those species which have the same
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number of electrons.
25.Electromagnetic radiations: The radiations which are associated with
electrical and magnetic fields are called electromagnetic radiations. When
an electrically charged particle moves under acceleration, alternating
electrical and magnetic fields are produced and transmitted. These fields
are transmitted in the form of waves. These waves are called
electromagnetic waves or electromagnetic radiations.
26.Properties of electromagnetic radiations:
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a. Oscillating electric and magnetic field are produced by oscillating
charged particles. These fields are perpendicular to each other and both
are perpendicular to the direction of propagation of the wave.
b. They do not need a medium to travel. That means they can even travel
in vacuum.
27.Characteristics of electromagnetic radiations :
a. Wavelength: It may be defined as the distance between two
neighbouring crests or troughs of wave as shown. It is denoted by.
b. Frequency (): It may be defined as the number of waves which pass
through a particular point in one second.
c. Velocity (v): It is defined as the distance travelled by a wave in one
second. In vacuum all types of electromagnetic radiations travel with the
same velocity. Its value is 3 X108 m sec-1. It is denoted by v
m
d. Wave number: Wave number ( ) is defined as the number of
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wavelengths per unit length.
28.Relationship between velocity, frequency and wavelength
Velocity = frequency x wavelength
c = ing
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29.Black body: An ideal body, which emits and absorbs all frequencies, is
called a black body. The radiation emitted by such a body is called black
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body radiation.
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30.Plancks quantum theory: Max Planck suggested that atoms and molecules
could emit or absorb energy only in discrete quantities and not in a
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continuous manner. Planck gave the name quantum, meaning fixed
amount to the smallest quantity of energy that can be emitted or absorbed
in the form of electromagnetic radiation.
pic
E v
w.
hc
E = hv
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Where:
E is the energy of a single quantum
is the frequency of the radiation
h is Plancks constant
h= 6.626 X 1034 Js
31.Quantisation of energy: Energy is always emitted or absorbed as integral
multiple of this quantum.
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E = nhv
Where n 1, 2,3, 4,.....
32. Photoelectric effect: The phenomenon of ejection of electrons from
the surface of metal when light of suitable frequency strikes it is called
photoelectric effect. The ejected electrons are called photoelectrons.
33. Experimental results observed for the experiment of Photoelectric effect
observed Hertz:
a. When beam of light falls on a metal surface electrons are ejected
immediately i.e. there is not time lag between light striking metal surface
and ejection of electrons
b. Number of electrons ejected is proportional to intensity or brightness of
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light
c. Threshold frequency ( vo ): For each metal there is a characteristic
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minimum frequency below which photoelectric effect is not observed. This
is called threshold frequency.
ing
d. If frequency of light is less than the threshold frequency there is no ejection
of electrons no matter how long it falls on surface or how high is its
intensity.
ch
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34.Photoelectric work function (Wo): The minimum energy required to eject
electrons is called photoelectric work function.
Wo hvo
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35.Energy of the ejected electrons :
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1
h(v v0 ) me v 2
2
pic
36.When a white light is passed through a prism, it splits into a series of
coloured bands known as spectrum.
w.
37.Spectrum is of two types: continuous and line spectrum
a. The spectrum which consists of all the wavelengths is called
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continuous spectrum.
b. A spectrum in which only specific wavelengths are present is known
as a line spectrum. It has bright lines with dark spaces between
them.
38.Electromagnetic spectrum is a continuous spectrum. It consists of a range
of electromagnetic radiations arranged in the order of increasing
wavelengths or decreasing frequencies. It extends from radio waves to
gamma rays.
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39. Spectrum is also classified as emission and line spectrum.
c. Emission spectrum: A substance absorbs energy and moves to a higher
energy state. The atoms, molecules or ions that have absorbed radiation
are said to be excited. Since the higher energy state is unstable they
return to the more stable energy state by emitting the absorbed
radiation in various regions of electromagnetic spectrum. The spectrum
of radiation emitted by a substance that has absorbed energy is called
an emission spectrum.
d. Absorption spectrum is the spectrum obtained when radiation is passed
through a sample of material. The sample absorbs radiation of certain
wavelengths. The wavelengths which are absorbed are missing and
come as dark lines.
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40.The study of emission or absorption spectra is referred as spectroscopy.
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41.Spectral Lines for atomic hydrogen:
ing
Series n1 n2 Spectral Region
Lyman 1 2, 3, 4, 5 Ultraviolet
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Balmer 2 3, 4, 5 Visible
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Paschen 3 4, 5 Infrared
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Brackett 4 5, 6 Infrared
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Pfund 5 6, 7 Infrared
pic
42.Rydberg equation: It allows the calculation of the wavelengths of all the
spectral lines of hydrogen.
w.
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43. Bohrs model for hydrogen atom:
a. An electron in the hydrogen atom can move around the nucleus in a circular
path of fixed radius and energy. These paths are called orbits or energy levels.
These orbits are arranged concentrically around the nucleus.
b. As long as an electron remains in a particular orbit, it does not lose or gain
energy and its energy remains constant.
c. When transition occurs between two stationary states that differ in energy, the
frequency of the radiation absorbed or emitted can be calculated.
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E E 2 -E1
v
h h
v = Frequency of radiation
h = Planck's constant
E1 Energy of lower energy state
E 2 Energy of higher energy state
d. An electron can move only in those orbits for which its angular momentum is
an integral multiple of h/2
m
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43.Bohrs theory for hydrogen atom:
a. Stationary states for electron are numbered in terms of Principal
Quantum numbered as n=1, 2, 3
= n2a0 where a0= 52.9 pm
c. Energy of stationary state
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b. For hydrogen atom: The radii of the stationary states is expressed as rn
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1
En R H 2
n
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where R H 2.18 1018 J(Rydberg cons tan t)
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n 1, 2,3,....
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1
E n 2.18 x1018 2 J
n
pic
d. For ions containing only one electron:
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Z2
E n 2.18 x1018 2 J
n
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where n 1, 2,3,....
rn = n2a0 pm
Z
Where Z is the atomic number
44.Limitations of Bohrs model of atom:
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a. Bohrs model failed to account for the finer details of the hydrogen
spectrum. For instance splitting of a line in the spectrum into two closely
spaced lines.
b. Bohrs model was also unable to explain spectrum of atoms containing
more than one electron.
c. Bohrs model was unable to explain Zeeman effect i.e. splitting of
spectral line in presence of magnetic effect.
d. Bohrs model also failed to explain Stark effect i.e. splitting of spectral
line in presence of electric field.
e. Bohrs model could not explain the ability of atoms to form molecules
by chemical bonds
45.Dual behavior of matter: de Broglie proposed that matter exhibits dual
behavior i.e. matter shows both particle and wave nature.
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1. de Broglies relation:
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h h
mv p
Where:
- Wavelength
p - Momentum
ing
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v - Velocity
h Plancks constant
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2. According to de Broglie, every object in motion has a wave character.
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Wavelengths of macroscopic objects cannot be detected but for
microscopic particles it can be detected. This is because for microscopic
objects, the mass is less. Since mass and wavelength are inversely
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proportional to each other, the wavelength will be more. But for
macroscopic objects, the mass is large. Therefore, wavelength will be
too short to be detected.
pic
3. Heisenbergs uncertainty principle: It states that it is impossible to
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determine simultaneously, the exact position and exact momentum (or
velocity) of an electron.
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h
x . p x
4
h
x . (m v x )
4
h
x . vx
4m
Where
x Uncertainty in position
vx - Uncertainty in velocity
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px - Uncertainty in momentum
This means that if the position of electron is known, the velocity of
electron will be uncertain. On the other hand, if the velocity of electron
is known precisely, the position of electron will be uncertain.
4. Heisenbergs uncertainty principle rules our the existence of definite
paths or trajectories of electrons and other similar particles
5. Failure of Bohrs model:
a. It ignores the dual behavior of matter.
b. It contradicts Heisenbergs uncertainty principle.
46.Classical mechanics is based on Newtons laws of motion. It successfully
describes the motion of macroscopic particles but fails in the case of
microscopic particles.
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Reason: Classical mechanics ignores the concept of dual behaviour of
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matter especially for sub-atomic particles and the Heisenbergs uncertainty
principle.
ing
47.Quantum mechanics is a theoretical science that deals with the study of the
motions of the microscopic objects that have both observable wave like and
particle like properties.
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48.When quantum mechanics is applied to macroscopic objects (for which
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wave like properties are insignificant) the results are the same as those
from the classical mechanics.
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49.Quantum mechanics is based on a fundamental equation which is called
Schrodinger equation.
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50.Schrodingers equation: For a system (such as an atom or a molecule
whose energy does not change with time) the Schrdinger equation is
pic
written as:
H E
w.
Where:
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H is the Hamiltonian operator
E is the total energy of the system
represents the wave function which is the amplitude of the electron
Wave
51.When Schrdinger equation is solved for hydrogen atom, the solution gives
the possible energy levels the electron can occupy and the corresponding
wave function(s) of the electron associated with each energy level.
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Out of the possible values, only certain solutions are permitted. Each
permitted solution is highly significant as it corresponds to a definite energy
state. Thus, we can say that energy is quantized.
That is, it can have only certain specific values.
52. gives us the amplitude of wave. The value of has no physical
significance.
53. 2 gives us the region in which the probability of finding an electron is
maximum. It is called probability density.
54.Orbital: The region of space around the nucleus where the probability of
finding an electron is maximum is called an orbital.
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55.Quantum numbers: There are a set of four quantum numbers which specify
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the energy, size, shape and orientation of an orbital.
These are:
a. Principal quantum number (n)
ing
b. Azimuthal quantum number (l)
c. Magnetic quantum number (ml)
d. Electron spin quantum number (ms)
ch
56.Principal quantum number (n): It determines the size and to a large extent
the energy of the orbital.
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n 1 2 3 4
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Shell no.: K L M N
Total number of orbitals in a 1 4 9 16
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shell = n2
Maximum number of 2 8 18 32
pic
electrons = 2n2
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It can have positive integer values of 1, 2, 3 and so on.
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It also identifies the shell.
As the value of n increases, the energy also increases. Hence, the
electron will be located far away from the nucleus.
57.Azimuthal quantum number (l): Azimuthal quantum number. l is also
known as orbital angular momentum or subsidiary quantum number. It
identified the sushell and the three dimensional shape of the orbital.
It also determines the number of subshells or sub levels in a shell.
Total number of subshells in a particular shell is equal to the value of
n.
l = 0, 1, 2 (n-1)
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Each subshell corresponding to different values of l are represented
by different symbols:
Value of l 0 1 2 3
Notation of s p d f
symbol
58.Magnetic quantum number or Magnetic orbital quantum number (ml): It
gives information about the spatial orientation of the orbital with respect to
standard set of co-ordinate axis.
For any sub-shell (defined by l value) 2l+1 values of ml are possible.
For each value of l,
ml = l, (l 1), (l2)... 0,1... (l 2), (l1), l
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59.Electron spin quantum number (ms): It refers to orientation of the spin of
the electron. It can have two values +1/2 and -1/2. +1/2 identifies the
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clockwise spin and -1/2 identifies the anti- clockwise spin.
60.An orbital is identified by the set of 3 quantum numbers: Principal quantum
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number, Azimuthal quantum number and magnetic quantum number.
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61.An electron is identified by a set of four quantum numbers: Principal
quantum number, azimuthal quantum number, magnetic quantum number
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and spin quantum number.
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62.Sub-shell notation: Notation of a sub-shell is written as the Principal
quantum number followed by the symbol of the respective sub-shell.
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63.Plots of the orbital wave function (r ) and probability density 2(r) Vs
distance r of the electron from the nucleus for 1s orbital:
pic
w.
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For 1s orbital the probability density is maximum at the
nucleus and it decreases sharply as we move away from it(which is not
possible).Hence plot of probability density 2(r) Vs distance r of the
electron from the nucleus was drawn as shown below.
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The orbital wave function for an electron in an atom has no physical
meaning. It is simply a mathematical function of the coordinates of the
electron.
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64.Plots of the orbital wave function (r ) and probability density 2(r) Vs
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distance r of the electron from the nucleus for 2s orbital:
ing
ch
oa
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For 2s orbital the probability density is maximum at the
pic
nucleus and it decreases sharply as we move away from it(which is not
possible).Hence plot of probability density 2(r) Vs distance r of the
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electron from the nucleus was drawn as shown below.
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For 2s orbital, the probability density first decreases sharply to zero and
again starts increasing. After reaching small maxima it decreases again and
approaches zero as the value of r increases further.
65.The region where this probability density function reduces to zero is called
nodal surfaces or simply nodes.
66.Charge cloud diagrams: In these diagrams, dots represent the electron
probability density. The density of the dots in a region represents electron
probability density in that region.
67.Boundary surface diagram: In this representation, a boundary surface or
contour surface is drawn in space for an orbital on which the value of
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probability density 2(r) is constant. However, for a given orbital, only that
boundary surface diagram of constant probability density is taken to be
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good representation of the shape of the orbital which encloses a region or
volume in which the probability of finding the electron is very high, say,
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90%.
68.Radial nodes: Radial nodes occur when the probability density wave
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function for the electron is zero on a spherical surface of a particular radius.
Number of radial nodes = n l 1
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69.Angular nodes: Angular nodes occur when the probability density wave
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function for the electron is zero along the directions specified by a
particular angle. Number of angular nodes = l
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70.Total number of nodes = n 1
pic
71.Degenerate orbitals: Orbitals having the same energy are called degenerate
orbitals.
w.
72.The stability of an electron in a multi electron system is because of:
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a. The repulsive interaction of the electrons in the outer shell with the
electrons in the inner shell.
b. The attractive interactions of electron with the nucleus.
These attractive interactions increase with increase of positive charge
(Ze) on the nucleus.
a. The stability of an electron in multi-electron atom is because total
attractive interactions are more than the repulsive interactions.
73. Shielding effect or screening effect: Due to the presence of electrons in
the inner shells, the electron in the outer shell will not experience the full
positive charge on the nucleus.
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So due to the screening effect, the net positive charge experienced by the
electron from the nucleus is lowered and is known as effective nuclear
charge.
Effective nuclear charge experienced by the orbital decreases with increase
of azimuthal quantum number (l).
74. Orbitals have different energies because of mutual repulsion between
electrons in a multi- electron atom.
75.Orbitals with lower value of (n+l) are filled first as they have lower energy.
76.If two orbitals have the same value of (n+l) then orbital with lower value of
n will have lower energy.
77.Energies of the orbitals in the same subshell decrease with increase in
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atomic number.
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78.Filling of electrons: The filling of electrons into the orbitals of different
atoms takes place according to Aufbau principle ,Paulis exclusion principle,
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the Hunds rule of maximum multiplicity
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79. Aufbau Principle: In the ground state of the atoms, the orbitals are filled in
order of their increasing energies. The order in which the orbitals are filled
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is as follows:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 4f, 5d, 6p, 7s...
It is based on (n+ l) rule. It states that the orbital with lower value of (n +
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l) has lower energy.
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80.Pauli Exclusion Principle: No two electrons in an atom can have the same
set of four quantum numbers. Only two electrons may exist in the same
orbital and these electrons must have opposite spin.
pic
81. Hunds rule of maximum multiplicity: Pairing of electrons in the orbitals
belonging to the same subshell (p, d or f) does not take place until each
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orbital belonging to that subshell has got one electron each i.e., it is singly
occupied.
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82. Electronic configuration of atoms: The electronic configuration of different
atoms can be represented in two ways.
a. sapbdc ...... notation: In the first notation, the subshell is represented
by the respective letter symbol and the number of electrons present in
the subshell is depicted, as the super script, like a, b, c, ... etc. The
similar subshell represented for different shells is differentiated by
writing the principal quantum number before the respective subshell.
b. Orbital diagram: In the second notation, each orbital of the subshell is
represented by a box and the electron is represented by an arrow () a
positive spin or an arrow () a negative spin.
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83. Stability of completely filled and half filled subshells:
a. Symmetrical distribution of electrons
b. Exchange energy
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ing
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oa
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pic
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