Atomic structure Grade 11 Chemistry

UNIT 3 ATOMIC STRUCTURE
 Atom ; The term atom was coined by John Dalton and derived from Greek word “Atomos”
means indivisible.
 An atom is the smallest particle of an element which can take part in chemical reaction and
determine the chemical properties of the compound.
Dalton Atomic Theory
John Dalton put forwarded a theory to describe about the structure of an atom which is called
Dalton atomic theory .The main postulates of his theory are,
a)All the matter are made of large number of minutes particle called as atom.
b)Atom can neither be created nor be destroyed by any physical or chemical means.
c)Atoms are indivisible and in destructive units. It means atom do not under go subdivision
during chemical reaction.
d)Atom of same element are identical in all respect.

e)Atom of different element are entirely different and have different properties.
f) Atom combine in a simple whole number ratio to give compound atom.
g) Compound atom of same elements are identical in all respect.
h)The property of compound is due to property of an atom.
Limitations of Daltons atomic theory
a)Unable to explain the cause of chemical combination.
b)Fail to explain the law of gaseous volume.
c)Does not give idea about isotope , isobar and isotone.
d)Unable to distinguish between ultimate particle of an element and an atom.

MODERN MODIFICATION ON DALTON'S ATOMICTHEORY
An atoms are no longer supposed to be indivisible.
Atoms of same elements my not be similar in all respect . For example Cl-35 and Cl-37.
Atom of different elements may be similar in one or more aspect .For e .g . Ca-40 and Ar -
40.
The ratio in which atoms combine may not be simple . For e .g . C:H:O in C 12 H 22 O 11 is
12:22:11 which is not simple.
An atom of one element can be changed into another atom by nuclear reaction.
Mass and energy of an atom are interconvertible .

CONCEPT OF ISOTOPE , ISOBAR AND ISOTONE
1. Isotope ; An isotope are the different atom of same element having same atomic number but having
different atomic mases .It is derived from Latin word isos means same and topos means atomic number.
E.g. a) 1 H 0 , 1 H 2 and1 H 3
b) 17 Cl 35 and 17 Cl 37
2 . Isobar ; An isobar are the different atom of different element having same atomic mass but having
different atomic number and number of neutron . It is derived from Latin word isos means same and baros
means heavy.
E.g. a)18 Ar 40 and 20 Ca 40
b) 92 U 235 and 93 Np 235
3. Isotone ;An isotone are the different atom of different element having same no of neutron but having
different atomic mass and atomic number . It is derived from Latin word isos mean same and tonous mean
neutron.
E.g. 6 C 14, 7 N 15 and 8 O 16
.

FUNDAMENTAL PARTICLE OF AN ATOM AND THEIR DISCOVERY
• An atom is composed from large number of microscopic particle like electron ,proton
,neutron, positron ,meson ,quark etc. which are called subatomic particle . Among them
electron ,proton and neutron are called fundamental particle of an atom as they are
considered as principal sub-atomic particle.
1 . Discovery of Electron
a) An electron was discovered by J .J Thomson by carrying out discharge tube experiment in
1879 A .D.
When high electric discharged is passed through the sample of gas under study at high
voltage (10,000 Volt) and low pressure (10-3 mmHg) in between two metal electrode through
the tube , cathode rays are emitted from the surface of cathode and move toward anode.
These rays on striking on the inner side of the tube produced flashes of faint greenish light.
On applying electric field these rays are deviated toward +ve electric field which implies that
particle in cathode rays consists of negatively charge particle called electron.

• So , electron is subatomic particle represented by symbol e- having
a)mass=9.1 × 10 -31 kg which is 1/1837th of mass of proton.
b)charge=-1.602 × 10 -19 coulomb
Fig ; Discovery of electron by discharge tube experiment

DISCOVERY OF PROTON
• A proton was discovered was E . Goldstein in 1886 A .D by carrying out discharge tube
experiment using perforated cathode .
• When high electric discharged is passed through the sample of gas under study at high
voltage (10,000 Volt) and low pressure (10-3 mmHg) in between two metal electrode
through the tube , cathode rays are emitted from the front surface of perforated cathode
and move toward anode are called cathode rays.
• From the backside of the cathode some rays are emitted which are moving opposite to
the cathode rays are called canal rays or anode rays . These rays are deflected toward –
ve plate of electric field .
• The simplest and lightest particle hydrogen ion were observed when hydrogen gas was
taken in a discharge tube which is called proton
• .So, a proton is a subatomic particle with symbol p+ or H having

a)mass=1.672×10 -27 Kg which is 1837 times the mass of electron.
b) Charge unit positive charge I .e. 1.602 × 10 -19 coulomb.
Fig ; Discovery of proton by discharge tube experiment

DISCOVERY OF NEUTRON
• A neutron was discovered by James Chadwick in 1932 A . D while bombarding a thin foil of
Beryllium with fast moving alpha particle.
4 Be 9 + 2 He 4
6 C 13
6 C 12 +0 n 1
alpha particle unstable neutron
So, a neutron is a neutral subatomic particle represented by n having mass 1.675 ×10 -
27 Kg comparable to the mass of electron.s

1 . THOMSON ATOMIC MODEL
J.J Thomson discovered a first model to describe about the structure of an atom known as
Thomson atomic model.
According to this model ,an atom is the sphere of uniform positive charge in which – vely
charge electron are embedded as like a seed in a water melon.so it is also called water
melon model of an atom.
This model was much like resin in a pudding also ti is called Thomson's plum pudding model.

Limitation : This model was unable to locate the position of an
electron and proton within atom so discarded.

RUTHERFORD ALPHA SCATTERING EXPERIMENT

In order to locate the position of an electron , proton and nucleus Rutherford and his co-
worker in 1911 A .D. performed a series of a experiment by taking alpha particle as
bombarding particle is called Rutherford alpha scattering experiment.
In his experiment he shoot a thin stream of highly energetic alpha particle from
radioactive source into thin foil pf gold covered outside by circular and movable zin
sulphide coated screen. The fate of flashes of light produced by striking of alpha
particle in screen were observed by using suitable microscope .
Observation of Rutherford alpha scattering experiment
1. Most of the ∝ - particle passed straight through the thin foil of gold as undeflected.
2. Few of the ∝ - particle deviated by small angle from thin foil of gold.
3. Very few or one of ten thousand ∝ − particle suffered from large deflection or even
reflected back by making angle of 180 degree.

CONCLUSION OF RUTHERFORD ALPHA SCATTERING EXPERIMENT
 Most of the space within atom is vacant since most of the ∝ - particle passed straight
through the thin foil of gold as undeflected.
Positively charge particle is located at center of an atom called nucleus since few of
the 𝐩𝐨𝐬𝐢𝐭𝐢𝐯𝐞𝐥𝐲 𝐜𝐡𝐚𝐫𝐠𝐞 ∝ - particle deviated by small angle from thin foil of gold.
A nucleus has appreciable mass and concentrated at the center of an atom since very
few or one of ten thousand ∝ − particle suffered from large deflection or even
reflected back by making angle of 180 degree after collision with nucleus.

RUTHERFORD NUCLEAR MODEL OF AN ATOM
Fig. Rutherford nuclear model of an atom

• On the basic of scattering experiment Rutherford in 1912 A.D. discovered a nuclear model
of an atom called Rutherford nuclear model of an atom. The main postulates of this model
are
Most of the space within an atom is vacant or empty.
The positively charge dense mass particle is located at the center is called nucleus.
The magnitude of +charge is different for different atom.
An atom is electrically neutral as it contain equal number of electron and proton.
- vely charge electron revolving continuously around the nucleus under the electrostatic
force of attraction which balance the positive charge on nucleus.
Electron are not stationary but they revolve around the nucleus continuously as the planet
revolve around the sun . So this model is also called planetary model of an atom.

LIMITATION OF RUTHERFORD NUCLEAR MODEL OF AN ATOM
Unable to explain the stability of an atom.
 Unable to explain the position and energy of an electron.
Unable to explain the spectra of Hydrogen like atom.

• In order to overcome the limitation of Rutherford atomic model ,Neil Bohrs in 1913 A.D.
developed a nuclear model of an atom based on the quantization of angular momentum
known as Bohrs atomic model of an atom. The main postulates of his theory are ,
 An atom consist of positively charge dense mass nucleus located at the center revolving
continuously by the negatively charge electron.
Electron revolve continuously around the nucleus in fixed circular path called orbit
represented by n . For the different value of n=1,2,3,………..etc. respective shell are
represented by K , L , M , N…..etc.
Each orbit or shell has their own fixed energy so they are also called stationary energy level
or shell.
As long as electron revolving continuously around the nucleus, it neither loose nor gain
energy continuously but only in discrete unit of h 𝝑.
i.e. ∆E=h 𝝑 where h= Planck constant, 𝝑(neu)=frequency of radiation and ∆E=amount of
energy loose or gain.

Only those circular path are allowed to revolve by electron in which angular momentum of
electron is equal to the integral multiple of h/2𝝅.
i.e. m V r = n ×
𝒉
2𝝅
Where , m=mass of an electron
V= velocity of an electron
r= distance of electron from center of nucleus(radius of orbit)
n= number of orbit(1 ,2 ……….etc.)
h= Planck constant .
The maximum number of electron that can be accommodated in a particular shell is given
by 2 n 2 rule where n=number of orbit . This is also called Bohr-burry rule . E.g. for k shell
,n=1 so maximum no of electron accommodated is 2 x 12=2 .
The centrifugal force of electron away from nucleus is completely balanced by centripetal
force of nucleus . As a result , electron revolve continuously around the nucleus without
falling into nucleus.

 Absorption or emission of energy take place only when electron jump from one energy
level to another energy level . Absorption of energy take place when electron jump from
lower to higher energy level and emission of energy take place when electron jump back
from higher to lower energy level.
∆E = E 2 - E 1
Where, ∆E=amount of energy absorbed or emitted.
E 2=energy of higher energy level
E 1=energy of lower energy level

LIMITATION OF BOHRS ATOMIC MODEL
 unable to explain the spectra of an atom containing more than one electron called
multielectron system.
Unable to describe the splitting of spectral line into group of hyperfine line structure.
Unable to describe about stark and Zeeman effect.
Unable to describe the dual nature of an electron i. e. electron behave as a particle as well
as wave.
 Unable to describe the Heisenberg uncertainty principle.
Unable to describe the cause of chemical combination and structure.

SUCCESSES OF BOHRS ATOMIC MODEL
Able to describe the stability of an atom.
Able to describe the spectra of hydrogen like atom.
Able to calculate the distance of an electron from the center of nucleus by using
formula . i .e . r = Τ
𝜺𝟎
𝟐
𝒏𝟐
𝒉 𝟐
𝒁𝒆𝟐𝝅𝒎
Able to calculate the energy of an electron in particular orbit of hydrogen.
I . e . E n= Τ
− 𝒁𝟐
𝒆𝟒
𝒎
𝟖 𝜺𝟎
𝟐
𝒏𝟐
𝒉 𝟐

ORIGIN OF SPECTRA OF HYDROGEN
Fig; Origin of Hydrogen spectra

In hydrogen atom there is presence of only one electron in its valence orbit.
When hydrogen is subjected to electric discharge at high voltage and low pressure , the
molecule gets dissociated into atom followed by the promotion of electron into different
energy level.
In sample of hydrogen there are different hydrogen atom , so electrons are excited into
different energy level based on the amount of energy absorbed.
At higher energy level electron are unstable and immediately tends back to their original
level by emitting excesses energy absorbed in the form of electromagnetic radiation .
Since different downward jump are associated with different amount of energy which
when observed through spectrometer six different spectral series of hydrogen are
observed is called hydrogen spectrum. They are Lyman , Balmer , Paschen , Brackett ,
P-fund and Humphries series .
The different spectral series of hydrogen , their origin in the region of spectrum and their
transition is tabulated as below.

Spectral series of
hydrogen
Lower energy
level(n 1)
Higher energy
level(n 2)
Spectral region
Lyman series 1 2,3,4…………. Ultraviolet
Balmer series 2 3,4,5…………… Visible
Paschen series 3 4,5,6…………… Near infrared
Brackett series 4 5,6,7…………….. Far infrared
P-fund series 5 6,7,8…………….. Far infrared
Humphries series 6 7,8,9…………….. Far infrared

The number of lines in each different spectral series is calculated by using Rydberg
equation as
Wave number( ҧ
𝑣 ) =1/ ሶ
𝜆 = 𝑅[1/ n 1
2 - 1/n 2
2]
Where, ሶ
𝜆 = wavelength of radiation.
R= Rydberg constant=1.09× 10-7 m-1
n
1
=lower energy level
n
2
=higher energy level
For Lyman series ഥ
𝑣 Lyman
=𝑅[1/ 12 - 1/n 2
2] where n 2=2,3,4………

de- Broglie's Hypothesis
de- Broglie in 1924 A.D. put forwarded a theory to describe about the dual nature of
electron is called de- Broglie's Hypothesis.
This hypothesis state that ,”Electron show the dual behaviour of matter(particle) as well
as wave(radiation)”.
According to de- Broglie ,the wavelength associated with a particle of mass ‘m’ moving
with velocity ‘v’ is given by
𝝀′ =
𝒉
𝒎𝒗
=
𝒉
𝑷
where 𝝀′ = de- Broglie wavelength.
h= Planck's constant
m=mass of electron
V=velocity of electron and P=momentum

• According to Einstein theory of radiation , the energy (E) of particle with mass(m) and
velocity (C) is given by E= m C 2……….eq(1).
• According to Bohrs theory ,the energy (E) of particle of mass (m) and frequency of
radiation(𝒗 ) is given by
E= h 𝒗 ……………….eq(2)
Now equating eq(10 and eq(2) we get,
or, m C 2 = h𝒗
Or, m C 2 = h × C/ 𝝀
Or, 𝝀 = h/m C………………………eq(3).For particle ‘C’ is replaced by ‘V ‘so
i.e. 𝝀 = h/p…………………………eq(4).this is de- Broglie equation. This equation
is Valid only for small size particle and fail in case of particle with large size.

HEISENBERG UNCERTAINTY PRINCIPLE
Werner Heisenberg in 1927 A.D. put forwarded a theory of uncertainty, after his name known
as Heisenberg uncertainty principle.
This principle state that, “Simultaneous determination of position and momentum(velocity) of
moving microscopic particle is not possible precisely". It means if position is determined with
great accuracy , there is uncertainty in velocity and vice-versa.
Mathematically, Δ𝑥 × Δ𝑃 ≥
ℎ
4𝜋
or, Δ𝑥 × 𝑚 Δ𝑉 ≥
ℎ
4𝜋
…………eq(1)
Where, Δ𝑥= uncertainty in position
Δ𝑃= uncertainty in momentum
Δ𝑉 = uncertainty in velocity and m= mass of particle

Case(I ) if position is determined with great accuracy, 𝜟𝒙=0 .Then from eq(1), we get
𝟎 × 𝜟𝑽≥
𝒉
𝟒𝝅𝒎
i.e. 𝜟𝑽= ∞(undetermined and uncertainty in velocity)
Case(ii)if velocity is measured with great accuracy, 𝜟𝑽=0 . Then from eq(1),we get
𝟎 × 𝜟𝑿≥
𝒉
𝟒𝝅𝒎
I .e . 𝜟𝑿= ∞(undetermined and uncertainty in position)
According to Bohrs theory ,an electron always revolve around the nucleus in fixed path called
orbit but according to Heisenberg, Simultaneous determination of position and
momentum(velocity) of moving microscopic particle like electron is not possible precisely. It
means if velocity is determined with great accuracy , there is uncertainty in position and
vice-versa . So , Bohrs theory of fixed orbit is seem to be defective on the light of uncertainty
principle.

QUANTUM NUMBER
The position ,state and energy of an electron in an atom is specified by using four
constant number are called quantum number .They are principal , azimuthal ,magnetic
and spin quantum number.
1 . Principal quantum number
It describe about the main energy level or shell in which electron belong.
It is represented by ‘n’.
Its value ranges from 1 to ∞ but never zero.
For the different value of n=1,2,3…..etc. , the respective shell are denoted by K, L , M ,
N……etc.
The maximum number of electron that can be accommodated in particular shell is given
by 2 n 2 rule where n= number of orbit.

Value of n Shell representation Maximum number of
electron(2 n 2)
1 K 2
2 L 8
3 M 18
4 N 32

2 . AZIMUTHAL QUANTUM NUMBER
It is represented by l.
It is also called subsidiary quantum number.
It describe about the shape of orbital(subshell).
Its value depend upon the value of n and ranges from o to n-1.
The different value of l =o ,1 ,2 and 3 represent the different subshell i.e. s , p , d and f
present within the shell respectively.
The number of subshell present within the shell is equal to value of n.
The total number of electron present within the each sub shell is given by 2(2 l + 1).

Value of n Shell Value of l=0
to n-1
Subshell
notation
Total number
of subshell
Total number
of electron in
subshell2(2l+
1)
1 K 0 1s 1 2
2 L 0 and 1 2s and 2 p 2 6

3 . MAGNETIC QUANTUM NUMBER
It is denoted by ‘m’.
It describe about the orientation of atomic orbital in three dimensional space.
Its value depends upon the value of l and ranges from –l to +l including zero.
The total number of orbital in particular subshell is given by 2 l + 1 value.
Value of n shell Value of l=0
to n-1
Value of m=-
l to +l
No. of
orbital in
subshell (2 l
+ 1)
No. of
orbital in
shell
1 K 0 0 1 1
2 L 0
1
0
-1 , 0 +1
1
3
4

4 SPIN QUANTUM NUMBER
It is denoted by ‘S’
It describe about the spinning of electron around its own axis.
It has only two values i.e. +1/2 for clockwise motion of an electron represented by up
headed arrow ( ) and -1/2 for anticlockwise motion of an electron represented by down
headed arrow ( ).

PAULI'S EXCLUSION PRINCIPLE
This principle stat that , “No two electron in atom can have same values of all four quantum
number .”
It means if they have same value of principal ,azimuthal ,and magnetic quantum number but
they have different value of spin quantum number +1/2 for clockwise motion electron and -1/2
for anticlockwise motion electron.
For e. g.in He molecule it has electronic configuration 1S 2 . For valence shell 1S 2
1st e- 2nd e-
They have same value of n , l and m(1,0,0) but different value of s i.e. +1/2 for I s t e- and -1/2 for 2nd e-.
Type of
electron
n l m s
1st 1 0 0 +1/2
2nd 1 0 0 -1/2

HUND'S RULES OF MAXIMUM MULTIPLICITY
This rule state that , “pairing of electron in atomic orbital of various subshell takes place only
when all the possible atomic orbital contain single electron.”
Since, this principle avoid the pairing of electron as far as possible so this principle is also called
principle of minimum pairing.
For e .g. in Nitrogen
7 N= 1S 2 , 2S 2 , 2 P 3
incorrect because pairing of electron takes
place before filling of all possible atomic orbital
correct because each orbital contain
single electron.

AUFBAU PRINCIPLE
Latin word Aufbau means to build up or to construct , so this principle is also called building
up or construction principle.
This principle state that, “The filling of electron in atomic orbital of various subshell take
place in the order of their increasing energy .”
It means atomic orbital of lower energy is filled first followed by atomic orbital of
subsequently increasing energy.
This principle is based on n + l subrule where n=principal and l=azimuthal quantum number.
Case(I )atomic orbital having lowest value of n + l is filled first.
for e. g. 3 d and 4s
n=3 and l=2 n=4 and l=0
So n + l=3 +2 =5 n + l=4+0=4 .Here , 4s has lowest value of n + l than 3 d so it is filled
first.

Case(ii) if two atomic orbital has same value of n + l then, atomic orbital having lowest value of
n is filed first.
For e .g. 3 P and 4S
n=3 and l=1 n=4 and l=0
n + l=3+1=4 n + l=4+0=4 .Here both atomic orbital has same value of n + l
i.e. 4 but 3 P has lowest value of n so it is filled first than 4S.
The sequence of filling of electron in various atomic orbital according to Aufbau principle is
given as below.

Fig ; Aufbau Sequence of filling of electron on various atomic orbital

For e. g. 20 Ca=1s 2 ,2s 2 ,2 p 6 ,3s 2 , 3 p 6 , 4 s 2
K + = 1s 2 ,2s 2 ,2 p 6 ,3s 2 , 3 p 6
Cl- =1s 2 ,2s 2 ,2 p 6 ,3s 2 , 3 p 6
Limitation of Aufbau principle
1. Fail in case of Cr , Cu ,Mo Ag………..
E .g. 24 Cr=1s 2 ,2s 2 ,2 p 6 ,3s 2 , 3 p 6 , 4 s 2,3 d 4 (incorrect)
=1s 2 ,2s 2 ,2 p 6 ,3s 2 , 3 p 6 , 3 d 5 ,4 s 1(correct)…… extra stability due to half filled
29 Cu =1s 2 ,2s 2 ,2 p 6 ,3s 2 , 3 p 6 , 4 s 2 ,3 d 9 (incorrect)
=1s 2 ,2s 2 ,2 p 6 ,3s 2 , 3 p 6 , 3 d 10 ,4 s 1 (correct)……. extra stability due to full filled
H .w. Cu 2+ and S 2-
Q . N . An element has outermost valence shell electronic configuration 3 d 5 , 4 s 1. Find out the
element and total s electron present.

Atomic structure Grade 11 Chemistry

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