chemistry notes for class 11 structure of atom download in pdf

7/25/2019 Chemistry Notes for Class 11 STRUCTURE of ATOM Download in PDF

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STRUCTURE OF ATOM

Atoms:

Atom is the smallest indivisible particle of the matter. Atom is made of electron, proton and neutrons.

Electrons were discovered using cathode ray discharge tube experiment. Nucleus was discovered

by Rutherford in 1911.

athode ray discharge tube experiment! A cathode ray discharge tube made of glass is

ta"en with two electrodes. At very low pressure and high voltage, current starts flowing

through a stream of particles moving in the tube from cathode to anode. #hese rays were

called cathode rays. $hen a perforated anode was ta"en, the cathode rays struc" the

other end of the glass tube at the fluorescent coating and a bright spot on the coating

was developed

Results of Rutherford experiments:

a. athode rays consist of negatively charged electrons.

b. athode rays themselves are not visible but their behavior can be observed with help

of fluorescent or phosphorescent materials.

c. %n absence of electrical or magnetic field cathode rays travel in straight lines. %n presence

of electrical or magnetic field, behavior of cathode rays is similar to that shown by electrons

e. #he characteristics of the cathode rays do not depend upon the material of the

electrodes and the nature of the gas present in the cathode ray tube.

harge to mass ratio of an electron was determined by #homson. #he charge to mass ratio

of an electron as 1.&'(()* x 1*11. "g1

harge on an electron was determined by R A +illi"an by using an oil drop experiment. #he value of

the charge on an electron is 1. x 1*19.

PARTICLE ELECTRON PROTON NEUTRON

Discove

ry

Sir. J. J.Thomson

1869

Goldstein(1886)

Chadwick(1932)

Nature

ofNegative Positive Neutral

Amount

of

chare

1.6 x 10

19Coloumb

1.6 x 10

19Coloumb

0

Mass 9.11 x 1031kg 1.672614 x

1027kg

1.67492 x10

27kg


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#he mass on an electron was determined by combining the results of #homson-s experiment

and +illi"an-s oil drop experiment. #he mass of an electron was determined to be 9.1*9 x

1*/1"g.

Discovery of protons nd cnl rys:

+odified cathode ray tube experiment was carried out which led to the discovery ofprotons.

Chrcteristicsof positively chr!ed prticles:

a. harge to mass ratio of particles depends on gas from which these origin ate b. #he

positively charged particles depend upon the nature of gas present in the cathode ray

discharge tube

b. 0ome of the positively charged particles carry a multiple of fundamental of electrical charge. d.

ehaviour of positively charged particles in electrical or magnetic field is opposite to that

observed for cathode rays

"eutrons were discovered by 2ames hadwic" by bombarding a thin sheet of beryllium

by 3 particles. #hey are electrically neutral particles having a mass slightly greater than

that of the protons.Atomic num#er $%& : the number of protons present in the

nucleus4+oseley191/5.

Mss "um#er $A& : 0um of the number of protons and neutrons present in the nucleus.

Thomson model of n tom:

#his model proposed that atom is considered as a uniform positively charged sphere and electrons

are embedded in it. An important feature of #homson model of an atom was that mass of atom is

considered to be evenly spread over the atom. #homson model of atom is also called as 6lum

pudding, raisin pudding or watermelon model #homson model of atom was is carded because it

could not explain certain experimental results li"e the scattering of 3 particles by thin metal foils.

O#servtions from ' prticles sctterin! experiment #y Rutherford:

a. +ost of the 3 particles passed through gold foil un deflected

b. A small fraction of 3 particles got deflected through small angles

c. 7ery few 3 particles did not pass through foil but suffered large deflection nearly1(*o

Conclusions Rutherford dre( from ' prticles sctterin! experiment:

a.0ince most of the 3particles passed through foil undeflected,

it means most of the space in atom is empty

b.0ince some of the 3particles are deflected to certain angles, it

means that there is positively mass present in atom

c.0ince only some of the 3particles suffered large deflections, the

positively charged mass must be occupying very small space

d.0trong deflections or even bouncing bac" of 3particles from metal

foil were due to direct collision with positively charged mass in atom


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Rutherford)s model of tom :

#his model explained that atom consists of nucleus which is concentrated in a very small volume.

#he nucleus comprises of protons and neutrons. #he electrons revolve around the nucleus in fixed

orbits. Electrons and nucleus are held together by electrostatic forces of attraction.

Dr(#c*s of Rutherford)s model of tom :

a.According to Rutherford-s model of atom, electrons which are

negatively charged particles revolve around the nucleus in fixed

orbits. #hus,

b.#he electrons undergo acceleration. According to electromagnetic

theory of +axwell, a charged particle undergoing acceleration

should emit electromagnetic radiation. #hus, an electron in an orbit

should emit radiation. #hus, the orbit should shrin". ut this does

not happen.c. #he model does not give any information about howelectrons are distributed around nucleus and what are energies of

these electrons

+sotopes: #hese are the atoms of the same element having the same atomic number but

different mass number. e g 181,18),18/

+so#rs: %sobars are the atoms of different elements having the same mass number but different

atomic number. e g 1(Ar* , )*a*

+soelectronic species! #hese are those species which have the same number of electrons.

Electrom!netic rditions:

#he radiations which are associated with electrical and magnetic fields are called

electromagnetic radiations. $hen an electrically charged particle moves under acceleration,

alternating electrical and magnetic fields are produced and transmitted. #hese fields are

transmitted in the form of waves. #hese waves are called electromagnetic waves or

electromagnetic radiations.

,roperties of electrom!netic rditions:

a.scillating electric and magnetic field are produced by

oscillating charged particles. #hese fields are perpendicular

to each other and both are perpendicular to the direction of

propagation of the wave.

b.#hey do not need a medium to travel. #hat means they can even travel in vacuum.

Chrcteristics of electrom!netic rditions:

a.-velen!th: %t may be defined as the distance between twoneighboring crests or troughs of wave as shown. %t is denoted

by :.


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b.Fre.uency $/&: %t may be defined as the number of waves

which pass through a particular point in one second.

c.0elocity $v&: %t is defined as the distance travelled by a

wave in one second. %n vacuum all types of electromagnetic

radiations travel with the same velocity. %ts value is / ;1*

(m sec1. %t is denoted by vd.$ave number! $ave number is defined as the number of

wavelengths per unit length.

7elocity < fre=uency ;wavelength c < >:

,lnc*1s 2untum Theory

o #he radiant energy is emitted or absorbed not

continuously but discontinuously in the form of small discrete

pac"ets of energy called ?=uantum-. %n case of light , the

=uantum of energy is called a ?photon-

o #he energy of each =uantum is directly proportional to

the fre=uency of the radiation, i.e. E 3 @ or E< h@

where h< 6lanc"-s constant < .) x 1*)& 2s

o Energy is always emitted or absorbed as integral multiple of this =uantum. E< nh@ $here

n


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Dul #ehvior of electrom!netic rdition

#he light possesses both particle and wave li"e properties, i.e., light has dual behavior .

whenever radiation interacts with matter, it displays particle li"e properties.4lac" body

radiation and photoelectric effect5 $ave li"e properties are exhibited when it

propagates4interference an diffraction5

Spectrum:

$hen a white light is passed through a prism, it splits into a series of colored bands "nown as

spectrum5

Spectrum is of t(o types:

4a5 Continuous nd line spectrum #he spectrum which

consists of all the wavelengths is called continuous spectrum.

4b5 6ine spectrumA spectrum in which only specificwavelengths are present is "nown as a line spectrum. %t

has bright lines with dar" spaces between them.

Electromagnetic spectrum is a continuous spectrum. %t consists of a range of electromagnetic

radiations arranged in the order of increasing wavelengths or decreasing fre=uencies. %t extends

from radio waves to gamma rays.

0pectrum is also classified as emission and line spectrum.

o Emission spectrum: #he spectrum of radiationemitted by a substance that has absorbed energy is called

an emission spectrum.

o A#sorption spectrum is the spectrum obtained when

radiation is passed through a sample of material. #he

sample absorbs radiation of certain wavelengths. #he

wavelengths which are absorbed are missing and come as

dar" lines.

#he study of emission or absorption spectra is referred as spectroscopy. 0pectral Cines for atomic

hydrogen!


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Rydberg e=uation

R < Rydberg-s constant < 1*9&& cm1

3ohr)s model for hydro!en tom:

a. An electron in the hydrogen atom can move around

the nucleus in a circular path of fixed radius and energy.

#hese paths are called orbits or energy levels. #hese 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. $hen transition occurs between two stationary states

that differ in energy, the fre=uency of the radiation absorbed

or emitted can be calculated

d. An electron can move only in those orbits for whichits angular momentum is an integral multiple of hD)

#he radius of the nth orbit is given byrn


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7eisen#er!)s uncertinty principle:

%t states that it is impossible to determine simultaneously, the exact position and exact

momentum 4or velocity5 of an electron. #he product of their uncertainties is always e=ual to orgreater than hD.

8eisenberg-s uncertainty principle rules out the existence of definite paths or traectories of

electrons and other similar particles

Filure of 3ohr)s model:a. %t ignores the dual behavior of matter.

b. %t contradicts 8eisenberg-s uncertainty principle.

Clssicl mechnics ! is based on Newton-s laws of motion. %t successfully describes the

motion of macroscopic particles but fails in the case of microscopic particles Reason!

lassical mechanics ignores the concept of dual behavior of matter especially for subatomic

particles and the 8eisenberg-s uncertainty principle.

2untum mechnics is a theoretical science that deals with the study of the motions of the

microscopic obects that have both observable wave li"e and particle li"e properties.

Guantum mechanics is based on a fundamental e=uation which is called 0chrodinger e=uation.

Schrodin!er)s e.ution:

Bor a system 4such as an atom or a molecule who seenergy does not change with time5

the 0chrHdinger e=uation is written as!

$hen 0chrHdinger e=uation is solved for hydrogen atom, the solution gives the possible

energy levels the electron can occupy and the corresponding wave function4s5 of the

electron associated with each energy level. ut of thepossible values, only certain

solutions are permitted. Each permitted solution is highly significant as it corresponds to a

definite energy state. #hus, we can say that energy is =uantiIed.


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,ro##ility density : J gives us the amplitude of wave. #he value of J has no physical

significance. K) gives us the region in which the probability of finding an electron is

maximum. %t is called pro##ility density.

Or#itl: #he region of space around the nucleus where the probability offending an electron ismaximum is called an orbital.

2untum num#ers:

#here are a set of four =uantum numbers which specify the energy, siIe, shape and

orientation of an orbital. #o specify an orbital only three =uantum numbers are re=uired while

to specify an electron all four =uantum numbers are re=uired.

,rincipl .untum num#er $n&: %t identifies shell, determines siIes and energy of orbitals

A8imuthl .untum num#er $l&:AIimuthal =uantum number. ?l- is also "nown as orbital angular

momentum orsubsidiary =uantum number. l. %t identifies subshell, determines the shape of orbitals, energy

of orbitals in multielectron atoms along with principal =uantum number and orbital angular

momentum, i.e., #he number of orbitals in a subshell < )l L 1. Bor a given value of n, it can

have n values ranging from * to n1. #otal number of subshells in a particular shell is e=ual to

the value of n.

M!netic .untum num#er or M!netic or#itl .untum num#er $ml&: %t gives information

about the spatial orientation of the orbital with respect to standard set of coordinate axis. Bor

any subshell 4defined by ?l- value5 )lL1 values of ml are possible. Bor each value of l, ml < M l,

M 4l M15, M 4lM)5... *,1...4l M )5, 4lM15, l

0ubshellnotation

s p d f g

7alue of ?l- * 1 ) / .

Number oforbitals

1 / ' & 9


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Electron spin .untum num#er $ms&: %t refers to orientation of the spin of the electron. %t

can have two values L1D) and 1D). L1D) identifies the cloc" wise spin and 1D) identifies the

anti cloc"wise spin.

"odl surfces or simply nodes :

#he region where this probability density function reduces to Iero is called nodal surfaces or

simply nodes.

Rdil nodes! Radial nodes occur when the probability density of wave function for the

electron is Iero on a spherical surface of a particular radius. Number of radial nodes < n M l M

1 An!ulr nodes:Angular nodes occur when the probability density wave function for the

electron is Iero along the directions specified by a particular angle. Number of angular nodes

< l

#otal number of nodes < n M 1

De!enerte or#itls! rbitals having the same energy are called degenerate orbitals. 0hape

of p and d orbitals

Shieldin! effect or screenin! effect:

ue 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. 0o, due to the screening effect, the net

positive charge experienced by the electron from the nucleus is lowered and is "nown as

effective nuclear charge. Effective nuclear charge experienced by the orbital decreases with

increase of aIimuthal =uantum number 4l5.


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Auf#u ,rinciple:

%n the ground state of the atoms, the orbitals are filled in order of their increasing energies

n9l rulerbitals with lower value of 4nLl5 have lower energy. %f two orbitals have the same

value of 4nLl5 then orbital with lower value of n will have lower energy. #he order in which

the orbitals are filled is as follows!

1s, )s, )p, /s, /p, s, /d, p, 's, d, 'p, f, 'd, p, &s...

,uli Exclusion ,rinciple:

No two electrons in an atom can have the same set of four =uantum numbers. nly two

electrons may exist in the same orbital and these electrons must have opposite spin.

7und)s rule of mximum multiplicity:

6airing of electrons in the orbital s belonging to the same subshell 4p, d or f5 does not ta"e

place until each orbital belonging to that subshell has got one electron each i.e., it is singly

occupied.

Electronic confi!urtion of toms:

a. Arrangement of electrons in different orbitals of an atom. #he electronic configuration

of different atoms can be represented in two ways. a. sapbdc...... notation.

b. rbital diagram!, each orbital of the subshell is represented by a box and the electron isrepresented by an arrow 4O5 a positive spin or an arrow 4P5 a negative spin.

St#ility of completely filled nd hlf filled su#shells:

a.0ymmetrical distribution of electrons the completely filled

or half filled subshells have symmetrical distribution of

electrons in them and are more stable.

b. Exchange energy #he two or more electrons with the

same spin present in the degenerate orbitals of a subshell

can exchange their position and the energy released due to

this exchange is called exchange energy. #he number of

exchanges is maximum when the subshell is either half filled

or completely filled. As a result the exchange energy is

maximum and so is the stability.

chemistry notes for class 11 structure of atom download in pdf

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