ATOMS[VARIOUS TYPES OF ATOM MODELS]

- B Y

R . R A M P R A S A N T H

I B . S C . P H Y S I C S

P E R I Y A R M A N I A M M A I U N I V E R S I T Y

1

2

In this presentation What made scientists to think of atoms?

(i)Law of conservation of mass. (ii)Law of constant proportion.

Dalton’s atomic theory of molecules. Drawbacks of Dalton’s atomic theory. J.J.Thomson model of an atom and its drawbacks. Rutherford atom model and its drawbacks. Bohr’s atom model. Zeeman and stark effect. Drawbacks of Bohr’s atom model. Sommerfield’s atom model. Drawbacks of sommerfield’s atom model. Erwin Schrödinger atomic theory. Warren Heisenberg prediction of position electron. What is an Atom?

3

INTRODUCTION TO ATOMS

Ancient Greek philosophers – Democritus and Leucippus suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further. Democritus called these indivisible particles atoms (meaning indivisible).

All this was based on philosophical considerations and not much experimental work to validate these ideas could be done till the eighteenth century. By the end of the eighteenth century scientists recognised the difference between elements and compounds and naturally became interested in finding out how and why elements combine and what happens when they combine.

Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.

THE LAW OF CONSERVATION OF MASS. THE LAW OF CONSTANT PROPORTIONS.

4

• Is there is any change in mass in chemical reaction ?

• Experiment:• Let x be copper sulphate or sodium

carbonate [To be kept in conical flask]• Then Y be sodium carbonate or sodium

sulphate. [To be kept in ignition tube]• prepare 5% solution of each• Setup the apparatus as shown in the figure.• Weigh the apparatus• Then swirl and tilt the apparatus such that

the chemical mixes well.• Now weigh it• Is any differences in weight?

THE LAW OF CONSERVATION OF MASS.

THE LAW OF CONSERVATION OF MASS STATES THAT: “Thou shall not create mass nor destroy them in chemical reaction”

5

THE LAW OF DEFINITE PROPORTIONLavoisier and various scientists found that compounds where made of combination of elements. At some proportionsi.e., in compounds such as water say 9g of said to decompose ,it must decompose at a definite ratio of 1:8 grams[1g hydrogen and 8 grams of oxygen]irrespective of where the compound came from or who prepared it.This is known as “the law of definite proportion” was proposed by proust

THE LAW OF DEFINITE PROPORTION STATES THAT:“In a chemical substance the elements are always present in definite proportions by mass”

6

The next problem faced by scientist was to explain these law. These problems were solved by these scientist.

7

John Dalton was born in a poor weaver’s family in 1766 in England.He began his career as a teacher at the age of twelve. Seven years later

he became a school principal. In 1793, Dalton left for Manchester to teach mathematics, physics and chemistry in a college. He spent

most of his life there teaching and researching.

In 1808, he presented his atomic theory which was a turning point in

the study of matter.

JOHN DALTON F.R.S

8

JOHN DALTON’S ATOMIC THEORY OF MOLECULES

(i) All matter is made of very tiny particles called atoms.

(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

(iii) Atoms of a given element are identical in mass and chemical properties.

(iv) Atoms of different elements have different masses and chemical properties.

(v) Atoms combine in the ratio of small whole numbers to form compounds.

(vi) The relative number and kinds of atoms are constant in a given compound.

9

Compare the previous example, in compounds such as water say 9g of said to decompose ,it must decompose at a definite ratio of 1:8 grams[1g hydrogen and 8 grams of oxygen]

in hydrogen peroxide 18 g decomposes in 2g of hydrogen and 16 g of oxygen.

in both the cases consider the oxygen 8g and 16g its ratio is 8:16 or 1:2 a small integral ratio.

This led Dalton to propose “The Law Of Multiple Proportion”

The Law Of Multiple Proportion states that: “when two elements combined to form more than one compound, the masses of one element that combine with a given mass of the other element in different compounds are in the ratio of small whole numbers”

10

DRAWBACKS OF JOHN DALTON’S ATOM MODEL AND HIS ACHIEVEMENTS According to Dalton atomic Theory, atoms are

indivisible. W.k.t. atoms can be divided in electron, protons, neutrons.

This theory has missed to explain why substances like charcoal, graphite and diamond have different properties when all these substances are made up of same type of atom called carbon.

In this theory says that all the atom of same element have exactly same mass. It is known that isotopes are atoms with same element but have different masses. whereas, isobars are its vice versa.

Though his theory had many snag . He was the first to explain the atom . He was awarded with royal medal.

J.J. Thomson (1856-1940), a Britishphysicist, was born inCheetham Hill, a suburbof Manchester, on18 December 1856. Hewas awarded the Nobelprize in Physics in 1906for his work on thediscovery of electrons.He directed the Cavendish Laboratory

atCambridge for 35 years and seven of

hisresearch assistants subsequently wonNobel prizes

SIR.JOSEPH JOHN THOMSON

11

12

J.J THOMSON ATOM MODEL Thomson proposed that: An atom consists of positively charged sphere and the

electrons are embedded in it. The atom was assumed to consists of a sphere of uniform distribution of about 10-10m

His model looked like raisins stuck on the surface of a lump of pudding.

The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

13

SHORTCOMING OF J.J.THOMSON ATOM MODEL AND HIS ACHIEVEMENTS This atom could not account the electrical

neutrality of atom. J.J.Thomson was awarded with Nobel Prize

in1906 for discovery of electron. He also received knighthood from British Empire.

He directed the Cavendish Laboratory at university of Cambridge for 35 years and seven of his research assistants subsequently won Nobel prizes.

14

Ernst Rutherford (1871–1937), a former research student of J. J. Thomson, was engaged in experiments on α-particles emitted by some radioactive elements.

In 1906,he proposed a classic experiment of scattering of these α-particles by atoms to

investigate the atomic structure.This experiment was later performed around 1911 by

HansGeiger (1882–1945) and Ernst Marsden (1889–1970,

who was 20 year-old student ). The details are discussed in Section. The explanation of the results led to the birth of Rutherford’s planetary model of atom (also called the nuclear model of the atom). According to this the entire positive charge and most of the mass of the atom is concentrated in a

small volume called the nucleus with electrons revolving around the nucleus just as planets revolve

around the sun.He is called as “Father of Nuclear Physics.” He

received noble prize for chemistry in 1908.

ERNEST RUTHERFORD

15

RUTHERFORD’S PLANETARY MODEL OF ATOM a beam of 5.5 MeV α-particles emitted from

83Bi214a radioactive source at a thin metal foil

made of gold. Alpha-particles emitted by a Alpha-particles

emitted from the radioactive source were collimated into a narrow beam by their passage through lead bricks. The beam was allowed to fall on a thin foil of gold of thickness 2.1 × 10-7 m.radioactive source were collimated into a narrow beam by their passage through lead bricks.

The beam was allowed to fall on a thin foil of gold of thickness 2.1 × 10–7 m.

The scattered alpha-particles were observed through a rotatable detector consisting of zinc sulphide screen and a microscope. The scattered alpha-particles on striking the screen produced brief light flashes or scintillations. These flashes may be viewed through a microscope and the distribution of the number of scattered particles may be studied as a function of angle of scattering.

Apparatus required

16

OUTCOMES OF RUTHERFORD’S EXPERIMENT The dots in this figure represent the data

points and the solid curve is the theoretical prediction based on the assumption that the target atom has a small, dense, positively charged nucleus. Many of the α-particles pass through the foil. It means that they do not suffer any collisions. Only about 0.14% of the incident α-particles scatter by more than 1; and about 1 in 8000 deflect by more than 90. Rutherford argued that, to deflect the α-particle backwards, it must experience a large repulsive force.

This force could be provided if the greater part of the mass of the atom and its positive charge were concentrated tightly at its centre.

Then the incoming α-particle could get very close to the positive charge without penetrating it, and such a close encounter would result in a large deflection.

This agreement supported the hypothesis of the nuclear atom. This is why Rutherford is credited with thediscovery of the nucleus.

17

RUTHERFORD ELECTRON ORBITS The Rutherford nuclear model of the atom

which involves classical concepts, pictures the atom as an electrically neutral sphere consisting of a very small, massive and positively charged nucleus at the centre surrounded by the revolving electrons in their respective dynamically stable orbits.

The electrostatic force of attraction, Fe between the revolving electrons and the nucleus provides the requisite centripetal force (Fc) to keep them in their orbits. Thus, for a dynamically stable orbit in a hydrogen atom

18

19

DEFECT IN RUTHERFORD ATOM MODEL The revolution of the electron in a circular

orbit is not expected to be stable. Any particle in a circular orbit would undergo acceleration.

During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus.

If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. We know that atoms are quite stable

20

WHAT MADE BOHR TO THINK OF A NEW ATOM MODEL According to classical electromagnetic theory, an accelerating

charged particle emits radiation in the form of electromagnetic waves. The energy of an accelerating electron should therefore, continuously decrease.

The electron would spiral inward and eventually fall into the nucleus . Thus, such an atom can not be stable.

Further, according to the classical electromagnetic theory, the frequency of the electromagnetic waves emitted by the revolving electrons is equal to the frequency of revolution.

As the electrons spiral inwards, their angular velocities and hence their frequencies would change continuously, and so will the frequency of the light emitted.

Thus, they would emit a continuous spectrum, in contradiction to the line spectrum actually observed.

Clearly Rutherford model tells only a part of the story implying that the classical ideas are not sufficient to explain the atomic structure.

21

Zeeman and stark effect Zeeman effect If a substance which gives a line

emission spectrum ,is placed in a magnetic field , the lines of the spectrum gets split up into a number of closely spaced lines .

Stark effect If a substance which gives a line emission spectrum ,is placed in a electric field , the lines of the spectrum gets split up into a number of closely spaced lines .

22

Plank and Einstein atom theory

In 1900 max Planck, a professor of theoretical physics in Berlin. Showed that when you vibrate atoms strong enough, when you heat an object until it glows. i.e., releases energy in form of packets is known as “quanta”.

In1905,Einstein said that these quanta behaved as discrete particle. Which was called as einstein’s discrete particle, a “photon”.

Atoms not only emit photon ,they do absorb them

This effect is known as photoelectric effect Einstein received noble prize for his work in

photoelectric effect in 1921

photon

Nuc

leus

electron

Photo-electric effect

23

Danish physicist who explained the

spectrum of hydrogen atom

based on quantum ideas.Neil bohr studied in

Rutherford laboratory since 1912.

Bohr sucessfully combined classical and quantum ideas from plank and

einstein on a typical atom model .

Then he published three postulates which is known

as bohr’s model of hydrogen atom.

He was awarded Nobel prize in 1922

PROF. NIELS HENRIK DAVID BOHR

24

NEIL BOHRS MODEL OF AN ATOM The electron revolves

around the nucleus only in certain circular orbit . These orbit are associated with definite energies called energy shells or quantum levels.

These are numbered as 1,2,3,4…from the nucleus as K,L,M,N..etc . These are known as azimuthal quantum numbers

25

POSTULATES NEIL BOHR MODEL OF AN ATOM Bohr’s first postulate was that an electron in an atom could revolve in

certain stable orbits without the emission of radiant energy, contrary to the predictions of electromagnetic theory. According to

this postulate, each atom has certain definite stable states in which it can exist, and each possible state has definite total energy. These are called the stationary states of the atom.

Bohr’s second postulate defines these stable orbits. This postulate states that the electron revolves around the nucleus only in those orbits for which the angular momentum is some integral multiple of h/2π where h is the Planck’s constant (= 6.6 × 10–34 J s).

Thus the angular momentum (L) of the orbiting electron is quantised. That is L = nh/2π

Bohr’s third postulate incorporated into atomic theory the early quantum concepts that had been developed by Planck and Einstein.

It states that an electron might make a transition from one of its specified non-radiating orbits to another of lower energy. When it does so, a photon is emitted having energy equal to the energy difference between the initial and final states. The frequency of the emitted photon is then given by

hν = Ei – Ef where Ei and Ef are the energies of the initial and final states and Ei >

Ef .

26

SHORTCOMINGS OF BOHR ATOM MODEL According to Bohr, the radiation results when

an electron jumps from one orbit to another it emits radiation . But how it happens it was not explained by him.

Bohr successfully explained the observed spectra of atoms such as He,Li,Be etc. but he couldn't explain atoms with large number of electrons.

Bohr atom model doesn’t obey Heisenberg’s Uncertainty principle

There is no explanation for zeeman and stark effect.

27

According to sommerfields atom model, the path of an

electron around the nucleus , in general is an

ellipse with a nucleus at one of the foci.

The velocity of the electron moving in an elliptical orbit varies at different parts of the orbit. This causes the relavistic variation in the mass of moving electron.

ARNOLD SOMMERFEILD’S MODEL OF AN ATOM

28

In elliptical orbit one has to deal with two variable quantities.

The varying distance of electron from nucleus. The varying angular position of electron from

nucleus. So for this a new quantum number was introduced

Orbital or azimuthal quantum number. This is to characterize the angular momentum of

the electron .its value varies from zero to [n-1]in steps of unity

This is used to find the possible elliptical orbit by [where a and b are semi-major and semi-minor

axes for an ellipse. Consider first energy level n=1 when n=1 : l=0.

i.e.,in this energy level . So, there is only one orbit for an electron . So when a=b the two axes are equal therefore it is circular orbit. So it is said to be as 1s.

Similarly,for second energy level n=2 . There is two permissible sub-shell/orbit for electron

When n=2 l=0,1

Yellow is nucleusRed is electron

29

Various sub-shell for electrons

2p

1s

2s2p

3d

3p

3s

30

SHORTCOMINGS OF SOMMERFEILD’S ATOM MODEL It could not explain the arrangement of

electrons in atom Sommerfeild’s could not explain the spectra

of alkali metals such as potassium , sodium etc.

It could not explain zeeman and stark effect. This model does not give any explanation for

intensitites of the spectral lines.

31

Schrodinger had an very interesting idea to go with particle waves and

try to form atom model.His theory worked kind of harmonic

theory for a violin string that the vibration traveled in circles.

The world of atoms, indeed began to appear strange . It proved difficult to form an accurate picture of atom because nothing in our world really

compares with it.Schrodinger’s wave mechanics did not question the makeup of waves . But he had to call something ,so he

gave it a symbol The electron is found to be in cloud

form neither in elliptical orbits.He received Nobel prize for his

famous equation

A SHORT NOTES ON SCHRÖDINGER ATOMIC THEORY

𝛹

32

Shape of 1s,2s orbitals

Shape of p orbital

Shape of d orbital

33

WERNER HEISENBERG THEORY OF ATOM[LAW OF UN-CERTAINITY] In 1927, Heisenberg formulated the an idea, which

agreed the test , that no experiment can measure the momentum of a quantum particle simultaneously this is known as “Heisenberg un-certainty principle”

This implies that as one measures the certainty of the position of a particle , the un-certainty in the momentum gets correspondingly larger, or with an accurate knowledge about the momentum, the idea about the particle will be correspondingly less.

Heisenberg law of un-certainty states that:“it is impossible to measure simultaneously both the position and velocity (or momentum) of a microscopic particle with absoluteaccuracy or certainty.”

34

WHAT IS AN ATOM? In greek it is known as “indivisible”. Or the

very small. The smallest part of an matter . Where it

comprises of sub-atomic particles such as electrons neutrons protons etc. Its radius is measured in nanometres.

35

THANK YOU