DEVELOPMENT OF THE DEVELOPMENT OF THE ATOMIC MODELATOMIC MODEL

From Democritus to RutherfordFrom Democritus to Rutherford

Dr, Anilkumar R. Dr, Anilkumar R. KoppalkarKoppalkar S.S.Margol College S.S.Margol College ShahabadShahabad

c.400 B.C.c.400 B.C.Ancient Greek PhilosopyAncient Greek Philosopy

Everything in the Everything in the universe is made of universe is made of one or more of the one or more of the basic “elements:”basic “elements:”

Earth, Fire, Water, AirEarth, Fire, Water, Air

What makes up the elements?What makes up the elements?

ARISTOTLEARISTOTLE Matter is infinitely Matter is infinitely

divisible; no matter divisible; no matter how small a piece is, it how small a piece is, it can always be divided can always be divided into smaller piecesinto smaller pieces

DEMOCRITUSDEMOCRITUS There exists a There exists a

“smallest piece” of “smallest piece” of matter, which cannot matter, which cannot be divided any further.be divided any further.

These pieces are These pieces are called “called “ατομοσατομοσ,” or ,” or “atoms”“atoms”

Aristotle Democritus

Whose Argument Prevails?Whose Argument Prevails?

Aristotle’s viewpoint enjoyed the support of Aristotle’s viewpoint enjoyed the support of most of the world because he was more most of the world because he was more well-known and because Democritus had well-known and because Democritus had no evidence to back up his claim since no evidence to back up his claim since these “atoms” would be too small to see.these “atoms” would be too small to see.

1803: John Dalton1803: John Dalton

Dalton was an Dalton was an English schoolteacherEnglish schoolteacher

Began teaching Began teaching mathematics and mathematics and chemistry at the age chemistry at the age of 12of 12

Revived the idea of Revived the idea of Democritus’ “smallest Democritus’ “smallest piece” of matterpiece” of matter

Dalton’s Atomic ModelDalton’s Atomic Model

All matter is made of tiny particles called All matter is made of tiny particles called “atoms”“atoms”

Atoms are indivisible and indestructibleAtoms are indivisible and indestructibleAtoms of the same element are identicalAtoms of the same element are identicalAtoms of different elements differ in some Atoms of different elements differ in some

fundamental wayfundamental wayAtoms combine in simple whole number Atoms combine in simple whole number

ratios to form compoundsratios to form compounds

Why did people believe Dalton?Why did people believe Dalton?

He had EVIDENCE!He had EVIDENCE! Dalton found that compounds always contained Dalton found that compounds always contained

the same mass ratio of one element to anotherthe same mass ratio of one element to another This is now known as the “Law of Definite This is now known as the “Law of Definite

Proportions”Proportions” Elements could combine in different ratios, but Elements could combine in different ratios, but

when they did, they would make different when they did, they would make different compoundscompounds

This is now known as the “Law of Multiple This is now known as the “Law of Multiple Proportions”Proportions”

So what?So what?

These results could only be explained by These results could only be explained by assuming that matter was made of atoms assuming that matter was made of atoms – tiny building blocks – and that these – tiny building blocks – and that these atoms only came in certain sizes. atoms only came in certain sizes.

Dalton’s View of an atom

Hooray, Dalton!Hooray, Dalton!

Besides giving us his Atomic Theory, Besides giving us his Atomic Theory, Dalton did much more in the newly-Dalton did much more in the newly-emerging field of chemistry:emerging field of chemistry:

He worked with mixtures of gases and determined He worked with mixtures of gases and determined how the pressure was related to their proportionshow the pressure was related to their proportions

He made a list of all the known elements from He made a list of all the known elements from lightest to heaviestlightest to heaviest

1869: Dmitri Mendeleev1869: Dmitri Mendeleev

Russian chemistRussian chemist Arranged elements in Arranged elements in

tabular form so that tabular form so that elements with similar elements with similar properties were in the properties were in the same columnsame column

When listed in order When listed in order by mass, elements by mass, elements generally repeat generally repeat properties in groups properties in groups of 8 (Law of Octaves)of 8 (Law of Octaves)

The First Periodic TableThe First Periodic Table Most tables at the time listed elements by massMost tables at the time listed elements by mass Mendeleev also arranged elements by mass, but Mendeleev also arranged elements by mass, but

left several “holes” in his table and occasionally left several “holes” in his table and occasionally reversed the order of elements to fit the reversed the order of elements to fit the properties of others in that columnproperties of others in that column

The “holes” were later filled in with newly The “holes” were later filled in with newly discovered elements that had the properties discovered elements that had the properties predicted by Mendeleev’s table.predicted by Mendeleev’s table.

The reason for the reversal of elements was The reason for the reversal of elements was explained later by Henry Moseley, who noted explained later by Henry Moseley, who noted that the elements were in order by atomic that the elements were in order by atomic number (number of protons) rather than by massnumber (number of protons) rather than by mass

1897: J.J. Thomson1897: J.J. Thomson

English physicistEnglish physicist Worked with Worked with

Cathode-Ray Tubes Cathode-Ray Tubes (CRTs)(CRTs)

Credited with the Credited with the discovery of the discovery of the electronelectron

CRTsCRTs

Mysterious particles Mysterious particles emanated from the emanated from the cathode endcathode end

These particles were These particles were deflected by magnetic deflected by magnetic and electric fieldsand electric fields

They were very small They were very small and negatively and negatively chargedcharged

These particles were called “electrons” and were assumed to be a part of all matter

Thomson’s Atomic ModelThomson’s Atomic Model

Electrons

Positively charged “goo”

“Plum-Pudding Model”

1909-11: Robert Millikan1909-11: Robert Millikan

Set out to discover Set out to discover the charge of a single the charge of a single electronelectron

Famous experiment Famous experiment called the “oil-drop called the “oil-drop experiment”experiment”

Using his results and Using his results and the charge-to-mass the charge-to-mass ratio from Thomson, ratio from Thomson, the mass of an the mass of an electron was foundelectron was found

Procedure:

(1) Oil drop is placed on the dry glass substrate;

(2) Solution of surfactant + NaCl is poured in the cuvette;

(3) The process of oil drop detachment is recorded by horizontal microscope and video-camera.

Scheme of the experimental cell:

1 – glass plate;

2 – oil droplet 1 l;

3 – glass holders;

4 – surfactant solution;

5 – syringe;

6 – cuvette.

The Oil Drop ExperimentThe Oil Drop Experiment

1910: Ernest Rutherford1910: Ernest Rutherford

Expert in radiationExpert in radiation Famous “Gold-Foil Famous “Gold-Foil

Experiment”Experiment” Discovered the Discovered the

presence of the presence of the nucleus by firing nucleus by firing alpha particles at a alpha particles at a sheet of gold foilsheet of gold foil

Ernest Rutherford Ernest Rutherford (movie: 10 min.)(movie: 10 min.)

Most particles passed through. So, atoms are mostly empty.

Some positive -particles deflected or bounced back!

Thus, a “nucleus” is positive & holds most of an atom’s mass.

Radioactive

substance

path of invisible -particles

Rutherford shot alpha (Rutherford shot alpha () particles at gold foil.) particles at gold foil.

Lead block

Zinc sulfide screen Thin gold foil

Rutherford’s Atomic ModelRutherford’s Atomic Model

Electrons

NucleusPositively chargedMade of “protons”

Empty Space

1932: James Chadwick1932: James Chadwick

A fellow researcher with Rutherford, A fellow researcher with Rutherford, Chadwick discovered years later that the Chadwick discovered years later that the nucleus was not made of only one particle nucleus was not made of only one particle – the proton – but of two particles.– the proton – but of two particles.

This second particle was called the This second particle was called the “neutron” because it had no electrical “neutron” because it had no electrical chargecharge

HISTORY OF THE ATOMHISTORY OF THE ATOM

1913 Niels Bohr

studied under Rutherford at the Victoria

University in Manchester.

Bohr refined Rutherford's idea by

adding that the electrons were in

orbits. Rather like planets orbiting the

sun. With each orbit only able to

contain a set number of electrons.

Bohr’s Atom

electrons in orbits

nucleus

HELIUM ATOM

+N

N

+-

-

proton

electron

neutron

Shell

What do these particles consist of?

ATOMIC STRUCTUREATOMIC STRUCTURE

Particle

proton

neutron

electron

Charge

+ ve charge

-ve charge

No charge

1

1

nil

Mass

THANK YOUTHANK YOU