Download DEVELOPMENT OF THE ATOMIC MODEL

DEVELOPMENT OF THE
ATOMIC MODEL
From Democritus to Rutherford
Dr, Anilkumar R. Koppalkar
S.S.Margol College Shahabad
c.400 B.C.
Ancient Greek Philosopy

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Everything in the
universe is made of
one or more of the
basic “elements:”
Earth, Fire, Water, Air
What makes up the elements?

ARISTOTLE

Matter is infinitely
divisible; no matter
how small a piece is, it
can always be divided
into smaller pieces
Aristotle

DEMOCRITUS

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There exists a
“smallest piece” of
matter, which cannot
be divided any further.
These pieces are
called “ατομοσ,” or
“atoms”
Democritus
Whose Argument Prevails?

Aristotle’s viewpoint enjoyed the support of
most of the world because he was more
well-known and because Democritus had
no evidence to back up his claim since
these “atoms” would be too small to see.
1803: John Dalton

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Dalton was an
English schoolteacher
Began teaching
mathematics and
chemistry at the age
of 12
Revived the idea of
Democritus’ “smallest
piece” of matter
Dalton’s Atomic Model
All matter is made of tiny particles called
“atoms”
 Atoms are indivisible and indestructible
 Atoms of the same element are identical
 Atoms of different elements differ in some
fundamental way
 Atoms combine in simple whole number
ratios to form compounds

Why did people believe Dalton?

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He had EVIDENCE!
Dalton found that compounds always contained
the same mass ratio of one element to another
This is now known as the “Law of Definite
Proportions”
Elements could combine in different ratios, but
when they did, they would make different
compounds
This is now known as the “Law of Multiple
Proportions”
So what?

These results could only be explained by
assuming that matter was made of atoms
– tiny building blocks – and that these
atoms only came in certain sizes.
Dalton’s View of an atom
Hooray, Dalton!

Besides giving us his Atomic Theory,
Dalton did much more in the newlyemerging field of chemistry:
 He
worked with mixtures of gases and determined
how the pressure was related to their proportions
 He made a list of all the known elements from
lightest to heaviest
1869: Dmitri Mendeleev



Russian chemist
Arranged elements in
tabular form so that
elements with similar
properties were in the
same column
When listed in order
by mass, elements
generally repeat
properties in groups
of 8 (Law of Octaves)
The First Periodic Table

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Most tables at the time listed elements by mass
Mendeleev also arranged elements by mass, but
left several “holes” in his table and occasionally
reversed the order of elements to fit the
properties of others in that column
The “holes” were later filled in with newly
discovered elements that had the properties
predicted by Mendeleev’s table.
The reason for the reversal of elements was
explained later by Henry Moseley, who noted
that the elements were in order by atomic
number (number of protons) rather than by mass
1897: J.J. Thomson



English physicist
Worked with
Cathode-Ray Tubes
(CRTs)
Credited with the
discovery of the
electron
CRTs



Mysterious particles
emanated from the
cathode end
These particles were
deflected by magnetic
and electric fields
They were very small
and negatively
charged
These particles were
called “electrons” and
were assumed to be a
part of all matter
Thomson’s Atomic Model
Electrons
Positively charged “goo”
“Plum-Pudding Model”
1909-11: Robert Millikan

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Set out to discover
the charge of a single
electron
Famous experiment
called the “oil-drop
experiment”
Using his results and
the charge-to-mass
ratio from Thomson,
the mass of an
electron was found
The Oil Drop Experiment
Scheme of the
experimental cell:
1 – glass plate;
2 – oil droplet  1 l;
3 – glass holders;
4 – surfactant solution;
5 – syringe;
6 – cuvette.
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.
1910: Ernest Rutherford
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Expert in radiation
Famous “Gold-Foil
Experiment”
Discovered the
presence of the
nucleus by firing
alpha particles at a
sheet of gold foil
Ernest Rutherford (movie: 10 min.)
 Rutherford
shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactiv
e
substance
path of invisible
-particles
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.
Rutherford’s Atomic Model
Electrons
Empty Space
Nucleus
Positively charged
Made of “protons”
1932: James Chadwick
A fellow researcher with Rutherford,
Chadwick discovered years later that the
nucleus was not made of only one particle
– the proton – but of two particles.
 This second particle was called the
“neutron” because it had no electrical
charge

HISTORY 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
Shell
proton
+
-
N
N
+
electron
What do these particles consist of?
-
neutron
ATOMIC STRUCTURE
Particle
Charge
Mass
proton
+ ve charge
1
neutron
No charge
1
electron
-ve charge
nil
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