PF1 Pathan F.V.
Slide 1
PF1 Pathan Farheen, 5/5/2020
Pathan F.V.
HISTORY OF THE ATOM
1808 John Dalton
suggested that all matter was made up of
tiny particles that were indivisible and
indestructible and called them
ATOMS
Pathan F.V.
Postulates of Dalton’s Atomic Theory
• The matter is made up of indivisible particles known as atoms.
• The properties of all the atoms of a given element are the same including
mass. This can also be stated as all the atoms of an element have identical
mass while the atoms of different elements have different masses.
• Atoms of different elements combine in fixed ratios to form compounds.
• Atoms are neither created nor destroyed. This implies that during chemical
reactions, no atoms are created nor destroyed.
• The formation of new products (compounds) results from the
rearrangement of existing atoms (reactants).
• Atoms of an element are identical in mass, size and many other chemical or
physical properties, but atoms of two-different elements differ in mass, size,
and many other chemical or physical properties.
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DALTONS ATOMIC THEORY
16 X + 8Y 8 X 2Y
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Atomic Structure
All matter is composed of atoms.
Subatomic Particles
Particle Charge Mass Location
Electron (e-) -1 0 Electron cloud
Proton (p+) +1 1 Nucleus
Neutron (no) 0 1 Nucleus
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HISTORY OF THE ATOM
1898 Joseph John Thompson
found that atoms could sometimes eject a far
smaller negative particle which he called an
ELECTRON
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Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a
negatively charged particle.
Cathode ray tubes pass electricity through a gas that is contained at a very
low pressure.
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Conclusions from the Study of the Electron
Cathode rays have identical properties regardless
of the element used to produce them. All elements
must contain identically charged electrons.
Atoms are neutral, so there must be positive
particles in the atom to balance the negative charge
of the electrons
Electrons have so little mass that atoms must
contain other particles that account for most of the
mass
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Thomson’s Atomic Model
Thomson believed that the electrons were like plums
embedded in a positively charged “pudding,” thus it
was called the “plum pudding” model.
-Watermelon Model
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Thomson proposed that
1. An atom consist of a positively charged sphere and the
electrons are embedded in it.
2. The negative and positive charges are equal in magnitude.
So, the atom as a whole is electrically neutral.
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Rutherford’s Gold Foil Experiment
Alpha () particles are helium nuclei
Particles were fired at a thin sheet of gold foil
Particle hits on the detecting screen (film) are
recorded
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HISTORY OF THE ATOM
1910 Ernest Rutherford
Oversaw Geiger and Marsden carrying out his famous
experiment.
they fired Helium nuclei at a piece of gold foil which was
only a few atoms thick.
they found that although most of them passed through.
About 1 in 10,000 hit
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Rutherford’s experiment.
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Rutherford’s Findings
Most of the particles passed right through
A few particles were deflected
VERY FEW were greatly deflected
“Like howitzer shells bouncing off of tissue paper!”
Conclusions:
The nucleus is small
The nucleus is dense
The nucleus is positively charged
Results of foil experiment if Plum
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Pudding model had been correct.
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Actual Results.
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Atomic Structure
Atoms are composed of
-protons – positively charged particles
-neutrons – neutral particles
-electrons – negatively charged particles
Protons and neutrons are located in the nucleus.
Electrons are found in orbitals surrounding the
nucleus.
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HELIUM ATOM
Shell
proton
+
N
-
+
- N
neutron
electron
