Lecture 5 & 6

Eelo American University, Borama, Awdal, 11/6/2008
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EELO AMERICAN UNIVERSITY
BORAMA, AWDAL Democritus
SOMALILAND 400 BC

• This is the Greek philosopher
Democritus who began the search for
a description of matter more than
2400 years ago.
Md. Faysal Ahamed Khan – He asked: Could matter be
Lecture 5-Structure of Atom divided into smaller and smaller
pieces forever, or was there a
& limit to the number of times a
Lecture 6 - Quantum theory & Atomic piece of matter could be divided?
structure

Welcome to the class of Chemistry I
Course No. CHEM 211
Credit hours 3

Atomos Atomos

• His theory: Matter could § To Democritus, atoms
not be divided into were small, hard particles
smaller and smaller pieces that were all made of the
same material but were
forever, eventually the
different shapes and
smallest possible piece
sizes.
would be obtained.
§ Atoms were infinite in
• This piece would be number, always moving
indivisible. and capable of joining
• He named the smallest together.
piece of matter “atomos,”
meaning “not divisible.”
This theory was ignored and forgotten
for more than 2000 years!

Prepared byMd. Faysal Ahamed Khan

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• The eminent
philosophers of
the time,
Aristotle and
Plato, had
more respect,
and ultimately
wrong theory.
Aristotle and Plato favored the earth,
fire, air and water approach to the
nature of matter. Their ideas was
believed because of their eminence as
philosophers. The atomos idea was
buried for approximately 2000 years.

Laws of conservation
Dalton’s Model
of Mass
In the early 1800s, the English
Chemist John Dalton
performed a number of
experiments that eventually Mass can neither be created nor destroyed in Chemical
led to the acceptance of the reaction
idea of atoms. His theory of
atom was based on the
following laws:
1. Laws of conservation of 2H2 + O2 = 2 H2O
mass 4x1.008 16x2 2x(2x1.008 + 16)
=4.032 gm =32 gm = 2x18.016
2. Laws of constant = 36.032 gm
composition 36.032 gm
3. Laws of multiple
proportion


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Laws of multiple proportion
Laws of Constant Composition
Same elements can combine in different
No matter what its source, a particular chemical ways to form different substances, whose
compound is composed of the same elements in mass ratios are small whole number
the same fraction by mass multiples of each other. That is if
elements A and B reacts to form two
compounds, the different masses of B
By Mass water is: 88.8% oxygen that combine with a fixed mass of A can
11.2% hydrogen be express as a ratio of small whole
numbers.

How Dalton’s theory explains the
Dalton’s Theory mass laws
• He deduced that all elements are • Mass Conservation: Since each type of atoms have
composed of atoms. Atoms are a fixed mass, a chemical reaction, in which atoms
indivisible and indestructible are just combined differently with each other, cannot
particles. possibly result a mass change.
• Atoms of one element cannot be • Definite Composition: A compound is a
This theory converted into atoms of another combination of specific ratio of different atoms,
became one elements each of which has a particular mass. Thus, each
of the
foundations • Atoms of the same element are element in a compound constitutes a fixed fraction
of modern identical in mass & properties. of total mass.
chemistry. Atoms of different elements are • Multiple Proportions: Atoms of an element have
different. the same mass and are indivisible. Because different
• Compounds are formed by the numbers of B atoms combine with each A atom in
joining of atoms of two or more different compounds, the mass of element B that
elements by a specific ratio. combine with a fixed mass of element A give a
small, whole-number ratio.


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Thomson’s Plum Pudding
Limitation of Dalton’s model Model
• In 1897, the English
• Atoms are divisible scientist J.J.
• In a nuclear reaction atoms of one Thomson provided
elements often changes to atoms of the first hint that an
other elements atom is made of
even smaller
• Isotopes & Isobars particles.
He proposed a
model of the atom
that is sometimes
called the “Plum
Pudding” model or
Raisin Bread
model.

Thomson Model Thomson Model
Where
did they
• Atoms were made from a • This surprised come
positively charged Thomson, because
from?
substance with the atoms of the gas
negatively charged
electrons scattered about, were uncharged.
like raisins in a pudding. Where had the
negative charges
• Thomson studied the come from?
passage of an electric
current through a
gas. Thomson concluded that the negative charges
came from within the atom.
• As the current passed
through the gas, it
A particle smaller than an atom had to exist.
gave off rays of
negatively charged
The atom was divisible!
particles.


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Rutherford’s Gold Foil Experiment

Thomson called the negatively charged “corpuscles,” • In 1910, after the
today known as electrons. discovery of radioactivity,
emission of particles
and/or radiation from
Since the gas was known to be neutral, having no atoms of certain elements,
charge, he reasoned that there must be positively the English physicist
charged particles in the atom. Ernest Rutherford used
one type of radioactive
But he could never find them. particle in a series of
experiment that that solved
the mysteries of the atomic
structure.

• Rutherford’s experiment Involved firing
a stream of tiny positively charged
particles at a thin sheet of gold foil
(2000 atoms thick)

– Most of the positively charged “bullets” passed
right through the gold atoms in the sheet of gold
foil without changing course at all.
– Some of the positively charged “bullets,”
however, did bounce away from the gold sheet as
if they had hit something solid. He knew that
positive charges repel positive charges.

Expect:

1. Mostly small angle scattering
2. No backward scattering events

Results:

1. Some small scattering events
2. Several backward scatterings!!!


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Rutherford
• This could only mean that the gold atoms in • Rutherford reasoned that all of an atom’s
the sheet were mostly open space. Atoms positively charged particles were contained
were not a pudding filled with a positively in the nucleus. The negatively charged
charged material.
particles were scattered outside the nucleus
• Rutherford concluded that an atom had a around the atom’s edge.
small, dense, positively charged center that
repelled his positively charged “bullets.”
• He called the center of the atom the A new question arises. If the electron is
“nucleus” negatively charged, won't the attraction
• The nucleus is tiny compared to the atom as for electrons by the nucleus cause the
a whole. electron to fall into the nucleus and
therefore atoms should collapse.

Rutherford attempts to explain his Limitation of Rutherford’s
experimental results. model
He knew about the solar system - the attraction • A nucleus and an electron attract each other,
of the planets by the sun - universal gravitation. so if they are to remain apart, the energy of
Yet planets are not pulled into the sun. They are electron’s motion (kinetic energy) must
in motion around the sun and this motion balance the energy of attraction (potential
prevents them from being pulled into the sun. energy). According to the laws of classical
physics says that a negative particle moving
So Rutherford puts the electron in motion in a curved path around a positive one must
around the nucleus. emit radiation and thus lose energy, so
orbiting electron will lose energy
continuously and spiral into nucleus.

Electron
crashes
into the
nucleus!?


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Limitation (Cont’d)

• In solar system, the planets are
electrically neutral but in atoms the
electrons and protons are electrically Quantum theory & Atomic
charged
• The shape and size of electrons structure
orbiting path is still undetermined and
Rutherford did not give any idea about Lecture 6
that also
• In case of more than one electrons
atom Rutherford did not explain how
they orbit around the nucleus

From the failure of Rutherford's
model Electromagnetic radiation
• Neils Bohr, a Danish Physicist, points
out that laws of physics do not apply for • Energy can be transmitted through
the submicroscopic world of ATOM space by Electromagnetic radiation
• In 1913 Bohr proposed a new model • Electro magnetic radiation consists of
based on the modern quantum theory of
energy waves which have both electrical &
• With this model he was able to explain magnetic properties
- why orbiting e- do not collapsed to nucleus Ex: stone thrown to the pond creates wave like
- how atomic spectra occurs electromagnetic waves
• To understand Bohr theory first we need • Waves convey the energy from one
to know place to another
- Nature of electromagnetic radiation • They travels through empty space at
- Atomic Spectra
the speed of light
- Quantum theory of energy


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wavelength
Visible light
Wave nature of Light

Amplitude
• Light is electromagnetic radiation - crossed
electric and magnetic waves: wavelength
Node
Ultaviolet radiation

Properties : Wavelength - distance between consecutive peaks -
Wavelength, (nm) crests - measured in m, nm, angstroms.
Frequency, n (s-1, frequency - (nu) - number of times per second a crest
passes a given point (cycles per second)
Hz)
Amplitude, A All waves have:
Magnetic vector
frequency and wavelength

symbol: ν (Greek letter “nu”) λ (Greek “lambda”)
units: “cycles per sec” = Hertz “distance” (nm)

• All radiation: λ•ν= c
where c = velocity of light = 3.00 x 108 m/sec

Note:
Long wavelength
→ small frequency
Short wavelength
→ high frequency
Reference: See Fig:7.3 of page 257 of Silberg’s Chemistry Book

• Wave nature of light is shown by classical
wave properties such as
increasing increasing
wavelength
• interference
frequency
• diffraction
Example:Red light has λ = 700 nm.Calculate the frequency, ν. • refraction
8
c = 3.00 x 10 m/s = 4.29 x 10 14 Hz
ν=
λ 7.00 x 10 -7 m


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Particle nature of light Quantum theory of
electromagnetic radiation
Blackbody radiation & quantization of
energy § In 1900 Max Planck observed the fact
§ He proposed that light radiation produced
When a solid body is heated it’s began to emit
from a hot body discontinuously, in small
visible light and with the increase in
unit of wave, each of which have a
temperature the light began to brighter
specific frequency, which increase with
Observed changes in intensity & wavelength
temperature.
of emitted light Continuous wave

- This is blackbody radiation –
Discontinuous waves or QUANTA
Attempts to explain this observed change by
classical wave theory failed § Unit of wave = Quantum (plural, quanta)
- because in a continuous wave wavelength § The energy of Quanta is given by the
/frequency is fixed and not changing in its path following relation
E = nhν

Photoelectric effect & photon theory of
- So, Energy is QUANTIZED = that is you light
get energy in certain no. quantity like
particle/ packet, that is you can count, it is When a beam of light of sufficiently high
not continuous frequency is allowed to strike a metal surface
- Where did the radiation/quantum energy in vacuum, e- ejected from the surface- this is
came from? Ans: Hot object’s radiation is Photoelectric effect
emitted by the atoms contains within it.
- It means atoms contains only certain
quantity of energy
- So the energy of atom is also
QUANTIZED
- So, change in atoms energy means gain or
loss of one or more “packets” of energy
- So, an atom changes its energy state by
emitting one or more quanta of energy


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Einstein takes Planck's theory and extends it to the
Observation 1
structure of light itself. He doesn't think of it as a
Violet light will cause potassium to eject continuous wave, but as chunks/parts of
electrons, no amount of red light, no matter 1wavelength. He proposed light itself like energy
how bright (intense) it is will not have any is also particulate, occurring as quanta. He calls
these chunks of light, photons.
effect. Einstein was surprised that the
Explanation of observation 1:
threshold energy was related to color rather In order to release an e from metal surface, the
than the intensity. incident photon has first to overcome the
They are only ejected when the frequency of attractive force exerted by the positive ion of the
the light exceeds a certain threshold value for metal
each particular metal. The energy of photon proportional to its
frequency of incident light
Observation 2
More electrons are ejected with brighter light Explanation of observation 2:
of a certain color, but the energy of each The more the intensity = the more no. of photon
= each photon releasing one e but the energy of
electron is the same.
each e is fixed as frequency of each photon is
fixed

Spectrum
• Visible light/white light is radiant energy coming from
sun or from incandescent lamps.
• It composed of light waves in the range of 400-
Planck’s quantum theory & Einstein’s 800nm. Each wave has a characteristic color
photon theory proved that • When a beam of white light from an incandescent
light bulb pass through a prism, diff wavelengths are
refracted at diff angles, when received on a screen these
- Energy/electromagnetic radiation/light form series of color bands: VIBGYOR, Shorter λ's are
has fixed quantity & it is discrete bent more than longer λ 's.
particle
• Thus a continuous spectrum results
But those were only true for matter only
previously.


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Pass a current through a hydrogen gas in a
discharge tube at low pressure. The gas glows. Bohr Model
When examined through a spectroscope, a line • In 1913, the
spectrum is observed, not a continuous one as Danish scientist
seen with the light bulb. Niels Bohr
proposed a model
for H atom and
explain the line
spectra of
Hydrogen.
• In his model, Bohr
used Planck’s and
Einstein’s idea
about quantized
energy and
proposed three
The wave theory of light could not explain postulates:
the line spectra of excited gases

Bohr Model Bohr model cont’d
1. Electrons travel around the nucleus in
specific orbits at fixed distance from 2. While in these orbits, an e does not
the nucleus and e- in each orbit have a radiate (or lose) energy. These orbit
definite energy. is called energy levels.
3. The electron can moves to another
orbit/energy level only by absorbing
or emitting a photon/quantum,
whose energy equals the difference
in energy between the two
orbits/levels.
Ephoton = Estate A – Estate B = E = hν


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Bohr explanation for line LIMITATION of BOHR
spectrum MODEL
• Bohr ‘s model explain
that an atomic But problems existed with Bohr theory —
spectrum is not – theory only successful for the H atom,
continuous because a that is it failed for more that one electron
atom has only certain atom
discrete energy levels
– Electrons do not travels in fixed orbits
or states
• A spectrum line
results when a photon
of specific energy
(and thus specific
energy) is emitted as
electron moves from a
higher energy states to
a lower energy states

THE WAVE MODEL The Wave Model
• Today’s atomic • In fact, it is impossible to determine the
model is based exact location of an electron. The probable
on the principles location of an electron is based on how
of wave much energy the electron has.
mechanics. • According to the modern atomic model, at
• According to the atom has a small positively charged nucleus
theory of wave surrounded by a large region in which there
mechanics, are enough electrons to make an atom
electrons do not neutral.
move about an
atom in a
definite
path/ORBIT.


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Electron Cloud:
• ELECTRON CLOUD is a space in
which electrons are likely to be
found.
• Electrons whirl about the nucleus
billions of times in one second
• They are not moving around in
random patterns.
• Location of electrons depends upon
how much energy the electron has.
• Depending on their energy they are
locked into a certain area in the
cloud.
• Electrons with the lowest energy
are found in the energy level
closest to the nucleus
• Electrons with the highest energy
are found in the outermost energy
levels, farther from the nucleus.


Lecture 5 & 6

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