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Chapter 4 - MODELS OF ATOM
DALTON’S ATOMIC THEORY
Dalton proposed that:
 An element is composed of tiny, indivisible, indestructible particles
called atoms.
 All atoms of an element are identical and have the same properties.
 Atoms of different elements combine to form compounds.
 Compounds contain atoms in small whole number ratios.
 Atoms can combine in more than one ratio to form different compounds
THOMSON MODEL OF THE ATOM.
After Dalton’s proposal, there was disturbing evidence that the atom was
divisible after all. The evidence was shown by passing electricity through
sealed glass tubes. These glass tubes glowed in different colors. This
eventually led to the discovery of negatively charged particles called
Electrons and positively charged particles called Protons. The electrons has a
charge of 1- while the proton has 1+
Thomson calculated the mass of electron as 9.11x 10-28 whiles the mass of
proton as 1.67x 10-24.
RUTHERFORD MODEL OF THE ATOM
Rutherford was working in the field of radioactivity. He coined out the
terms alpha-ray, beta-ray and later gamma-ray. Rutherford proposed a new
model of the atom. He suggested that negatively charged electrons were
distributed about a positively charged nucleus. He was able to estimate the
size of the atom and its nucleus.
James Chadwick, Rutherford’s student (1932) discovered the Neutron.
Subatomic
Particle
Electron
Symbol
Proton
P+
Neutron
n
e-
Relative
Charge
Outside
nucleus
Inside
nucleus
Inside
nucleus
1
Relative
Charge
1-
Mass
1+
1
0
1
1/1836
ATOMIC NOTATION
Atomic number is the number of proton in atomic nucleus.
Mass number is the total number of protons and neutrons in the nucleus.
zSy
Where A is the mass number (number of protons+ number of neutrons)
Z is the atomic number (number of protons)
Sy is the symbol of the element
Since atoms are neutral, the number of protons = number of electrons.
ISOTOPES
Twenty elements occur naturally with fixed number of neutrons, while for
most elements, the number of neutrons vary.
Isotopes are atoms of the same element that have different numbers of
neutrons in the nucleus. Number of neutrons varies as well as mass number.
Example:
1
H - Protium
2
H - Deuterium
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H – Tritium (unstable/ radioactive)
We often refer to an isotope by stating the name and the mass number, for
example, Carbon-14, Cobalt-60. Carbon atom with mass number (Z) of 14
and atomic number (A) of 6, Cobalt atom with mass number (Z) of 60 and
atomic number (A) of 27.
ATOMIC MASS
Due to the minute mass of atoms, scientists determine the mass relative to
each other. With a special magnetic field instrument, the mass of an atom
can be composed to the mass of Carbon-12 atom. The Carbon-12 was
chosen as the reference standard and is assigned a value of exactly 12 atomic
mass units. Atomic mass unit (amu) is equal to 1/12 the mass of Carbon-12.
ATOMIC MASS OF AN ELEMENT
Atomic mass of an element is the weighted average mass of all naturally
occurring isotopes.
Isotope
Mass
Abundance
12
C
12.000amu
98.89%
13
C
13.003amu
1.11%
12
C : 12.000 amu x 0.9889 = 11.87
2
13.003 amu x 0.011 = 0.144
12.014
PERIODIC TABLE
We often refer to atomic number and atomic mass of an element.
43
Tc
(99)
43 = atomic number
(99) = mass number in bracket because the element is radioactive and
unstable
THE WAVE NATURE OF LIGHT
Wavelength is the distance the light wave travels to complete one cycle.
Frequency refers to the number of wave cycles completed in each second.
The velocity of light is constant – 3.00 x 108 m/s
The wavelength depends on the frequency and energy. The longer the
wavelength, the lower the energy and frequency and vice versa.
Light refers to radiant energy that is visible. Visible spectrum is the visible
light that our eyes can see (400nm- 700nm) but not the ultraviolet and
infrared region
A spectrum is an arrangement of electromagnetic radiations in the
increasing order of their wavelengths. Radiant energy spectrum is an
uninterrupted band, or continuous spectrum, of visible and invisible light
that ranges from short through long wavelength. Light from the sun, a lit
candle or an incandescent bulb is called white light. When resolved through
a prism gives a continuous spectrum.
A rainbow forms when sunlight passes through rain drops. Each rain drop
acts as miniature prism and separates sunlight into various bands of color.
QUANTUM CONCEPT
Max Plancks introduced the quantum concept, he stated that the energy
radiated by an object is not continuous, but rather the radiation is emitted in
small bundles. When an object radiates light, it releases a unit of radiant
energy called PHOTON.
BOHR MODEL OF THE ATOM
Bohr speculated that electrons orbital around the atomic nucleus just as
planets circle round the sun.
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Energy Level – A fixed-energy orbit in which electron travel. Bohr
suggested that electrons travel in circular orbits about the nucleus. The
electron possesses a specific energy and is said to occupy energy level. If an
electron changes orbit in the Bohr atom, there is a quantum change. Bohr
argued that an emission line spectrum results from electrons dropping from
higher energy levels to lower levels. Each time an electron drops, a photon
light in released whose energy corresponds to the difference in energy
between the two levels. The concept of electron of electron energy levels
was supported by the line spectrum of hydrogen.
ATOMIC FINGERPRINT
A line spectrum is sometimes referred to as ‘Atomic Fingerprint’ because
different elements exhibit unique set of spectral lines.
ENERGY LEVELS AND SUBLEVELS
A closer look at the emission line spectra from gases revealed energy
sublevels within main levels. The number of sublevels corresponds to the
number of energy level.
Energy level 1- 1S
Energy Level 2- 2S 2P
Energy Level 3- 3S 3P 3d
Energy Level 4- 4S 4P 4d 4f
DISTRIBUTION OF ELECTRONS BY ENERGY LEVEL
Energy Level
Energy sublevel Maximum
Maximum
electron in
electron in
sublevel
Energy level
1
1S
2e
2e
2
2S
2e
2P
6e
8e
3
3S
2e
3P
6e
3d
10e
18e
4
4S
2e
4P
6e
4d
10e
4f
14e
32e
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ELECTRON CONFIGURATION
Each energy level is filled sublevel by sublevel. The inner shell (s) filled
and the others. The first electron fills the energy closest to the nucleus. A
partial list of sublevels in order of increasing energy:
1S<2S<2P<3S<3P<4S<3d<4P<5S<4d<5P<6S
Configuration
2P2
2= energy level
P= energy sublevel
Subscript 2= number of electrons
For example, writing the configuration of an element,

Find atomic number from periodic table

Write sublevels according to increasing energy
For example, iron, the atomic number is 26, the configuration will be,
1S2 2S2 2P6 3S2 3P6 4S2 3d6
QUANTUM MECHANICAL MODEL OF ATOM
In the 1920’s our understanding of electrons in atoms became very
sophisticated. Werner Heisenberg suggested the uncertainty principle- that
is, it is impossible to simultaneously know both the precise location and
energy of an electron. Instead, the energy of an electron can be known only
in terms of its probability of being located somewhere within an atom. This
description gave rise to the Quantum Mechanical atom. A location within
the atom where there is a high probability of finding an electron having
certain energy is called an ORBITAL. An orbital is a region about the
nucleus having a given energy, size, and shape. The shape of an S orbital is
spherical, and a P orbital resembles the shape of a dumbbell.
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