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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 electron has a charge of –1 while the proton has a
charge of +1 (the actual charges in coulombs are –1.602 x 10–19 C and +1.602 x
10–19 C, but are written “–1” and “+1” for simplicity).
Thomson calculated the mass of electron as 9.11 x 10–28 gram. The mass of the
proton as 1.67 x 10-24 gram.
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
Symbol
Location in
the Atom
Relative
Charge
Electron
e or e–
–1
Proton
p or p+
+1
1
Neutron
n
Outside
nucleus
Inside
nucleus
Inside
nucleus
Approximate
mass ass in
Atomic Mass
Units (amu)
1/1836
0
1
1
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.
“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
3
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 spectrometer), the mass
of an atom can be obtained relative to the mass of the carbon-12 atom. The carbon12 was chosen as the reference standard and is assigned a value of exactly 12
atomic mass units. One atomic mass unit (1 amu) is equal to 1/12 the mass of the
carbon-12 atom.
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 amu exactly
98.89%
13
C
13.003amu
1.11%
2
12
C : 12 amu x 0.9889
= 11.87 amu
13.003 amu x 0.011 = 0.144 amu
12.014 amu
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.
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
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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
In a configuration, “2p2” means,
2 = energy level
p = energy sublevel
Superscript 2 = number of electrons
In writing the configuration of an element,
 Find the 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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