Atoms and the Periodic Table I: Wave Properties
At the
beginning, the atom was suggested to have a spherical shape (Dalton, ca 1700).
After that, there have been a lot of researches to describe what happen inside
the atom. The early modern structure of atom was suggested by Ernst Rutherford.
He suggested an atom is someway similar to the solar system but there is a
problem in this model. If the electron orbits the nucleus, so the interaction
between electron and nucleus is electrostatic force. By this interaction, the
electron will be accelerated along its orbit and it causes the electron will
form have spiral orbit, and it will collapse to the nucleus with lifetime
almost 0. If we refer to this model, the result in spectroscopy would show a
continuous spectrum, but the result is a discrete or line spectrum. Spectroscopy
itself shows interaction between matter and radiation. The theory behind
spectroscopy cannot apply the classical mechanics, but another mechanics theory
should be applied for object that very small such as atomic particle, and it is
called the quantum mechanics.
Spectroscopy
basically operates as shown in schematic diagram below.
A sample is
prepared and it is shined by a light source. Then, as the sample is radiated,
there are interaction between sample and the light that will be detected by the
detector. If the detector measures the intensity of the light that is absorbed
by sample, the result will be absorption spectrum. In the other sides, if the
detector measures the emission from the sample it will be a emission spectrum.
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From this
experiment it is shown that an atom has wave properties, besides its particle
properties. This phenomenon is called wave-particle duality. As shown in H atom
spectrum, it is emitted electromagnetic radiation, and it travels in waves and
it carries energy. To begin the discussion about wave-particle duality, let’s
see the wave properties first.
WAVE
PROPERTIES
1.
Wavelength (λ)
Wavelength can be
described as the shortest distance between two equivalent points in the wave
and it is measured in unit of length. Mostly, for practical reason it is commonly
measured in micrometre (μm) or nanometre (nm). Distance from A to B is a wavelength.
2.
Frequency (ν)
Frequency is the number
of oscillation every unit of time. Since number of oscillation is
dimensionless, so the unit is s-1 or Hz.
3.
Wavenumber (
)
)
Wavenumber is the number
of oscillation per unit of length and it is commonly measured in cm-1
or m-1. The relationship between wavenumber and wavelength is:
4.
Amplitude
There is no definite
description of amplitude, because it depends on the type of wave. Generally, it
is the maximum or the minimum point at the wave, or mostly called the height of
the crest or the depth of the trough of the wave.
5.
Velocity
In this case, velocity
can be defined as how far the wave travel in unit of time and it is measured in
m s-1. Velocity is a vector quantity, so it has direction and
magnitude. Electromagnetic wave travels at the speed of light (c) in
vacuum (in vacuo). Therefore, the relationship between c and the other
wave properties is:
From the description
above, all the wave properties are not independent with each other.
As electromagnetic wave, Maxwell
describes the propagation of electromagnetic wave as shown below:
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| http://www.aethro-kinematics.com/EmWaves.jpg |
The wave is synchronously
oscillating which means it is mutually perpendicular varying sinusoidal wave
with the position of time. Moreover, a monochromatic radiation is a single
frequency.
The wave properties of
electromagnetic radiation are also experimentally proven from the diffraction
experiment. A beam of electromagnetic wave shine through a small gap which has
the separation of λ and it produces a pattern bright and dark.
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| http://wiki.chemeddl.org/mediawiki/images/3/34/Chapter_21_page_3.jpg |
The electromagnetic wave also has
particle properties. This was proven by an experiment that a plate of alkali metal
(e.g. Cs) in vacuum was shot by light. In a light there are numbers of
wavelength which carries different energy in a package that called photon.
Then, a detector was placed and it confirmed that there are electrons were
knocked out from the metal plate. This phenomenon is called Einstein’s
photoelectric effect. Moreover, not all wavelengths can knock out the electron
from an atom. Only the precise wavelength (so it implies the precise energy)
can knock out the electron from an atom, it will not work if the energy is too
low or too high. The energy that is required can be calculated as:
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