charge on the two is the same.
electrons. The atom as a whole is, therefore, electrically neutral.
hydrogen has 1 proton, carbon has 6 protons, and silver has 47 protons.
the size of the atom. He called this concentration of positive charge the nucleus of the
outside the nucleus. To explain why these electrons where not pulled into the nucleus
by the attractive electric force, Rutherford modeled them as moving in orbits around
the nucleus in the same manner as the planets orbit the Sun. For this reason, this model
is often referred to as the planetary model of the atom (Figure 1).
neutron, which has approximately the same mass as the proton but is electrically
the total atomic volume. The diameter of the whole atom is on the order of 10−8 cm,
but the diameter of the nucleus is only about 10−13 cm.
electromagnetic radiation and no others, which Rutherford′ s model cannot explain
acceleration, but centripetally accelerated charges revolving with frequency ƒ should
radiate electromagnetic waves of the same frequency ƒ.
electric force of attraction as shown in Figure 2.
only in specific discrete radii.
called them, the electron does not emit energy in the form of radiation. Hence the total
describe the electron’s motion. This representation claims that the centripetally
accelerated electron does not continuously emit radiation, losing energy and
eventually spiraling into the nucleus, as predicted by Rutherford ′s planetary model.
energetic initial orbit to a lower-energy orbit. This transition can not be visualized.
Bohr was able to calculate the radii of these allowed orbits and shows that, the
radiation or spectral lines are emitted as a consequence of orbital restrictions. The
orbital restrictions are most easily illustrated with the simplest atom hydrogen,
which has a single proton nucleus and one electron orbiting around it. Unless energy
is added to the atom, the electron is found in the allowed orbit closet to the nucleus. If
energy is added to the atom, the electron may “jump” to one of the higher allowed
orbits farther away from the nucleus, but the electron can never occupy the regions
between the allowed orbits (Figure 3).
discrete orbits with radii 1, 2, 3, and so on.
observations for the simple hydrogen atom. But to describe the behavior of atoms
with more than one electron, it was necessary to impose an additional restriction on
the structure of the atom; the number of electrons in a given orbit cannot be greater
than 2n2, where n is the order of the orbit from the nucleus, trough which the
maximum number of electrons in each orbit can be calculated according to the
has two electrons, and, therefore, its first orbit is filled. Lithium has three electrons,
two of which fill the first orbit; the third electron, therefore, must be in the second
orbit. This simple sequence is not completely applicable to the very complex atoms,
but basically this is the way the elements are constructed.
the electron. Therefore, instead of speaking of the electron as being in a certain orbit,
we can refer to it as having a corresponding amount of energy. Each of these allowed
values of energy is called an energy level. An energy level diagram for an atom is
shown in Figure 4. Note that every element has its own characteristic energy level
structure. The electrons in the atom can occupy only specific energy states; that is, in a
given atom the electron can have energy E1, E2, E3, and so on, but cannot have energy
between these two values. This is a direct consequence of the restrictions on the
allowed electron orbital configurations.
state is associated with the orbital configuration closest to the nucleus. The higher
allowed energy levels, called excited states, are associated with larger orbits and
different orbital shapes. Normally the electron occupies the lowest energy level but it
can be excited into a higher energy state by adding energy to the atom.
ways. The two most common methods of excitation are:
current is passed through a gas of atoms, the colliding electron is slowed down and
the electron in the atom is promoted to a higher energy configuration. When the
excited atoms fall back into the lower energy states, the excess energy is given off
as electromagnetic radiation. Each atom releases its excess energy in a single
photon. Therefore, the energy of the photon is simply the difference between the
energies of the initial state Ei and the final state Ef of the atom. The frequency ƒ of
the emitted radiation is given by
specific frequency, called transition or resonance frequency. Therefore, a group of
highly excited atoms of a given element emit light at a number of well-defined
frequencies which constitute the optical spectrum for that element.
specific frequency. The frequency must be such that each photon has just the right
amount of energy to promote the atom to one of its higher allowed energy states.
Atoms, therefore, absorb light only at the specific transition frequencies, given by
Equation 1. Light at other frequencies is not absorbed. If a beam of white light
(containing all the frequencies) is passed through a group of atoms of a given species,
the spectrum of the transmitted light shows gaps corresponding to the absorption of
the specific frequencies by the atoms. This is called the absorption spectrum of the
atom. In their undisturbed state, most of the atoms are in the ground state. The
absorption spectrum, therefore, usually contains only lines associated with transitions
from the ground state to higher allowed states (Figure 5). Optical spectra are produced
by the outer electrons of the atom. The inner electrons, those closer to the nucleus, are
bound more tightly and are consequently more difficult to excite. However, in a
highly energetic collision with another particle, an inner electron may be excited.
When in such an excited atom an electron returns to the inner orbit, the excess energy
is again released as a quantum (photon) of electromagnetic radiation. Because the
binding energy here is about a thousand times greater than for the outer electrons, the
frequency of the emitted radiation is correspondingly higher. Electromagnetic
radiation in this frequency range is called X - rays.
formation of chemical compounds and matter in bulk is due to the distribution of
electrons in the atomic orbits. When an orbit is not filled to capacity (which is the
case for most atoms), the electrons of one atom can partially occupy the orbit of
another. This sharing of orbits draws the atoms together and produces bonding
between atoms. As an example we show in Figure 6, the formation of a hydrogen
molecule from two hydrogen atoms.
in the binding of the two atoms into a molecule.
hydrogen atoms are close together, they share each other’s electrons, in this way
the orbit of each atom is completely filled part of the time. This shared orbit can
be pictured as a rubber band pulling the two atoms together.
sharing of electrons pulls the atoms together, the repulsion force of the nuclei tends
to keep them apart. The equilibrium separation between atoms in a molecule is
determined by these two counter forces. In a similar way, more complex molecules,
and ultimately bulk matter, are formed. Atoms with completely filled orbits (these
are atoms of the so-called noble gases) as helium, neon, argon, krypton, and xenon,
cannot share electrons with other elements and are, therefore, chemically most inert.
Because molecules are more complicated than atoms, their spectra are
correspondingly more complex. In addition to the electronic configuration, these
spectra also depend on the motion of the nuclei. Still the spectra can be interpreted
and are unique for each type of molecule.
atoms and molecules in the body. From a spectroscopic analysis of urine, for example,
one can determine the level of mercury in the body. Blood sugar level is measured by
first producing a chemical reaction in the blood sample which results in a colored
product. The concentration of this colored product, which is proportional to the blood-
sugar level, is then measured by absorption spectroscopy.
components in the substance. The absorption spectra can also provide information about the concentration of all the components in a substance. In absorption spectroscopy, the amount of absorption can be related to the concentration.
section of the spectrum is detected at a time.
position of the prism is calibrated to correspond with the wavelength impinging on the
electrical signal proportional to the light intensity. The intensity of the signal as a
function of wavelength can be displayed on a chart recorder.
time. Contrary to classical mechanics, one can never make simultaneous predictions of conjugate variables, such as position and momentum, with arbitrary accuracy. For instance, electrons may be considered to be located somewhere within a region of space, but with their exact positions being unknown. Contours of constant probability, often referred to as “clouds” may be drawn around the nucleus of an atom to conceptualize where the electron might be located with the most probability (Figure 8).
development of electron microscopes that can observe objects 1000 times smaller than are visible with light microscopes.
into a beam.
electrons in much the same way as light is diffracted in an optical microscope. But
because of their short wavelength, the electrons are influenced by much smaller
structures within the sample.
image onto film or a fluorescent screen.
provide the continuous curve in Figure 11, which shows the cutoff of x-rays below a minimum wavelength value that depends on the kinetic energy of the incoming electrons. X-ray radiation with its origin in the slowing down of electrons or continuous spectrum shown in the Figure is called Bremsstrahlung, the German word for “braking radiation.”
of sharp lines, which are due to characteristic x-rays. The data shown were obtained when 37-
ke V electrons bombarded a molybdenum target.
= 1) of the atom.
The time interval for this to happen is very short, less than 10-9 s.
the energy difference between the two levels. Thus the photon emitted has an energy
corresponding to the K α, the characteristic x-ray line on the curve in Figure 11.
the initial level as the first one above the final level. Thus Kα indicates that, a
transition occur from energy level L (n = 2) to energy level K (n = 1)].
= 3), the Kβ line in Figure 11 is produced.
A higher orbital electron fills the empty position, releasing its excess energy as a photon.
are slowed or even stopped in passing near the positively charged nuclei of the anode
material. This is the Brehmsstrahlung radiation.
incoming electrons from the cathode knock electrons near the nuclei out of orbit and
they are replaced by other electrons from outer orbits. X - rays produced in this way
have definite energies just like other line spectra from atomic electrons. They are
Characteristic x-rays. The spectral lines generated depend on the target (anode)
element used and thus are called characteristic lines.
physicians, Oudin and Barthelemy, obtained x-rays of bones in a hand. Since then, x-
rays have become one of the most important diagnostic tools in medicine. With
current techniques, it is even possible to view internal body organs that are quite
transparent to x-rays. This is done by injecting into the organ a fluid opaque to x-
rays. The walls of the organ then show up clearly by contrast.
provide depth information. The image represents the total attenuation as the x-ray
beam passes through the object in its path. For example, a conventional x-ray of the
lung may reveal the existence of a tumor, but it will not show how deep in the lung the
tumor is located. Several tomographic techniques (CT scans) have been developed to
produce slice-images within the body which provide depth information. Presently the
most commonly used of these is x-ray computerized tomography (CT scan) developed
in the 1960. The basic principles of the technique in its simplest form illustrated in
Figure 14 are:
a diametrically opposing detector.
detector combination is moved laterally scanning the region of interest as shown by the
arrow in Figure 14 (a).
transmission properties of the full path in this case A−B.
circle around the object.
within the plane of the object to be studied. These signals are stored and by a rather complex computer analysis a point by point image is constructed of the thin slice scanned within the body.
transmission properties of each point within the plane of the object to be studied.
thick. In the more recent versions of the instrument, a fan rather than a beam of x-rays
scans the object, and an array of multiple detectors is used to record the signal. Data
acquisition is speeded up in this way yielding an image in a few seconds.
regions of high and low x-ray intensity which when photographed show spots of varying brightness (Figure 15).
rays to prevent or to slow the proliferation of malignant cells by decreasing the rate of
mitosis or impairing DNA synthesis].
still x-ray pictures, or can be followed on x-ray motion-picture films as it moves through the body or part of the body to record body processes.
(white) coats the inner lining.
portions of the upper and lower teeth together (Figure19).
rays are very effective at discovering tooth decay. It is called Bitewing because the x-ray
film holder provides a surface to bite down on and hold the x-ray securely in place.
to root (Figure 20).
crown to root. These types of x-rays provide a complete side view and typically a complete
set consists of 14 films with each tooth appearing in two different films from two different
while the film rests on the biting surface of the teeth (Figure 21).
x-rays are taken to show the lower or the upper jaw.
The x-ray captures the entire jaws and teeth in one shot. It's used to plan treatment for
dental implants, check for impacted wisdom teeth, and detect jaw problems. A
panoramic x-ray is not good for detecting cavities, unless the decay is very advanced
and deep( Figure 22).
of a dentist. As the name suggests, a Panoramic X-ray makes a complete half circle from ear
to ear to produce a complete two dimensional representation of all teeth. Panoramic X-rays
give the dentist an overall picture of all your teeth and jaw bones.
characteristics of cancer cells is that they grow and divide faster than normal cells. Radiation also damages normal cells, but because normal cells are growing more slowly, they are better able to repair radiation damage than are cancer cells. In order to give normal cells time to heal and reduce side effects, radiation treatments are often given in small doses over a six or seven week period. It is used in more than half of all cancer cases. Radiation therapy can be used in different ways:
relationship with each other.
Initially, the atom is in the excited state. The incoming photon stimulates the atom to emit a
second photon of energy given by hƒ = E2 – E1.
excited state than in the ground state. That must be true because the number of
photons emitted must be greater than the number absorbed.
lifetime must be long compared with the usually short lifetimes of excited state, which
are typically 10-8s. In this case, the population inversion can be established and
stimulated emission is likely to occur before spontaneous emission as shown in
make a transition to a lower energy level, because this process happens naturally, it is
known as spontaneous emission].
When the atom falls to the ground state, it emits a photon of energy hƒ = E2 – E1.
stimulate further emission from other excited atoms. That is achieved by using
reflecting mirrors at the ends of the system. One end is made totally reflecting, and the
other is partially reflecting. A fraction of the light intensity passes through the
partially reflecting end, forming the beam of laser light (Figure 25).
example, an optical or electrical device) pumps the atoms to the excited states. The parallel end
mirrors confine the photons to the tube, but mirror 2 is only partially reflective.
fluorescent strip light, only it's coiled around the ruby crystal and it flashes every so often like a camera's flash gun.
makes inject energy into the crystal in the form of photons.
process. When an atom absorbs a photon of energy, one of its electrons jumps
from a low energy level to a higher one. This puts the atom into an excited state, but
makes it unstable. Because the excited atom is unstable, the electron can stay in the
higher energy level only for a few milliseconds. It falls back to its original level,
living off the energy it absorbed as a new photon of light radiation (small blue blob).
This is the spontaneous emission process.
at the speed of light.
the excited atom gives off two photons of light instead of one. This is the stimulated
emission described earlier. Now one photon of light has produced two and the light
has been amplified (increased in strength). In other words, "light amplification" (an
increase in the amount of light) has been caused by "stimulated emission of radiation"
(hence the name "laser", because that's exactly how a laser works!)
crystal but lets some escape.
Permissible Exposure (MPE). When you use a laser, it should be labeled with one of these four classes designations:
such as compact disc and DVD players, in which the laser (a few millimeters in size) scans the surface of the disc. Other common applications of lasers are bar code readers, laser printers and laser pointers.
diabetes. Glaucoma is a widespread eye condition characterized by a high fluid
pressure in the eye, a condition that can lead to destruction of the optic nerve. A simple
laser operation, (iridectomy) can “burnˮ open a tiny hole in a clogged membrane relieving the destructive pressure. A serious side effect of diabetes is neovascularization, the proliferation of weak blood vessels, which often leak blood. When neovascularization occurs in the retina, vision deteriorates (diabetic retinopathy) and finally is destroyed. Today, it is possible to direct the green light from an argon ion laser through the clear eye lens and eye fluid, focus on the retina edges, and photocoagulate the leaky vessels. Even people who have only minor vision defects such as nearsightedness are benefiting from the use of lasers to reshape the cornea,
changing its focal length and reducing the need for eyeglasses.
light bending part of the eye.
with water-soluble ink to guide replacement of the corneal flap.
and reflected exposing the cornea beneath (A keratectomy is a procedure that uses a
small instrument that makes a cut in the cornea as it moves across it).
improve the precision of your treatment while minimizing pain and recovery time (Figure 31).
benign tumors from the gums, palate, sides of cheeks, and lips.
detection of cavities by providing a reading of the by-products produced by decay.
see inside tooth and gums in real time.
minimize healing time.
healthier tooth structure. Such reshaping called crown lengthening; provides a stronger
foundation for a restoration.
injection and the traditional turbine drill. Lasers used in dental filling procedures are
capable of killing bacteria located in a cavity and this may lead to better long term
removal of soft tissue folds often caused by ill-fitting dentures.
bleaching process associated with teeth whitening and with the use of a whitening gel.
The translucent bleaching gel is applied to the teeth and a laser light is used to activate
the crystals to absorb the energy from the light and penetrate the teeth enamel to
increase the lightening effect on the teeth. The length of time in the cosmetic dentist's
chair depends on the degree of discoloration you have (Figure 32).
pain and inflammation of the temporomandibular jaw joint.
and improve the appearance of a gummy smile.
tooth) that are responsible for hot and cold tooth sensitivity.
2007.ppt#281,3, How X-rays interact with the tissues in your body
process provide that ordinary X-ray images do not?
[mass of proton = 1.67 X 10-27 kg]
inverted population distribution.
7 th Edition, Thomson , Brooks/COLE 2008.