Charged particles

CHAREGED PARTICLES
Rutherford’s experiment demonstrated that the total positive charge in an atom is localized in a very
small region of space (the nucleus). The majority of α particles simply passed through the gold foil,
indicating that they did not come near a nucleus. In other words, most of the atom is empty space.
The diffuse cloud of electrons (which has a size on the order of 10  8 cm) did not exert enough force on the α particles to defl ect them. The plum pudding model simply did not explain the observationsfrom the experiment with α particles.
Although the work of Thomson and Rutherford had provided a view of atoms that was essentially correct,there was still the problem of what made up the remainder of the mass of atoms. It had been postulated that there must be an additional ingredient in the atomic nucleus, and this was demonstrated in 1932 by James Chadwick. In his experiments a thin beryllium target was bombarded with α particles.Radiation having high penetrating power was emitted, and it was initially assumed that they were highenergy
γ rays. From studies of the penetration of these rays in lead, it was concluded that the particles
had an energy of approximately 7 MeV. Also, these rays were shown to eject protons having energies
of approximately 5 MeV from paraffi n. However, in order to explain some of the observations, it wasshown that if the radiation were γ rays, they must have an energy that is approximately 55 MeV. If an α particle interacts with a beryllium nucleus so that it becomes captured, it is possible to show that the energy (based on mass difference between the products and reactants) is only about 15 MeV. Chadwick studied the recoil of nuclei that were bombarded by the radiation emitted from beryllium when it was a target for α particles and showed that if the radiation consists of γ rays, the energy must be a function of the mass of the recoiling nucleus, which leads to a violation of the conservation of momentum andenergy. However, if the radiation emitted from the beryllium target is presumed to carry no charge and consist of particles having a mass approximately that of a proton, the observations could be explained
satisfactorily. Such particles were called neutrons, and they result from the reaction
Atoms consist of electrons and protons in equal numbers and, in all cases except the hydrogen atom,
some number of neutrons. Electrons and protons have equal but opposite charges, but greatly different
masses. The mass of a proton is 1.67  10  24 grams. In atoms that have many electrons, the
electrons are not all held with the same energy; later we will discuss the shell structure of electrons in
atoms. At this point, we see that the early experiments in atomic physics have provided a general view
of the structures of atoms.
THE NATURE OF LIGHT
From the early days of physics, a controversy had existed regarding the nature of light. Some prominent
physicists, such as Isaac Newton, had believed that light consisted of particles or “ corpuscles. ”
Other scientists of that time believed that light was wavelike in its character. In 1807, a crucial experiment
was conducted by T. Young in which light showed a diffraction pattern when a beam of light was
passed through two slits. Such behavior showed the wave character of light. Other work by A. Fresnel
and F. Arago had dealt with interference, which also depends on light having a wave character

The Electron
In the 1890s, many scientists became caught up in the study of radiation , the emission and transmission of energy through space in the form of waves. Information gained from this research contributed greatly to our understanding of atomic structure. One device used to investigate this phenomenon was a cathode ray tube, the forerunner of the television tube It is a glass tube from which most of the air has been evacuated. When the two metal plates are connected to a high-voltage source, the negatively charged plate, called the cathode, emits an invisible ray. The cathode ray
is drawn to the positively charged plate, called the anode, where it passes through a hole and continues traveling to the other end of the tube. When the ray strikes the specially coated surface, it produces a strong fl uorescence, or bright light.In some experiments, two electrically charged plates and a magnet were added to the outside of the cathode ray tube . When the magnetic fi eld is on and
the electric fi eld is off, the cathode ray strikes point A. When only the electric fi eld is on, the ray strikes point C. When both the magnetic and the electric fi elds are off or when they are both on but balanced so that they cancel each other’s infl uence, the ray strikes point B. According to electromagnetic theory, a moving charged body behaves like a magnet and can interact with electric and magnetic fi elds through which it passes.Because the cathode ray is attracted by the plate bearing positive charges and repelled by the plate bearing negative charges, it must consist of negatively charged particles.We know these negatively charged particles as electrons .An English physicist, J. J. Thomson, † used a cathode ray tube and his knowledgeof electromagnetic theory to determine the ratio of electric charge to the mass of an individual electron. The number he came up with was 21.76 3 108 C/g, where C
stands for coulomb, which is the unit of electric charge. Thereafter, in a series of experiments carried out between 1908 and 1917, R. A. Millikan ‡ succeeded in measuring the charge of the electron with great precision. His work proved that the charge on each electron was exactly the same. In his experiment, Millikan examined the motion of single tiny drops of oil that picked up static charge from ions in the air.He suspended the charged drops in air by applying an electric fi eld and followed their
The nature of light and the nature of matter are intimately related. It was from the study of light emitted
when matter (atoms and molecules) was excited by some energy source or the absorption of light
by matter that much information was obtained. In fact, most of what we know about the structure of
atoms and molecules has been obtained by studying the interaction of electromagnetic radiation with
matter or electromagnetic radiation emitted from matter. These types of interactions form the basis of
several types of spectroscopy, techniques that are very important in studying atoms and molecules.
In 1864, J. C. Maxwell showed that electromagnetic radiation consists of transverse electric and magnetic
fi elds that travel through space at the speed of light (3.00  10 8 m/sec). The electromagnetic spectrum
consists of the several types of waves (visible light, radio waves, infrared radiation, etc.) that form
a continuum as shown in Figure 1.3 . In 1887, Hertz produced electromagnetic waves by means of an
apparatus that generated an oscillating electric charge (an antenna). This discovery led to the development
of radio.
Although all of the developments that have been discussed are important to our understanding of the
nature of matter, there are other phenomena that provide additional insight. One of them concerns
the emission of light from a sample of hydrogen gas through which a high voltage is placed. The basic experiment is shown in Figure 1.4 . In 1885, J.J. Balmer studied the visible light emitted from the gas
by passing it through a prism that separates the light into its components.

where λ is the wavelength of the line, n is an integer larger than 2, and R H is a constant known as
Rydberg’s constant that has the value 109,677.76 cm  1 . The quantity 1/ λ is known as the wave number
(the number of complete waves per centimeter), which is written as ν ( “ nu bar ” ). From Eq. (1.2) it can
be seen that as n assumes larger values, the lines become more closely spaced, but when n equals infi nity,
there is a limit reached. That limit is known as the series limit for the Balmer series. Keep in mind
that these spectral lines, the fi rst to be discovered for hydrogen, were in the visible region of the electromagnetic
spectrum. Detectors for visible light (human eyes and photographic plates) were available
at an earlier time than were detectors for other types of electromagnetic radiation.
Eventually, other series of lines were found in other regions of the electromagnetic spectrum. The Lyman
series was observed in the ultraviolet region, whereas the Paschen, Brackett, and Pfund series were
observed in the infrared region of the spectrum. All of these lines were observed as they were emitted
from excited atoms, so together they constitute the emission spectrum or line spectrum of hydrogen atoms.
Another of the great developments in atomic physics involved the light emitted from a device known
as a black body. Because black is the best absorber of all wavelengths of visible light, it should also be
the best emitter. Consequently, a metal sphere, the interior of which is coated with lampblack, emits
radiation (blackbody radiation) having a range of wavelengths from an opening in the sphere when it
is heated to incandescence. One of the thorny problems in atomic physics dealt with trying to predict
the intensity of the radiation as a function of wavelength. In 1900, Max Planck arrived at a satisfactory
relationship by making an assumption that was radical at that time. Planck assumed that absorption
and emission of radiation arises from oscillators that change frequency. However, Planck assumed that
the frequencies were not continuous but rather that only certain frequencies were allowed. In other
words, the frequency is quantized . The permissible frequencies were multiples of some fundamental
frequency, ν 0 . A change in an oscillator from a lower frequency to a higher one involves the absorption






where λ is the wavelength, ν is the frequency, and c is the velocity of light (3.00  10 10 cm/sec). By
making these assumptions, Plank arrived at an equation that satisfactorily related the intensity and frequency
of the emitted blackbody radiation.
The importance of the idea that energy is quantized is impossible to overstate. It applies to all types
of energies related to atoms and molecules. It forms the basis of the various experimental techniques
for studying the structure of atoms and molecules. The energy levels may be electronic, vibrational, or
rotational depending on the type of experiment conducted.
In the 1800s, it was observed that when light is shined on a metal plate contained in an evacuated
tube, an interesting phenomenon occurs. The arrangement of the apparatus is shown in Figure 1.5 .
When the light is shined on the metal plate, an electric current fl ows. Because light and electricity are
involved, the phenomenon became known as the photoelectric effect . Somehow, light is responsible for
the generation of the electric current. Around 1900, there was ample evidence that light behaved as a
wave, but it was impossible to account for some of the observations on the photoelectric effect by considering
light in that way. Observations on the photoelectric effect include the following:
1. The incident light must have some minimum frequency (the threshold frequency ) in order for
electrons to be ejected.
2. The current fl ow is instantaneous when the light strikes the metal plate.
3. The current is proportional to the intensity of the incident light.
In 1905, Albert Einstein provided an explanation of the photoelectric effect by assuming that the incident
light acts as particles. This allowed for instantaneous collisions of light particles ( photons ) with
electrons (called photoelectrons), which resulted in the electrons being ejected from the surface of
the metal. Some minimum energy of the photons was required because the electrons are bound to
the metal surface with some specifi c binding energy that depends on the type of metal. The energy
required to remove an electron from the surface of a metal is known as the work function ( w 0 ) of the
metal. The ionization potential (which corresponds to removal of an electron from a gaseous atom) is
not the same as the work function. If an incident photon has an energy that is greater than the work
function of the metal, the ejected electron will carry away part of the energy as kinetic energy. In other
words, the kinetic energy of the ejected electron will be the difference between the energy of the incident
photon and the energy required to remove the electron from the metal. This can be expressed by
the equation
H=MC2
By increasing the negative charge on the plate to which the ejected electrons move, it is possible to stop the electrons and thereby stop the current fl ow. The voltage necessary to stop the electrons is known as the stopping potential . Under these conditions, what is actually being determined is the kinetic energy of the ejected electrons. If the experiment is repeated using incident radiation with a different frequency, the kinetic energy of the ejected electrons can again be determined. By using light having several known incident frequencies, it is possible to determine the kinetic energy of the electrons corresponding to each frequency and make a graph of the kinetic energy of the electrons versus ν . , the relationship should be linear with the slope of the line being h , Planck’s constant,and the intercept is ZERO . There are some similarities between the photoelectric effect described here and photoelectron spectroscopy of molecules that is described in Although Einstein made use of the assumption that light behaves as a particle, there is no denying the
validity of the experiments that show that light behaves as a wave. Actually, light has characteristics of both waves and particles, the so-called particle-wave duality . Whether it behaves as a wave or a particldepends on the type of experiment to which it is being subjected. In the study of atomic and molecularstructure, it necessary to use both concepts to explain the results of experiments.



REFERENCES

·  Jensen, W.B. (1980). The Lewis acid-base concepts : an overview. New York:.

·  Yamamoto, Hisashi(1999). Lewis acid reagents : a practical approach. New York: Oxford University Press.

  ·  Christian Laurence and Jean-François Gal "Lewis Basicity and Affinity Scales : Data and Measurement" Wiley, 2009.

·  Lewis, G.N., Valence and the Structure of Atoms and Molecules (1923) p. 142.

.  Miessler, L. M., Tar, D. A., (1991) p166 - Table of discoveries attributes the date of publication/release for the Lewis theory as 1923.

•   Brown HC and Kanner B. "Preparation and Reactions of 2,6-Di-t-butylpyridine and Related hindered Bases. A case of Steric Hindrance toward the Proton." J. Am. Chem. Soc. 88, 986 (1966).