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IntroductionThe idea that matter is composed of discrete units and can not be divided into any arbitrarily
tiny or small quantities has been around for thousands of years. In around 450 BC, Democritus
coined the term atomos, which meant "uncuttable". Though both the Indian and Greek concepts of
the atom were based purely on philosophy, modern science has retained the name coined by
Democritus.
In 1803, John Dalton used the concept of atoms to explain why elements always reacted in
simple proportions, and why certain gases dissolved better in water than others. He proposed that
each element consists of atoms of a single, unique type, and that these atoms could join to each
other, to form compound chemicals.
In 1897, JJ Thomson through his work on cathode rays discovered the electron and its
subatomic nature, which destroyed the concept of atoms as being indivisible units. Later, Thomson
also discovered the existence of isotopes through his work on ionized gases.
Thomson believed that the electrons were distributed evenly throughout the atom, balanced by
the presence of a uniform sea of positive charge. However, in 1909, the gold foil experiment was
interpreted by Ernest Rutherford as suggesting that the positive charge of an atom and most of its
mass was concentrated in a nucleus at the center of the atom, with the electrons orbiting it like
planets around a sun. In 1913, Niels Bohr added quantum mechanics into this model, which now
stated that the electrons were locked or confined into clearly defined orbits, and could jump
between these, but could not freely spiral inward or outward in intermediate states.
In 1926, Erwin Schrodinger, using Louis De Broglie’s 1924 proposal that all particles behave
to an extent like waves, developed a mathematical model of the atom that described the electrons as
three-dimensional waveforms, rather than point particles. A consequence of using waveforms to
describe electrons, pointed out by Werner Heisenberg a year later, is that it is mathematically
impossible to obtain precise values for both the position and momentum of a particle at any point in
time; this became known as the uncertainty principle. In this concept, for any given value of
position one could only obtain a range of probable values for momentum, and vice versa. Although
this model was difficult to visually conceptualize, it was able to explain many observations of
atomic behavior that previous models could not, such as certain structural and spectral patterns of
atoms bigger than hydrogen. Thus, the planetary model of the atom was discarded in favor of one
that described orbital zones around the nucleus where a given electron is most likely to exist.
Atom structure
Though the word atom originally denoted a particle that cannot be cut into smaller particles,
in modern scientific usage the "atom" is composed of various subatomic particles including:
- electrons, which have a negative charge, a size which is so small as to be currently
unmeasurable, and which are the least heavy (i.e., massive) of the three types of basic
particles, with an mass of 9.11x10-31kg.
- protons, which have a positive charge, with a free mass about 1836 times more than
electrons (mass of 1.67x10-27kg though binding energy changes can reduce this).
- neutrons, which have no charge, have a free mass about 1839 times the mass of
electrons, and about the same physical size as protons (which is on the order of
2.5x10-15 m in diameter).
So the atom is composed of three basic subatomic particles:
Particle/Symbol Relative (absolute) mass Relative (absolute) charge
Electron 1/1840 -1
e- ( −10 e ) ( 9.11 x 10-31kg or 5.488 x 10-4 u.) ( -1.602 x 10-19C )
Proton +1 +1
-27
1
p (1 p ) (1.67 x 10 kg or 1.0073 u.) (+1.602 x 10-19 C)
Neutron 1 0
1 -27
n (0n) (1.67 x 10 kg or 1.0087 u.)
Atomic mass unity = 1 a.m.u. = 1 u. = 1.6605 x 10-27 kg
, Protons and neutrons make up a dense, massive atomic nucleus, and are collectively called
nucleons. The electrons form the much larger electron cloud surrounding the nucleus. Both protons
and neutrons are themselves now thought to be composed of even more elementary particles,
quarks.
Experiments that proved the existence of the electron:
- electrolysis (Faraday ‘s laws, 1833)
- electrical discharges in gases (cathode and channel rays) (Plucker, Stoney)
- mass / charge ratio (e/m) determination (J.J.Thomson)
- determination of the electronic charge (Millikan)
- X-ray discovery (Roentgen)
Experiments that proved the existence of the nucleus:
- natural radioactivity
- Rutherford’s experience
- discovery of the neutron (Chadwick)
- discovery of the isotopes
Experiments that proved a special distribution of the electrons in the atom:
- emission spectra of hydrogen atom (Balmer)
- X-rays spectra (Moseley)
- photoelectric effect
- determination of ionization energy
In 1879, Crookes discovered that when a high voltage (V=10,000V) is applied to a gas at low
pressure (< 0.01 torr), streams of particles moved from the cathode to the anode. These particles
called cathode rays are common to all elements.
Cathode rays have the following properties:
- travel in straight lines
- can be deflected by magnetic and electrical fields, the direction of deflection showing
them to be negatively charged
- cause many substances to fluorescence (e.g. the familiar ZnS coated television tube)
- cause mechanic effects having the energy
- can penetrate thin sheets of metal.
Then, in 1897 J.J. Thomson determined that these electrons had a negative electric charge and
compared to the atom had very little mass. Thus he proposed that atoms consisted of a large
massive positively charged body with a number of small negatively charged electrons scattered
throughout it. The total charge of the electrons exactly balanced the positive charge of the large
mass, so the total electric charge was zero. This was called the plum pudding model of the atom.
The number of electrons determines the particular chemical element. Hydrogen, for example, has
one electron; helium has two; carbon has six, etc.
The apparatus, experiment and mathematic calculations that have been used by J.J.Thomson
in 1897 for determining the charge/mass ratio (e/m) of the electron (cathode rays):
The apparatus for determining the velocity and the charge/mass ratio (effective charge) of the
stream of electrons used by J.J.Thomson (1897)
, The particles from the cathode were made to pass through a slit in the anode and then through
a second slit. They then passed between two aluminum plates spaced about 5 cm apart and
eventually fell on to the end of the tube, producing a well-defined spot. The position of the spot was
noted and the magnetic field was then switched on, causing the electron beam to move in a circular
arc while under the influence of this field. an electric field was now applied in opposing to the
magnetic field and gradually increased until the spot returned to its original position.
If: B – magnetic flux density
e – charge on the electron
v - velocity of the electron
m - mass of the electron
r – radius of the arc in which the electron moves,
then the magnetic force, Bev, acting on each electron causes it to accelerate in the direction of
the force, and thus to move along the arc of a circle.
Thus : Bev = (mv2) / r or (e / m ) = (v / rB) (1)
When an electric field (E) is applied in opposition so that the electric force on each electron
balances the magnetic force :
Ee = Bev or v = E / B (2)
The velocity of the electrons can be calculated from equation (2). It is found that they travel at
about 3 x 107 m sec-1 , i.e. about the velocity of light.
Substituting for v in equaton (1) gives
e / m = E / (rB2) (3)
and since r can be determined from simple geometry knowing the dimensions of the
apparatus, the value e / m can be evaluated.
Its value, 1.7588 x 1011 C Kg-1 , is quite independent of the nature of the residual gas in the
apparatus, suggesting that electrons are negatively charged constituents of all matter.
The conclusive proof that electrons are discrete particles was obtained by Millikan during the
years 1910-1914 when he undertook a series of very careful experiments to determine the value of
the electronic charge.
oil droplet
Millikan’s apparatus for determining the value of the electronic charge
Small droplets of oil from an atomized are sprayed into a still thermostatted air space between
parallel plates of a condenser and two cases are studied:
a) the fall of one of these droplets ( which is uncharged ) under gravity (in the absence of a
potential) ; the droplet has an uniform motion given by the equality between gravity force
and friction (rubbing) force
mg = fv0
where v0 – the velocity of droplet in the absence of the electric field
m – apparently mass = the oil droplet’s mass – the air’s mass
m = apparent V (V- volume of the particle)
f = 6 r where: - coefficient of air viscosity;
r – the radius of droplet
g – gravitational acceleration
If the droplet is perfectly spherical , then : 4/3 r3 g (oil - air) = 6 r v0 and
r = (9v0) / (2g) (1)
, b) the fall of the droplet which is charged in the field presence
The air space (between the plates) is now ionized with an X-ray beam, or with cathode or
UV rays, or with the rays from a radioactive element) enabling the oil droplets to pick up charge
by collision with the ionized air molecules.
By applying a potential of several thousand volts across the parallel metal plates, the oil
droplet can either be speeded up or made to rise depending upon the direction of the electric
field.
If the droplet is negatively charged and the superior plate is positively charged then, the
equilibrium of forces is:
eE = e (U / l) = mg + fv1 = fv0 + fv1
e (U / l) = f ( v0 + v1) = 6 r ( v0 + v1) and
e = (6l / U ) (v0 + v1) (9v0) / (2g) (2)
where: e – the oil droplet charge
E – electric field intensity
U – the potential (the tension voltage)
l – the distance between the plates
Milllikan observed that droplets of oil could pick up several different charges, but that the
total charge was always an exact integral multiple of the smallest charge,
i.e. the charge on the electron, e = 1.602 x 10-19 C
From the values e = 1.602 x 10-19 C and
e / m = 1.76 x 1011 C Kg-1
m = 9.11 x 10- 31 Kg
In 1886, Goldstein used a discharge tube containing a perforated cathode and had observed
the formation of channel rays behind the cathode
The formation of channel rays in a discharge tube
These rays (positive ions) carries a positive charge equal in magnitude to that on the electron
They are formed by the loss of electrons from the residual gas in the discharge tube.
X-rays were discovered by Roentgen in 1895 when he noticed that a penetrating radiation was
emitted from discharges tubes and appeared to originate from the anode. The true nature of X-
radiation was not discovered until 1912, when it became apparent that its properties could be
explained by assuming it to be wavelike in character, i.e. similar to light but of much smaller
wavelength.
X-rays are produced whenever fast-moving electrons are stopped in their tracks by impinging
on a target, the excess energy appearing mainly in the form of X-radiation
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