Chapter 2: Main constituents of matter Dr. Zitouni.

k
Department ST

Second chapter: MAIN CONSTITUENTS OF MATTER

1. The relationship between matter and electricity

The credit for discovering the relationship between matter and electricity goes to the
scientist Michael Faraday in 1833, as a result of a series of experiments that he conducted
on different metals inside different solutions, which he called electrolysis experiments.
• Faraday's experiments and laws
Electrolysis is a method of carrying out a non-spontaneous reaction with the help of
electric current. It is a set of changes at the interface between a metallic conductor and an
ionic conductor, taking place under the influence of a sufficiently high electric voltage
applied to the metallic conductors of the electrolyser.
The electrolyser is a system in which the electrolysis process is carried out. It consists of
two electrodes (metallic phases, plates) immersed in an electrolyte solution. The
electrolysis process begins when the electrodes of the electrolyser are connected to a DC
source. A simplified diagram of the electrolyser is shown in Figure. 1.

Figure 1. Schematic diagram of the electrolyser

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Chapter 2: Main constituents of matter Dr. Zitouni. k
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In an electrolyser, the electrode connected to the positive pole of the current source is
called the anode, and the electrode connected to the negative pole of the current source
is called the cathode. The oxidation process takes place at the anode, the reduction
process takes place at the cathode. Galvanic cells enable the transfer of energy from the
cell to the environment, while in the electrolysis, an external source of current transfers
energy to the reactants of the electrolyser.

Faraday's laws

An important issue in the electrolysis process is to calculate the amount of product

formed by a certain amount of electricity. This is possible thanks to the calculations
made by Michael Faraday, the English physicist and chemist who created the laws of
Faraday electrolysis. Two Faraday's laws describe the electrolysis process quantitatively.

The first Faraday's law says that the mass of the substance (m) separated or deposited
on one of the electrodes is directly proportional to the intensity of the current I and the
duration of electrolysis t:

me = K . I. t

where:

me: mass of substance released or deposited on one of the electrodes [g];

k: electrochemical equivalent (this is the mass of the substance that is released on the
electrode when a unit charge passes through the electrolyte, for example 1 C) [g/C];

I: the intensity of the electric current flowing through the electrode [A];

t: duration of electrolysis [s].

Faraday's second law says that the ratio of the molar mass M of the substance that is
released or deposited at the electrode to the product of its electrochemical equivalent (k)
and the charge number (z) of the electrode reaction, which is recorded for 1 mole of a
substance with a molar mass M, is a constant value for all electrode processes and
amounts to 96500 C/mol (it is the Faraday constant, symbol F):

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Chapter 2: Main constituents of matter Dr. Zitouni. k
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𝑴
=𝑭
𝒌𝒛

where: M: molar mass of substance evolving at the electrode [g/mol];

z: the charge number of the reaction (positive, dimensionless quantity, equal to the
stoichiometric electron coefficient in the electrode reaction equation);

k: electrochemical equivalent [g/C];

F: Faraday constant of approximately 96 500 [C/mol].

Faraday's second law also says that the same amount of electricity flowing through
different electrolytes will release on the electrodes masses of substances proportional to
𝒎𝟏 𝒌𝟏
their electrochemical equivalents according to the relationship: =
𝒎𝟐 𝒌𝟐

After transforming the two Faraday laws, an equation is obtained, which is the basis for
quantitative calculations of the electrolysis process:

𝑴
𝒎𝒆 = . 𝑰. 𝒕
𝒛𝑭

Faraday's results can be summarized as follows:

• The mass of the reactant (liberated or deposited) on an electrode during electrolysis is
directly proportional to the quantity of electricity which passes through the solution of
an electrolyte.
• For a given quantity of electricity, the weight of the substance reacting at one of the
electrodes is proportional to its equivalent weight.
In 1874, G. Johnston Stoney announced that this electrical exchange that occurs in
the form of impulses leads us to think that electricity is not continuous and that it
consists of electricity-carrying units that are extremely small and separate from each
other and exist with the atoms of matter. In 1891, Stoney proposed naming the
hypothetical unit of electricity the electron.

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Chapter 2: Main constituents of matter Dr. Zitouni. k
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• Electrostatic discharge emission experiments

In 1850, Faraday placed a gas under normal pressure in a 50 cm tube, then applied a
high electrical voltage of about 10,000 volts between its electrodes. He did not record any
observations.

But when the pressure in the tube was reduced to 10 mm Hg by withdrawing the gas
from it using a vacuum pump, he noticed that the remaining gas emits light emissions as
the electric current passes between the cathode and anode of the tube in a process he
called electrical discharge.

In 1869, Johann Hittorf placed an opaque body between the cathode and a bright
glass. He observed a clear shadow of the body appearing on the bright glass. The place
where the shadow appeared showed that the source of these lights was the cathode.
The clear boundaries of the shadow revealed the straight paths in which these rays
traveled.

These two observations led Goldstein in 1876 to name these lights cathode rays.

2. Discovering the components of the atom and some of its

properties
2.1. The Electron
• Cathode rays: Crookes experiment

William Crookes in 1879 studied the electrical discharge in partially evacuated tubes
known as cathode ray discharge tubes (Which was discovered by Faraday).

In 1895, Jean Perrin was able to detect the charge of the electromagnetic radiation,
which he found it be negative.

Among the properties that Crookes deduced about cathode rays are:

▪ They produce sharp shadow of the solid object in their path suggesting
that they travel in straight line.
▪ They are deflected towards the positive plate in an electric field suggesting
that they are negatively charged.

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Chapter 2: Main constituents of matter Dr. Zitouni. k
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▪ They can move a light paddle wheel to rotate placed in their path. This
means they possess kinetic energy and are material particles.

Figure 2. cathode ray discharge tubes.

▪ They ionize gases through which they travel.

▪ The characteristics of cathode rays (electrons) do not depend on the
material of electrodes and nature of the gas present in the cathode ray tube.

𝒆
• Thompson's experiment and determining the ratio
𝒎

In October 1897, The British physicist J. J. Thomson presented his case for the
particulate interpretation of cathode rays in more detail. He reported a novel result
favoring that interpretation: the deflection of cathode rays by an electric field.

Figure 3. (a) Mr. J. J. Thomson produced. (b) This is an early cathode ray tube.

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Chapter 2: Main constituents of matter Dr. Zitouni. k
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𝒆
Furthermore, he reported a series of measurements of the mass to charge ratio ( )
𝒎

of cathode ray particles, whose purpose was to enable him to figure out their identity.
He obtained those measurements by means of two different approaches. The first one
was based on measurements of the charge carried by cathode rays, the heat produced by
their impact on a target, and the effect of a magnetic field on their trajectory (figure 4).

Figure 4. The effect of the magnetic field on the path of cathode rays

𝒆
A combination of those data led to an estimate of . The guiding idea behind the
𝒎

second approach was to place cathode rays under the influence of an electric and a
magnetic field and to adjust the intensity of the latter “so that the electrostatic deflexion
was the same as the magnetic”. It was then possible to calculate e/m on the basis of
𝒆
directly measurable parameters. Furthermore, the value of ( = 1,759 1011 𝐶/𝐾𝑔) was
𝒎

independent of the material of the cathode and the chemical composition of the gas
within the cathode ray tube. This independence suggested to Thomson that the
“corpuscles” were universal constituents of all material substances.

Figure 5. Diagram showing Thompson's experiment

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Chapter 2: Main constituents of matter Dr. Zitouni. k
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• Millikan experiment: Measurement of electron charge

In 1909, more information about the electron was uncovered by American physicist
Robert A. Millikan via his “oil drop” experiments. Millikan created microscopic oil
droplets, which could be electrically charged by friction as they formed or by using X-rays.
These droplets initially fell due to gravity, but their downward progress could be slowed or
even reversed by an electric field lower in the apparatus. By adjusting the electric field
strength and making careful measurements and appropriate calculations, Millikan was able
to determine the charge on individual drops (Figure 6)

Figure 6. Millikan’s experiment measured the charge of individual oil drops.

Looking at the charge data that Millikan gathered, you may have recognized that the
charge of an oil droplet is always a multiple of a specific charge, 1.6 × 10−19 C. Millikan
concluded that this value must therefore be a fundamental charge (the charge of a single
electron) with his measured charges due to an excess of one electron (1 times 1.6 × 10−19 C),
two electrons (2 times 1.6 × 10−19 C), three electrons (3 times 1.6 × 10−19 C), and so on, on
a given oil droplet.

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Chapter 2: Main constituents of matter Dr. Zitouni. k
Department ST

Since the charge of an electron was now known due to Millikan’s research, and the charge-
to-mass ratio was already known due to Thomson’s research (1.759 × 1011 C/kg), it only
required a simple calculation to determine the mass of the electron as well.

𝟏𝒌𝒈
Mass of electron = 1.602 × 10−19 C × = 9.107× 10−31 kg
𝟏.𝟕𝟓𝟗 × 𝟏𝟎𝟏𝟏 𝑪

• Thomson atomic model

Scientists had now established that the atom was not indivisible as Dalton had believed,
and due to the work of Thomson, Millikan, and others, the charge and mass of the negative,
subatomic particles (the electrons) were known. However, the positively charged part of
an atom was not yet well understood. In 1904, Thomson proposed the “plum pudding”
model of atoms (Figure 7), which described a positively charged mass with an equal
amount of negative charge in the form of electrons embedded in it, since all atoms are
electrically neutral.

Figure 7. Thomson suggested that atoms resembled plum pudding, an English

dessert consisting of moist cake with embedded raisins (“plums”).

A competing model had been proposed in 1903 by Hantaro Nagaoka, who postulated a
Saturn-like atom, consisting of a positively charged sphere surrounded by a halo of electrons
(Figure 8).

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Chapter 2: Main constituents of matter Dr. Zitouni. k
Department ST

Figure 8. Nagaoka proposed that atoms resembled the planet Saturn, with a ring of
electrons surrounding a positive “planet.”

2.2. The Proton and the Nucleus

• Goldstein's Experiment
In 1886, the scientist Goldstein conducted an experiment similar to Crookes'
experiment, through which he discovered the presence of positively charged particles in
atoms. Goldstein noticed that if the cathode was perforated, the area behind it would glow.
He confirmed that there were particles that had an electrical charge opposite to the charge
of the cathode rays, which he called channel light rays (Anode ray).

Figure 9. Production of Anode ray

He explained what happened by saying that when the cathode rays were generated, they
collided with the atoms of the inert gas used, forming positive ions that headed in the

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Chapter 2: Main constituents of matter Dr. Zitouni. k
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𝑸
opposite direction. The value of these particles was calculated exactly as in Thomson's
𝒎

experiment, but it was found to be much lower than it.

• Rutherford Experiment
The next major development in understanding the atom came from Ernest Rutherford,
a physicist from New Zealand who largely spent his scientific career in Canada and
England. He performed a series of experiments using a beam of high-speed, positively
charged alpha particles (α particles) that were produced by the radioactive decay of radium;
α particles consist of two protons and two neutrons. Rutherford and his colleagues Hans
Geiger and Ernest Marsden aimed a beam of α particles, the source of which was
embedded in a lead block to absorb most of the radiation, at a very thin piece of gold foil
and examined the resultant scattering of the α particles using a luminescent screen that
glowed briefly where hit by an α particle.

Figure 10. Rutherford and his colleagues fired α particles at a piece of gold foil and detected
where those particles went

What did they discover? Most particles passed right through the foil without being
deflected at all. However, some were diverted slightly, and a very small number were
deflected almost straight back toward the source (Figure 10). Rutherford described finding
these results: “It was quite the most incredible event that has ever happened to me in my

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Chapter 2: Main constituents of matter Dr. Zitouni. k
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life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and
it came back and hit you”.
Here is what Rutherford deduced: Because most of the fast-moving α particles passed
through the gold atoms undeflected (Figure 11), they must have traveled through
essentially empty space inside the atom. Alpha particles are positively charged, so
deflections arose when they encountered another positive charge (like charges repel each
other). Since like charges repel one another, the few positively charged α particles that
changed paths abruptly must have hit, or closely approached, another body that also had a
highly concentrated, positive charge. Since the deflections occurred a small fraction of the
time, this charge only occupied a small amount of the space in the gold foil.

Figure 11. Magnified view of particles passing through and being deflected by nuclei.

Analyzing a series of such experiments in detail, Rutherford drew two conclusions:

1. The volume occupied by an atom must consist of a large amount of empty space.
2. A small, relatively heavy, positively charged body, the nucleus, must be at the center of
each atom.
• Rutherford's atomic model

Figure 12. Rutherford's planetary model of 1911

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Chapter 2: Main constituents of matter Dr. Zitouni. k
Department ST

On the basis of above observations and conclusions, Rutherford proposed the nuclear
model of atom (figure 12), according to which:
- The whole positive charge and entire mass of an atom is concentrated at its Centre in
sphere of radius 10-14 m, called the nucleus.
- The electrons move in circular orbits around the nucleus. The necessary centripetal force
counteracting the electrostatic force of attraction between the nucleus and the electrons is
provided by the rapid circular motion of the electrons around the nucleus.
- Atom is electrically neutral as the total positive charge on the nucleus is equal to the
total negative charge of the electron in the atom.

• Discovery of the proton

In 1919, Rutherford subjected nitrogen gas to a stream of accelerated alpha particles and
observed the formation of oxygen nuclei with the appearance of a particle with a positive
electrical charge, which he called the proton, derived from the Greek word protos, which
means ֞the first֞. Because it is the smallest positive particle that has been observed and is
considered the basic component of the atoms′s nuclei.

A proton is a nuclear particle having a positive charge equal to that of the electron and a
mass more than 1800 times that of the electron.

2.3. The Neutron

• Chadwick experiment

Rutherford’s model of atomic structure left one major problem unsolved. It was known
that hydrogen, the simplest atom, contains only one proton and that the helium atom
contains two protons. Therefore, the ratio of the mass of a helium atom to that of a hydrogen
atom should be 2:1. (Because electrons are much lighter than protons, their contribution
can be ignored.) In reality, however, the ratio is 4:1. Rutherford and others postulated that
there must be another type of subatomic particle in the atomic nucleus; the proof was

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provided by another English physicist, James Chadwick, in 1932. When Chadwick

bombarded a thin sheet of beryllium with particles, a very high energy radiation similar to
rays was emitted by the metal. Later experiments showed that the rays actually consisted
of electrically neutral particles having a mass slightly greater than that of protons.
Chadwick named these particles neutrons.
The mystery of the mass ratio could now be explained. In the helium nucleus there are
two protons and two neutrons, but in the hydrogen nucleus there is only one proton and no
neutrons; therefore, the ratio is 4:1.

3. Description of the atom

𝐴
The atom is symbolized by the symbol. 𝑍𝑋
We characterize a nucleus by its atomic number (Z) and its mass number (A). The
number of protons in the nucleus of each atom of an element is called the atomic number
(Z). In a neutral atom the number of protons is equal to the number of electrons, so the
atomic number also indicates the number of electrons present in the atom. The chemical
identity of an atom can be determined solely by its atomic number.
The mass number (A) is the total number of protons and neutrons in a nucleus.
In general, the mass number is given by
mass number = number of protons + number of neutrons
= atomic number + number of neutrons
A nuclide is an atom characterized by a definite atomic number and mass number. The
shorthand notation for any nuclide consists of the symbol of the element with the atomic
number written as a subscript on the left and the mass number as a super script on the left.
You write the nuclide symbol for the naturally occurring sodium nuclide as follows:

𝐴 23
𝑍𝑋 ≈ 11𝑁𝑎
When the numbers of these subatomic particles (number of protons and number of
electrons) are not equal, the atom is electrically charged and is called an ion. The
charge of an atom is defined as follows:

Atomic charge = number of protons − number of electrons

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Atoms (and molecules) typically acquire charge by gaining or losing electrons. An

atom that gains one or more electrons will exhibit a negative charge and is called an
anion. Positively charged atoms called cations are formed when an atom loses one or
more electrons.

There are other subatomic particles, but the electron, the proton, and the neutron
are the three fundamental components of the atom that are important in chemistry.
Table 1 shows the masses and charges of these three elementary particles.

Table 1. Properties of Subatomic Particles

Name Location Charge (C) Unit Charge Mass (g) Mass (amu)
electron outside nucleus −1.602 × 10−19 -1 0.00091 × 10−24 0.00055
proton nucleus 1.602 × 10−19 +1 1.67262 × 10−24 1.00727
neutron nucleus 0 0 1.67493 × 10−24 1.00866

4. Isotopes and their natural abundance

Isotopes are atoms whose nuclei have the same atomic number but different mass
numbers; that is, the nuclei have the same number of protons but different numbers of
neutrons.
The symbol for a specific isotope of any element is written by placing the mass number
as a superscript to the left of the element symbol. The atomic number is sometimes written
as a subscript preceding the symbol, but since this number defines the element’s identity,
as does its symbol, it is often omitted. For example, magnesium exists as a mixture of
three isotopes, each with an atomic number of 12 and with mass numbers of 24, 25, and
26, respectively. These isotopes can be identified as 24Mg, 25Mg, and 26Mg.

Natural abundance (fractional abundance) is the measure of the average amount

of a given isotope naturally occurring on Earth. The abbreviation for natural abundance
is NA.
The mass of an element shown in a periodic table or listed in a table of atomic masses
is a weighted, average mass of all the isotopes present in a naturally occurring sample of

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that element. This is equal to the sum of each individual isotope’s mass multiplied by its
fractional abundance.
average mass = ∑𝒊(𝒇𝒓𝒂𝒄𝒕𝒊𝒐𝒏𝒂𝒍 𝒂𝒃𝒖𝒏𝒅𝒂𝒏𝒄𝒆 × 𝒊𝒔𝒐𝒕𝒐𝒑𝒊𝒄 𝒎𝒂𝒔𝒔) 𝒊 / 100
For example, the element boron is composed of two isotopes: About 19.9% of all
boron atoms are 10B with a mass of 10.0129 amu, and the remaining 80.1% are 11B with
a mass of 11.0093 amu. The average atomic mass for boron is calculated to be:
boron average mass = (0.199 × 10.0129 amu) + (0.801× 11.0093 amu) = 1.99 amu +8.82
amu = 10.81 amu
- isotopes of the same elements tend to behave the same in chemical reactions, which only
involve the electrons surrounding the nucleus. However, there are several differences in
isotopes that make them very useful in many applications.
- Different isotopes can differ dramatically in stability. Some isotopes are very stable,
while others are unstable and decay spontaneously and emit radiation (energy) when the
decay happens. Thus, the unstable isotopes are called radioactive isotopes.
- For example, 31P is a stable isotope of phosphorus, while 32P is a radioactive isotope.
- Because radioactivity can be easily detected, radioactive isotopes are very useful in
labeling and tracing chemical species in biochemistry or medical applications, for
example to track the spread of a drug in the body.
- The rate at which the radioactive isotope decays is given by its half-life, the interval
after which half of the material breaks down. Half-life varies between a fraction of a
second and thousands of years.
- 14C, with a half-life of roughly 5700 years, has been used to determine the approximate
age of an artifact or fossil. Isotopes with very short half-life cannot be used for this
because after thousands of years, all the radioactive isotope would have been decayed and
there would be no difference in samples from different ages.

• Mass Spectrometry and Isotope Separation

The occurrence and natural abundances of isotopes can be experimentally
determined using an instrument called a mass spectrometer. Mass spectrometry (MS) is
widely used in chemistry, forensics, medicine, environmental science, and many other
fields to analyze and help identify the substances in a sample of material. In a typical

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mass spectrometer (Figure 13), the sample is vaporized and exposed to a high-energy
electron beam that causes the sample’s atoms (or molecules) to become electrically
charged, typically by losing one or more electrons.

These cations then pass through a (variable) electric or magnetic field that
deflects each cation’s path to an extent that depends on both its mass and charge. The ions
are detected, and a plot of the relative number of ions generated versus their mass-to-
charge ratios (a mass spectrum) is made. The height of each vertical feature or peak in a
mass spectrum is proportional to the fraction of cations with the specified mass-to-charge
ratio. Since its initial use during the development of modern atomic theory, MS has
evolved to become a powerful tool for chemical analysis in a wide range of applications.
Isotopes have similar chemical properties because they have the same atomic number.
To separate them, devices called mass spectrometers are used.

Figure 13. Analysis of zirconium in a mass spectrometer produces a mass spectrum with
peaks showing the different isotopes of Zr.

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• Bainbridge Spectrometer

Kenneth Tompkins Bainbridge an American Physicist designed an elegant

instrument for the determination of isotopic masses of element known as Bainbridge
mass spectrometer. A Bainbridge mass spectrometer basically consists of (figure 14):

1. Ionization Chamber:

The ionization chamber is used to ionize the gas whose mass or isotopes are to be
determined, and positive ions are produced in the discharge tube by the electron collision
method.

2. Velocity Selector:

Velocity selector has two fields electric and magnetic field, both are applied perpendicular
to the moving ion beam. A potential is applied between two parallel plates to produce the

uniform electric field. A magnetic field B is applied at right angles to the electric field E
in such a way that the electric and magnetic forces acting on the ions act in the directions

opposite to each other.

3. Vacuum / Analyzing Chamber: Vacuum / Analyzing Chamber is a semi-

spherical cavity in which another magnetic field B' is applied perpendicular to the moving

positive ion.

A beam of positive ions produced in a discharge tube is collimated into a fine beam by
two narrow slits S1 and S2. This fine beam enters into a velocity selector.

The velocity selector consists of two plane parallel plates , which produces a uniform
electric field E and an electromagnet, to produce uniform magnetic field B (represented
by the dotted circle).

The velocity selector allows the ions with a particular velocity to come out of it, under
the combined action of an electric and a magnetic field. These two fields are at right
angles to each other and to the direction of the ions beam.

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Figure 14. Bainbridge′s mass spectrometer diagram

The electric field and magnetic field are so adjusted that the deflection produced by one
field is opposite to the other, so that the ions do not suffer any deflection within the
velocity selector. Let E and B be the electric field intensity and magnetic induction
respectively and q be the charge of the positive ion.
The force exerted by the electric field is equal to q E and the force exerted by the magnetic
field is equal to B q v where v is the velocity of the positive ion,

q. E = B. q. v
𝑬
v=
𝑩

Only those ions having this velocity v, pass out of the velocity selector and then through
the slit S3, enter the evacuated chamber. These positive ions having the same velocity are
subjected to another strong uniform magnetic field of induction B′ at right angles to the
plane of the paper acting outwards. These ions are deflected along circular path of radius
R and strike the photographic plate. The force due to magnetic field B′qv provides the
centripetal force.

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𝒗𝟐
B′. q. v = m.
𝑹
𝐁′ 𝐪 𝐑
m=
𝒗
𝑬
Substituting v =
𝑩

𝐁 𝐁′𝐪 𝐑
m=
𝑬

Ions with different masses trace semi-circular paths of different radii and produce dark
lines on the plate. Greater is the particle mass, larger is the radius of circular path. If m1
and m2 are two different masses of ions and m2 > m1 then R2 > R1. The distance between
the opening of the chamber and the position of the dark line gives the diameter 2R from
which radius R can be calculated. Since, B, B′, E and R are known, the mass of the
positive ions and hence isotopic masses can be calculated.

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