Atomic
Structure
Chapter # 2 (For Class XI)
According to New Syllabus 2022
2.3 Brief History of Atomic Structure
In history,
first time two Greek philosopher Leucippus and Democritus (440 B.C) gave
the idea that matter is composed of invisible, indivisible and infinite number
of building blocks called ‘atomos’ (Greek ‘a’; non and ‘tom’; break). This
ancient theory of atom was based on philosophical reasoning rather than
scientific basis. They said that different atoms and their different
combinations produce different types of matters (solids, liquids and gases).
Most philosophers of that time did not give weightage to their concepts about
matter and composition but favoured the Aristotelian concept that
“Matter is composed of four elements; earth, water, air and fire”.
The idea of
atom was revisited and studied upon many scientists and philosophers, however
it was John Dalton (1803) who recognized as the introducer of modern
atomic theory.
After 12
years of discovery of electron as a fundamental entity that dwell in an atom, Robert
Andrews Millikan, in 1909 performed an ingenious oil drop experiment with
charged oil drop getting suspended in electric field having its weight balanced
by the electrostatic force, he found out that the charge on every drop was an integral
multiple of 1.6x10−19 coulomb which encouraged him to deduce
that this must be the charge of Thomson’s electrons.
In 1911,
Ernest Rutherford laid the foundation that the atom consist of an
extremely charged region which he called nucleus wherein the most of the
mass of an atom is concentrated while electrons revolve in the extra-nuclear
part.
By 1920,
scientists knew and believed that most of the mass of atom was located in the
central nuclear core and 1932, English chemist James Chadwick informed
the world about the third subatomic particle that was neutral and a part of
nucleus which he named neutron.
2.2 Subatomic Particles and Their Characteristics
Subatomic
Particles
More than 100 subatomic
particles have been discovered such as electron, proton, neutron, positron,
mesons, hyperons, neutrino, antineutrino etc.
Fundamental
particles
Out of 100 subatomic particles,
only electron, proton and neutron are considered to be fundamental particles of
an atom as they play an important role for the determination of physical and
chemical properties of element.
Almost all of the mass of an
atom exists in nucleus and nucleus was discovered by Rutherford (in 1911 A.D.).
Except protium (lightest isotope of hydrogen), nuclei of all other atoms
contain neutrons.
Properties
of Subatomic Particles
Electrons
The subatomic
particle electron was discovers by J.J. Thomson in 1897 A.D, while studying on
cathode rays experiment. He called the cathode rays beam as electrons.
Thereafter, Millikan with his “Oil Drop Experiment” calculated the charge on an
electron.
Electron
revolve in orbits and occupy about 100,000 times greater volume than nucleus
but form less than one percent total mass of an atom. They carry negative
charge whose magnitude is equal to positive charge of protons. Electrons are
attracted by protons and this attractive force (electrostatic force) keeps
electrons constantly moving around nucleus. The mass of electron is nearly 1836
times less than proton and 1839 times less than neutrons.
Electronic configuration
refers to the presence of electrons in various shells and sub-shells. This
arrangement of electrons affects the atomic stability, melting points, boiling
points, density etc. They are also involved in chemical bond formation.
Protons
Protons are
positively charged sub-atomic particles which are located in the nuclei of all
elements. They were discovered by a German Physicist Goldstein (1886 A.D), by
using perforated cathode in Crook’s discharge tube. He observed that positive
ions were formed when cathode rays hit the gaseous atoms in discharge tube.
Rutherford called these positively charged particles as ‘PROTONS’. With respect
to mass and charge, the protons of all elements are identical to each other.
Atomic number of elements is related only to protons while mass number is the
sum of protons and neutrons (About 99.94% mass of an atom exists in nucleus).
Protons and electrons have some magnitude of charge but have different masses.
All elements have different number of protons.
Neutrons
The discovery
of neutrons was made by James Chadwick (1932 A.D), in an artificial
radioactivity experiment. He bombarded alpha particles from Polonium on a
stable element Beryllium, and noted that some highly penetrating neutral
particles were produced. These particles were named as neutrons.
Like protons,
neutrons are also the part of atomic mass but neutron has no charge. Neutrons
of all elements have same nature and mass. They are bounded with each other and
protons by the nuclear forces. Under certain circumstances, when electrostatic
force (between protons) overcomes the nuclear force, nucleus becomes unstable
and fission reaction is occurred.
Neutrons are
slightly heavier than protons. They are not deflected by electric or magnetic
fields. However, the isotopes of same element always have the same number of
protons but different number of neutrons. Further, the stability of nucleus
depends upon the neutrons. The number of neutrons is determined by subtracting
the number of protons from the mass number.
Number
of neutrons = Mass number – Number of protons
2.3
Radioactivity (Confirmation of Electrons and Protons)
Discovery and First Radioactive element discovered
The phenomenon of radioactivity was discovered by a French professor,
Henry Becquerel in 1896 A.D. while working on uranium mineral called
Pitch-blende (an oxide of uranium; U3O8). He observed
that there was continuous emission of some invisible radiations which producing
bright spots on (fogging) photographic plates, ionizing gases, penetrating
through thin metal sheets and producing fluorescence on zinc sulphide screen. This
process of emitting invisible radiations was termed as radioactivity.
Discovery polonium and radium By Marie
Currie and Pierre Currie
Marie Currie and her husband, Pierre Currie isolated the radioactive
component of the pitch blend mineral and separated two new radioactive elements
polonium and radium.
Definition
Radioactivity is the nuclear phenomenon in which there is a spontaneous
and continuous emission of nuclear radiations from atom whose atomic number is
greater than 83 due to the splitting of atomic nuclei.
Radioactive Elements
The elements which emit invisible nuclear
radiations or radioactive radiations spontaneously are
known as natural radioactive elements and the phenomenon is termed as natural
radioactivity. Natural radioactive elements have
atomic number greater than 83 emits nuclear.
Natural radioactive elements after spontaneous emission of rays, break
down to more stable elements. This emission of radiations continues till the
stable element lead (lead-207) is formed.
Nuclear radiations are also produced when a stable element is bombard or
truck by nuclear particle, this is known as artificial radioactivity.
Radioactive Radiations
The radiations emitted by radioactive elements are called Radioactive
Radiations. They are of three types: Alpha rays, Beta rays and Gamma rays. Initially
these rays were named as Becquerel rays but later on, Marie Currie
coined the term radioactive rays.
Experiment for Separation and Detection of
Radiation
Experiment
To study the nature of radiations, Rutherford placed a small piece of
radioactive material is in a Lead Block having a small hole in it. The
radiations emitted by radioactive substance were passed through an electric
field. (In fact, they are first subjected to pass through a vacuum chamber with
a photographic plate in which a magnetic or electric field is applied).
Distinguishing
Properties of Alpha Particles, Beta Particles and Gamma Rays
Uses of Nuclear Radiations
Following are the important uses of
nuclear radiations
1. In medical field, nuclear radiations
are used to diagnose, monitor and treat various diseases. These radiations are used to study
the bone formation in mammals. Radiotherapy is most commonly used for the treatment of
cancer. Radioisotopes has immense role in the growing field called nuclear medicine.
2. In agriculture
field, radioisotopes are used to treat the seeds in the production of new
varieties of crops.
3. They
are used for the production of energy.
Nuclear power stations like Karachi Nuclear Power Plant produce
electrical energy.
4. In
industries they are used to monitor the quality of products.
Radioisotopes are also used to measure the density of metals and thickness of
plastics.
5. In the field of geology, these
are used to study the rocks.
6. Carbon-14 isotope is used to measure
the age of fossils and artefacts (archeology).
Radioactive Dating
Radioactive Dating is a method for determining the age of an object
containing organic material by using the properties of radiocarbon, a
radioactive isotope of carbon. It is usually used to predict date ancient
objects.
2.4
Planck’s Quantum Theory of Light (Quantization of Energy)
Introduction and Definition
This theory was proposed by the German physicist Max Planck in 1900 A.D.
to describe the emission and absorption of radiations from heated bodies. He
was awarded Nobel Prize in 1918 A.D. This theory explains the nature of light
in terms of Quanta which is the smallest unit of radiation energy.
Basic Postulates of PQT
1. Atoms cannot absorb or
emit energy continuously.
2. The
emission or absorption of energy (light) occurs in small packets of energy or
specified amount called quanta which is defined as the smallest unit of
radiation energy which can exist independently. A quantum of radiant energy in
the form of light is called Photon.
3. The
energy of quantum (photon) is not fixed. The amount of energy of quantum is
directly proportional to the frequency of the radiations emitted or absorbed by
the body. i.e.
E
𝛂 u or E = hu (This is called as Planck’s equation)
Where,
E = Energy gained or lost by body.
h = Planck’s constant = 6.625 x 10–34 J.s (6.625 x 10–27
Ergs.sec).
u = Frequency of radiation
2.5
Spectrum and its Types
Definition
The definite pattern of band of colours obtained after breaking
(dispersing) of polychromatic light into its component radiations or several
colours in order of increasing or decreasing wavelengths by glass prism (placed
in spectrometer) is called SPECTRUM.
Monochromatic and Polychromatic Light
The light of single wavelength, which consists of only one colour, is
called monochromatic light. It is a light which is composed of only one kind of
ray or wavelength.
A light which is composed of more than one type of rays or wavelengths
or colours is known as polychromatic light such sunlight, bulb light etc.
Spectroscope
It is the apparatus used to see spectrum.
Spectroscopy
The word spectroscopy is derived from Latin word spectrum, which means
image and Greek word skopia, which means observation. It is the study of the
absorption and emission of light other radiation by matter. It is the study of
spectrum produced by different elements.
Spectrometer
It is the instrument used to
measure the intensity and frequency of radiations
Types of Spectrum
There are two types of Atomic
Spectrum:
1. Atomic Emission Spectrum.
2. Atomic Absorption Spectrum.
1. Atomic Emission Spectrum
They are obtained when light from such atoms, that has already absorbed
energy, is passed through a prism.
2. Atomic Absorption Spectrum
They are obtained when light is passed through a gaseous sample of an
element (Radiolucent object) which absorbs some component and then through
prism.
Types of Emission Spectrum
Emission spectrums are of two
types:
1. Continuous Spectrum.
2. Line
Spectrum.
Continuous Spectrum
Definition
The spectrum which contains a continuous band of colours is called
continuous spectrum.
Explanation
1. The continuous spectrum
is composed of different colours which are represented by a light of specific wave length and are not
sharply demarcated.
2. When
sunlight passes through prism, it deviates and splits into continuous band of
seven colours . Spectrum consists of red colour at one end which is least deviated and
violet colour at the other end which is maximum deviated. The composition of
spectrum may be abbreviated as VIBGYOR.
3. In between red and
violet colours, the other colours are orange, yellow, green, blue and indigo.
4. Each colour is associated
with specific wave length. Violet colour has shortest wave length of about 4000 Å while red has the longest wave length of about 7000 Å.
5. Continuous spectrum is
produced by polychromatic light.
6. It does not help in
determining the structure of atoms.
7. In rainy season, rainbow in sky is common example of
continuous spectrum.
Line Spectrum
Definition
When light emitted from electrically excited atoms is passed through
prism, certain distinct lines separated by dark space are obtained, this is
known as line spectrum.
For example
When light from a gaseous source in the excited state is allowed to pass
through the prism, a dis-continuous spectrum (line spectrum) is produced
consisting of discrete sharp lines (each corresponds to a definite wavelength)
separated by dark spaces. A gas is excited by strongly heating or
passing through electric discharge tube at low pressure.
Characteristics of Line Spectrum
1. Samples of a same
element always emit radiations of same wave length.
2. Under the right
conditions, samples of same element always produces same characteristic spectrum. Each element emit
of light of specific wavelength therefore the number of lines and the distance between them depends
upon the nature of element, so line spectra is used as “Finger Print” for the identification of elements.
For example, line
spectrum of sodium contains two yellow coloured lines separated by a definite distance.
Information obtained from Line Spectrum about Structure of
Atom
Line spectra of the elements give
the information that electron around the nucleus have definite amount of energy
and are arranged in definite energy levels E1. After absorbing
energy, they jump to an appropriate energy level E2 and then return
back. The difference is emitted energy of electrons is E2–E1
is equal to the energy absorbed. (Each
spectral line is produced due to downward jumping of electron from higher energy
level to lower energy level. Dark spaces are due to the space between two
energy levels).
Types of Line Spectrum
Line Spectrum can be seen by the following two ways:
(i) Absorption Line Spectrum
(ii) Emission Line Spectrum
(i) Absorption Line Spectrum
When a beam of light is passed through an absorbing material like
gaseous sample of an element, the element absorbs certain wavelengths while
rest of wavelengths pass through it. The spectrum obtained consists of a series
of dark lines with a bright background and known as absorption line spectrum.
(ii) Emission Line Spectrum
Sodium vapours lamp (street light), mercury vapours lamp, electrical
discharge tube, hot solids or elements emits radiations of certain wavelengths.
The spectrum which is formed form such radiations Is called emission line
spectrum.
2.6
X-Rays/Roentgen Rays
Introduction and Origin of X-Rays
In 1895 A.D, W. Roentgen accidently observed that when fast moving high
energy electrons (cathode rays) collide with metal anode in discharge tube
(Crooke’s discharge tube), highly penetrating invisible rays of short
wavelength are produced. Initially these rays were names as Roentgen rays
(after the name of its discoverer) but later on these were called X-rays. These
rays can penetrate through paper, glass, rubber, metal and human flesh.
Nature of X-Rays
The X-rays are highly energetic electromagnetic radiations of very high
frequency and very short wavelength (as u = 1/l). The frequency (wavelength) of X-rays
depends upon the material used as anode.
Types of X-rays
Mosely used different anode and analyzed
the intensity of X-rays. On the basis of wavelengths there are two types of
spectral lines of X-rays:
(a) K-series
These are produced by the elements
(anodes) having large atomic number. These spectral lines have short
wavelengths and high energy because transition of electrons occurs from high
energy level to low energy (means there is big difference between two energy
levels).
(b) L-series
These are produced by the elements
(anodes) having small atomic number. These spectral lines have long wavelengths
and low energy because there is small difference between two energy levels.
Production of X-rays/Mechanism of X-ray’s Formation
X-rays are produced in a special
discharge tube in which cathode is heated filament. Under high voltage (5000
volt) and vary low pressure (0.001 mm) cathode rays are emitted from cathode
and travel towards anode where they strike with high speed to give X-rays. The
transition of electron occurs in anode atoms which cause the production of
X-rays photon.
Relationship between wavelength of
X-rays and nuclear charge (Z) of atom
In 1911, Henry Moseley studied the different wavelengths (or frequencies) of X-rays produced from anodes of different metals. He found that the wavelength of X-rays emitted, decreases regularly with increasing nuclear charge of an atom. He called the number of positive charges (or protons) as the atomic number of an atom. i.e.
l of X-rays a 1/Z or u of X-rays a Z
Properties of X-rays
1. They are short wave
length high-energy invisible electromagnetic radiations
2. Their range of
wavelength lies between 0.1-10 Å.
3. They travel with speed
of light.
4. Penetration power
increases as energy of X-rays increases.
5. They travel in straight
line like cathode rays.
6. They are unaffected by
electric or magnetic field.
7. They affect the
photographic film.
8. They ionize gases and
damage and destroy the living tissues due to high energy.
Uses of X-rays
Initially, X-rays were used to assist in the setting of broken arm of a
person. The use of X-rays are increased with the passage of time.
1. These are used for the analysis of
metallic substance or bullets in flesh.
2. They are used by the dentist to examine
the defective or damage teeth.
3. They are used for destroying the cancer
cells.
4. They are used for checking the baggage
containing metallic knife, blade or weapon, transport of illegal goods etc at the air ports.
5. They are used for the determination of
structures of crystals in crystallography. Thus X-ray diffraction technique was developed.
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