Structure of Atom

Basic Physics
Structure of Atom
1 Structure of MatterBasic Physics
Reference Books:
F.M.Khan-The Physics of Radiation Therapy,
H.E.Jones-The Physics Of Radiology
By
P.Velliangiri M.Sc., MBA
Medical Physicist
North Bengal Oncology Centre,
Rangapani Near Siliguri
Darjeeling Dist, West Bengal
India
Mail ID – p.velliangirii@gmail.com

The Atom :
All matter is composed of individual entities called elements.
Each atom consists of a small central core, the nucleus, where
most of the atomic mass is located, and a surrounding “cloud” of
electrons moving in orbits around the nucleus.
Whereas the radius of the atom (radius of the electronic orbits)
is approximately 10-10 m, the nucleus has a much smaller radius,
namely about 10-15 m.
The properties of atoms are derived from the constitution of
their nuclei and the number and the organization of the orbital
electrons.
Basic Physics Structure of Matter2

The Nucleus:
The nucleus contains two kinds of fundamental particles:
protons and neutrons.
Protons are positively charged, neutrons have no charge
Electron has a negative unit charge (1.60 × 10-19 Coulombs)
The number of protons in the nucleus is equal to the number of
electrons outside the nucleus
An atom is completely specified by the formula A
ZX,
X is the chemical symbol for the element; A is the mass
number, Z is the atomic number

The Nucleus Cont….
Basis of different proportions of neutrons
and protons in the nuclei, atoms have been
classified into the following categories:
isotope same Nproton, different Nneutron
(1H1, 1H2 and 1H3)
isotone same Nneutron, different Nproton
(6C14 and 8O16)
isobar same (Nneutron+ Nproton)(nucleons),
different Nproton (8O16 and 7N16)
isomer  same Nproton, same Nproton,
different nuclear energy state

Atomic Mass and Energy Units :
Masses of atoms and atomic particles are conveniently given in
terms of atomic mass unit (amu).
amu :
An amu is defined as one twelfth of the mass of a 12
6C atom, a
carbon isotope. Thus, the atom of 12
6C is arbitrarily assigned the
mass equal to 12 amu.
1amu = 1.66×10-27 kg
The mass of an atom expressed in terms of amu is known as
atomic mass or atomic weight.
Avogadro's law :
Every gram atomic weight of a substance contains the same
number of atoms. Avogadro's number (NA), is 6.0228 × 1023 atoms
per gram atomic weight.

From the previous definitions, one can calculate other quantities
of interest such as the number of atoms per gram, grams per
atom, and electrons per gram. Helium as an example, its atomic
weight (AW) is equal to 4.0026.
The masses of atomic particles, according to the atomic mass
unit, are electron = 0.000548 amu, proton = 1.00727 amu, and
neutron = 1.00866 amu.

Distribution of Orbital Electrons :
The electrons revolve around the nucleus in specific orbits and are
prevented from leaving the atom by the centripetal force of attraction
between the positively charged nucleus and the negatively charged
electron.
The innermost orbit or shell is called the K shell. The next shells are
L, M, N, and O.
The maximum number of electrons in an orbit is given by 2n2, where
n is the orbit number.
For example, a maximum of two electrons can exist in the first orbit,
eight in the second, and 18 in the third.

Electron orbits can also be considered as energy levels.
The energy in this case is the potential energy of the electrons.
With the opposite sign it may also be called the binding energy of
the electron.

Atomic Energy Levels :
The binding energies of the electrons in various shells depend on the
magnitude of Coulomb force of attraction between the nucleus and
the orbital electrons.
Thus, the binding energies for the higher-Z atoms are greater
because of the greater nuclear charge.
In the case of tungsten (Z = 74), the electrons in the K, L, and M
shells have binding energies of about 70,000, 11,000, and 2,500 eV,
respectively.

The electron will fall back to its normal position with the emission of
energy in the form of optical radiation. The energy of the emitted
radiation will be equal to the energy difference of the orbits between
which the transition took place.

K, L, and M shells where the electrons are more tightly bound
(because of larger Coulomb forces), the absorption or emission
of energy will involve higher-energy radiation.
Also, if sufficient energy is imparted to an inner-orbit electron
so that it is completely ejected from the atom, the vacancy or the
hole created in that shell will be almost instantaneously filled by
an electron from a higher-level orbit, resulting in the emission of
radiation.
This is the mechanism for the production of characteristic x-
rays.

Nuclear Forces :
There are four different forces in nature. These are, in the order
of their strengths: (a) strong nuclear force, (b) electromagnetic
force, (c) weak nuclear force, and (d) gravitational force.
Of these, the gravitational force involved in the nucleus is very
weak and can be ignored.
The electromagnetic force between charged nucleons is quite
strong, but it is repulsive and tends to disrupt the nucleus. A
force much larger than the electromagnetic force is the strong
nuclear force that is responsible for holding the nucleons
together in the nucleus.

The weak nuclear force is much weaker and appears in certain
types of radioactive decay (e.g., β decay).
The strong nuclear force is a short-range force that comes into
play when the distance between the nucleons becomes smaller
than the nuclear diameter (~10-15 m).
In the case of a positively charged particle approaching the
nucleus, there will be a potential barrier due to the Coulomb
forces of repulsion, preventing the particle from approaching the
nucleus.

Nuclear Energy Levels :
The shell model of the nucleus assumes that the nucleons are
arranged in shells, representing discrete energy states of the
nucleus similar to the atomic energy levels.
Energy-level diagram with a decay scheme for a cobalt-60
(60
27Co) nucleus that has been made radioactive in a reactor by
bombarding stable 59
27Co atoms with neutrons. The excited
60
27Co nucleus first emits a particle, known as a β- particle and
then, in two successive jumps, emits packets of energy, known
as photons.
The emission of a β- particle is the result of a nuclear
transformation in which one of the neutrons in the nucleus
disintegrates into a proton, an electron, and a neutrino. The
electron and neutrino are emitted instantaneously and share the
released energy with the recoiling nucleus.

Energy-level diagram for the decay of 60Co nucleus β-particle
emission followed by two γ-ray photons emitted per disintegration
with energies of 1.17 MeV and 1.33 MeV.

Particle Radiation :
The term radiation applies to the emission and propagation of
energy through space or a material medium.
By particle radiation, we mean energy propagated by traveling
corpuscles that have a definite rest mass and within limits have a
definite momentum and defined position at any instant.
These particles can travel with high speeds, depending on their
kinetic energy, but never attain exactly the speed of light in a
vacuum. Also, they interact with matter and produce varying
degrees of energy transfer to the medium.

Elementary Particles :
 The study of the structure of atom reveals that the fundamental
particles electron, proton and neutron are the building blocks of
an atom.
 But the extensive studies on cosmic rays have revealed the
existence of numerous new nuclear particles like mesons.
 These particles are classified into four major groups as photons,
leptons mesons and baryons.
Photon :
Photon is a quantum of radiation with no charge and no mass,
but it is a carrier of energy. It travels with velocity of light.

Lepton :
 Leptons are lighter particles having mass equal to or less than
about 207 times the mass of an electron except neutrino and
antineutrino.
 This group contains particles such as electron, positron,
neutrino, antineutrino, positive and negative muons.
 The electron and positron are the antiparticles. Neutrino and
antineutrino are also associated with β-ray emission.
 The neutrinos and antineutrinos are massless and chargeless
particles, but carrier of energy and spin. Muons were discovered
in cosmic ray studies.

Mesons :
Mesons are fundamental particles carrying a single unit of
charge and possessing mass intermediate between electron and
proton (me and mp).
The name meson was given by Yukawa in 1935.
The three types of mesons are (i) π-meson (pion) (ii) K−meson
(kaon) and (iii) η- meson.
The mesons are the interaction agents between nucleons.
The rest mass of mesons vary between 250 me and 1000 me

Baryons :
Baryons form the heavier particle group.
Protons and neutrons are called nucleons and the rest of the
heavier particles other than nucleons are known as hyperons.
There are four types of hyperons which are lambda, sigma, xi
and omega hyperons.
Protons and neutrons are around 1836 times the mass of the
electron, whereas the mass of the hyperons vary from 2180 me
and 3275 me.

Electromagnetic Radiation:
Wave Model :
In an electromagnetic wave, electric and magnetic field vectors
are at right angles to each other and both are at right angles to
the direction of propagation.
They possess the wave character and propagate through free
space without any material medium.
These waves are transverse in nature.
The variation of electric field E
along Y direction and magnetic
field B along Z direction and wave
propagation in + X direction.

Wavelength :
Means pretty much what it says - the length of one wave.
More precisely, it means the distance from the peak of one wave to the
peak on the next wave.
Frequency :
With waves, it's "how many waves per second"

Electromagnetic spectrum :
The electromagnetic spectrum is the range of all possible
frequencies of electromagnetic radiation.
Uses of electromagnetic spectrum :
Radio waves :
These waves are used in radio and television communication
systems. AM band is from 530 kHz to 1710 kHz. Higher
frequencies upto 54 MHz are used for short waves bands.
Television waves range from 54 MHz to 890 MHz. FM band is
from 88 MHz to 108 MHz. Cellular phones use radio waves in
ultra high frequency (UHF) band.

Microwaves : Due to their short wavelengths, they are used in
radar communication system. Microwave ovens are an
interesting domestic application of these waves.
Infra red waves :
(i) Infrared lamps are used in physiotherapy.
(ii) Infrared photographs are used in weather forecasting.
(iii) As infrared radiations are not absorbed by air, thick fog, mist
etc, they are used to take photograph of long distance objects.
(iv) Infra red absorption spectrum is used to study the molecular
structure.

Visible light : Visible light emitted or reflected from objects
around us provides information about the world. The
wavelength range of visible light is 4000 Å to 8000 Å.
Ultra−violet radiations :
(i) They are used to destroy the bacteria and for sterilizing surgical
instruments.
(ii) These radiations are used in detection of forged documents,
finger prints in forensic laboratories.
(iii) They are used to preserve the food items.
(iv) They help to find the structure of atoms.

X rays :
(i) X rays are used as a diagonistic tool in medicine.
(ii) It Is used to study the crystal structure in solids.
γ−rays :
Study of γ rays gives useful information about the nuclear
structure and it is used for treatment of cancer.

Quantum Model :
To explain the results of certain experiments involving
interaction of radiation with matter, such as the photoelectric
effect and the Compton scattering, one has to consider
electromagnetic radiations as particles rather than waves. The
amount of energy carried by such a packet of energy, or photon,
is given by:
E = hv
where E is the energy (joules) carried by the photon, h is the
Planck's constant (6.62 × 10-34 J-sec), and ν is the frequency
(cycles/second).

Structure of Atom

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Structure of Atom