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Introduction (Atomic and
Nuclear StructureRadioactivity)
Matter &The Atom
 The term matter describes all of the physical
substances around us
 Matter is anything that has mass and takes up space
 The Universe is made up of matter and energy
Matter is made up of atoms.
An atom is the smallest whole particle of
matter.
Early Models of the Atom
Rutherford
 Mostly empty space
 Small, positive nucleus
 Contained protons
 Negative electrons scattered
around the outside
Early Models of the Atom
Bohr
 Electrons move in
definite orbits around
the nucleus
Modern Model of the Atom
The electron cloud
 Sometimes called the wave
model
 Spherical cloud of varying
density
Which force holds the electrons in orbit?
The Coulomb force
ATOMIC STRUCTURE
 The nucleus of an atom is made up of tightly bound
protons and neutrons, which are called nucleons.
 The nucleus contains most of the atomic mass.
Protons
 Much larger and heavier than electrons
 Protons have a positive charge (+)
 Located in the nucleus of the atom
Neutrons
 Large and heavy like protons
 Neutrons have no electrical charge
 Located in the nucleus of the atom
Electrons
 Tiny, very light particles
 Have a negative electrical charge (-)
 Move around the outside of the nucleus in shells
Fig1.2.
 Each shell is assigned a principal quantum number (n),
beginning with one for the K-shell, two for the L-shell,
and so on.
 The number of electrons each shell can contain is 2n2.
Electrons are arranged in Energy Levels or
Shells around the nucleus of an atom.
• first shell K
a maximum of 2 electrons
• second shell L
a maximum of 8 electrons
• third shell M
a maximum of 18 electrons
The Atom
The atom consists of two parts:
1. The nucleus which contains:
protons
neutrons
2. Orbiting electrons.
Atoms in nature are electrically neutral so
The number of electrons orbiting the nucleus equals the number
of protons in the nucleus.
Mass number = number of protons + number of neutrons
A
X
Z
Atomic number = number of protons
Element symbol
A
X
Z
A
= number of protons + number of neutrons
Z
= number of protons
N
= A – Z = number of neutrons
Number of neutrons = Mass Number – Atomic Number
Electron binding energy
 The work that is required to remove an electron from an
atom is called the electron binding energy.
 The binding energy of outer-shell electrons is smaller than
the binding energy of the inner-shell.
 Energetic particles can knock out inner-shell electrons
only if their energy is equal to or greater than the
electron binding energy.
 A vacancy in the K-shell will be filled by an electron
from a higher shell.
 Electrons moving from an outer shell to an inner shell
may emit excess energy as electromagnetic radiation.
Electromagnetic radiation
Radiation is the transport of energy through space.
Electromagnetic radiation represents a transverse
wave, in which the electric and magnetic fields oscillate
perpendicular to the direction of the wave motion.
 Wavelength (λ) is the distance between successive crests of
waves.
 Amplitude is the intensity defined by the height of the wave.
 Frequency (f) is the number of wave oscillations per
unit of time expressed in cycles per second, or in hertz
(Hz).
 The period is the time required for one wavelength to
pass (1/f).
X-rays are an example of electromagnetic radiation.
 The product of the wavelength (λ) and frequency (f) of
electromagnetic radiation is equal to the speed of light c
 (c = f .λ ).
 Fig. 1.3 shows the electromagnetic spectrum from radio
waves (long wavelength) to x-rays and gamma rays (short
wavelength).
Photons
 Electromagnetic radiation is quantized, meaning that it
exists in discrete quantities of energy called photons.
Photons may behave as waves or particles but have
no mass.
There are many types of uranium:
235
238
A
A
Z
Z
Number of protons
Number of protons
Number of neutrons
Number of neutrons
U
92
U
92
235
238
U
92
U
92
A
235
A
238
Z
92
Z
92
Number of protons
92
Number of protons
92
Number of neutrons
143
Number of neutrons
146
Isotopes
of any particular element contain the same
number of protons, but different numbers of neutrons.
Most of the isotopes which occur naturally are stable.
A few naturally occurring isotopes and all of the
man-made isotopes are unstable.
Unstable isotopes can become stable by releasing
different types of particles.
This process is called radioactive decay and the
elements which undergo this process are called
radioisotopes/radionuclides.
Radioactive Decay
The word decay means to "break down."
Radioactive decay results in the emission of either:
o an alpha particle (a),
o a beta particle (b),
o or a gamma ray(g).
Radiation
• Radiation: Energy in the
form of particles or
electromagnetic waves.
• Radiation is the transport of
energy through space
Classification of radiation
 Radiation is classified into two main categories, non-ionizing
and ionizing, depending on its ability to ionize matter.
Ionizing Radiation:
o energy to remove an electron from an atom or molecule
Radiation with sufficient .
Non-ionizing radiation:
o Is radiation without enough energy to separate molecules or
remove electrons from atoms. Examples are visible light, radio
and television waves, ultra violet (UV), and microwaves with a
large spectrum of energies.
Radioactivity
 The process by which unstable
atoms spontaneously transform to
new atoms* and in the process
emit radiation.
Units of Activity
o Curie (Ci)
o Bequerel (Bq)
$-
Half-Life
1.00
 Half-life is the amount
f
e
lt
l
0.80
ln ( 2 )
T1/2
of time needed for the
0.60
activity to reach one half
One half-life
0.40
of the original amount.
f
1
2
t
T1/2
Two half-lives
0.20
0.007
0.00
0
20
40
Days
60
80
100
Common Types of Radiation
Common Types of Radiation
Alpha Particle
An alpha particle is identical to that of a helium nucleus.
It contains two protons and two neutrons.
Alpha Particle
Alpha Decay
A
X
Z
A-4
4
Y
He
+
Z-2
2
unstable atom
alpha particle
more stable atom
Alpha Decay
222
226
Ra
88
Rn
86
4
He
2
Alpha Decay
A
A-4
4
226
222
4
X
Z
Ra
88
Y
+
Z-2
Rn
+
86
He
2
He
2
Beta Particle
A beta particle is a fast moving electron which is
emitted from the nucleus of an atom undergoing
radioactive decay.
Beta decay occurs when a neutron changes into a
proton and an electron.
Beta Decay
As a result of beta decay, the nucleus has one less
neutron, but one extra proton.
The atomic number, Z, increases by 1 and the mass
number, A, stays the same.
Beta Decay
As a result of beta decay, the nucleus has one less
neutron, but one extra proton.
The atomic number, Z, increases by 1 and the mass
number, A, stays the same.
Beta Decay
218
218
Po
84
At
85
b
-1
0
Beta Decay
A
X
Z
218
Po
84
A
b
-1
218
b
-1
Y
+
Z+1
Rn
+
85
0
0
Photons
A photon is an x or gamma ray
 Has no weight
 Has no charge
Gamma Rays
Gamma rays are not charged particles like a and b
particles.
Gamma rays are electromagnetic radiation with high
frequency.
Photon Decay
99Tc
99mTc
Gamma ray
Excited Nucleus
Stable Nucleus
Photon Decay
 When atoms decay by emitting a or b particles to form a
new atom, the nuclei of the new atom formed may still
have too much energy to be completely stable.
 This excess energy is emitted as gamma rays (gamma ray
photons have energies of ~ 1 x 10-12 J).