Atoms, Isotopes,
Alpha, Beta, Gamma Decay
Dr Mark Marais
May 2025
The Atom
The atom consists of two parts:
1. The nucleus which contains:
2. Orbiting electrons.
protons
neutrons
Dr Mark Marais
May 2025
All matter is made up of elements (e.g. carbon, hydrogen, etc.).
The smallest part of an element is called an atom.
Atoms of different elements contain different numbers of protons.
The mass of an atom is almost entirely due to the number of protons and neutrons.
The Atom
Elements are systematically arranged on the Periodic Table according to number of protons in their atoms.
Dr Mark Marais
May 2025
X
A
Z
Mass number
Atomic number
Element symbol
= number of protons + number of neutrons
= number of protons
Dr Mark Marais
May 2025
X
A
Z
A = number of protons + number of neutrons
Z = number of protons
A – Z = number of neutrons
Number of neutrons = Mass Number – Atomic Number
Dr Mark Marais
May 2025
Periodic Table
U
235
92
U
238
92
There are many types of Uranium:
A | |
Z | |
Number of protons | |
Number of neutrons | |
A | |
Z | |
Number of protons | |
Number of neutrons | |
Dr Mark Marais
May 2025
U
235
92
U
238
92
There are many types of uranium:
Isotopes of any particular element contain the same number of protons, but different numbers of neutrons.
A | 235 |
Z | 92 |
Number of protons | 92 |
Number of neutrons | 143 |
A | 238 |
Z | 92 |
Number of protons | 92 |
Number of neutrons | 146 |
Dr Mark Marais
May 2025
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 isotopes which undergo this process are called radioisotopes/radionuclides.
Dr Mark Marais
May 2025
Radioactive decay commonly results in the emission of
one of the following types:
Radioactive Decay
Dr Mark Marais
May 2025
An alpha particle is identical to that of a helium nucleus.
It contains two protons and two neutrons.
Alpha Decay
Dr Mark Marais
May 2025
X
A
Z
Y
A - 4
Z - 2
+
He
4
2
Alpha Decay
unstable atom
more stable atom
alpha particle
Dr Mark Marais
May 2025
Ra
226
88
Rn
222
86
He
4
2
Alpha Decay
Dr Mark Marais
May 2025
X
A
Z
Y
A - 4
Z - 2
+
He
4
2
Ra
226
88
Rn
222
86
+
He
4
2
Alpha Decay
Dr Mark Marais
May 2025
Rn
222
86
He
4
2
+
Po
218
84
He
4
2
Rn
222
86
+
Y
A
Z
He
4
2
Alpha Decay
Dr Mark Marais
May 2025
He
4
2
U
234
92
+
Th
230
90
He
4
2
X
A
Z
+
Th
230
90
He
4
2
Alpha Decay
Dr Mark Marais
May 2025
Th
230
90
+
Y
A
Z
He
4
2
He
4
2
+
Ra
226
88
He
4
2
Th
230
90
Alpha Decay
Dr Mark Marais
May 2025
X
A
Z
+
Pb
214
82
He
4
2
He
4
2
+
Pb
214
82
He
4
2
Po
218
84
Alpha Decay
Dr Mark Marais
May 2025
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
Dr Mark Marais
May 2025
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
Dr Mark Marais
May 2025
Po
218
84
β
0
-1
At
218
85
Beta Decay
Dr Mark Marais
May 2025
X
A
Z
Y
A
Z + 1
+
β
0
-1
Po
218
84
Rn
218
85
+
β
0
-1
Beta Decay
Dr Mark Marais
May 2025
Th
234
90
Y
A
Z
+
β
0
-1
Th
234
90
Pa
234
91
+
β
0
-1
Beta Decay
Dr Mark Marais
May 2025
X
A
Z
Pb
210
82
+
β
0
-1
Tl
210
81
Pb
210
82
+
β
0
-1
Beta Decay
Dr Mark Marais
May 2025
Bi
210
83
Y
A
Z
+
β
0
-1
Bi
210
83
Po
210
84
+
β
0
-1
Beta Decay
Dr Mark Marais
May 2025
X
A
Z
Bi
214
83
+
β
0
-1
Pb
214
82
Bi
214
83
+
β
0
-1
Dr Mark Marais
May 2025
Gamma Decay
Gamma rays are not charged particles like α and β particles.
Gamma rays are electromagnetic radiation with high frequency.
When atoms decay by emitting α or β 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).
Dr Mark Marais
May 2025
Penetrating Ability of Particles
Dr Mark Marais
May 2025
Penetrating Ability of Particles
Dr Mark Marais
May 2025