1. The Atom The atom consists of two parts:
1. The nucleus which contains:
protons
neutrons
2. Orbiting electrons.

2. The Atom
All matter is made up of elements (e.g. carbon, hydrogen, etc.).
The smallest part of an element is called an atom.
Atom of different elements contain different numbers of protons.
The mass of an atom is almost entirely due to the number of protons and neutrons.

3. X A Z Mass number = number of protons + number of neutrons
Element symbol Z
Atomic number
= number of protons

4. 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

5. U U 235 92 238 92 There are many types of uranium: A Z
Number of protons
Number of neutrons A Z
Number of protons
Number of neutrons

6. U U 235 92 238 92 There are many types of uranium: A 235 Z 92
Number of protons
Number of neutrons
143 A
238 Z 92
Number of protons
Number of neutrons
146
Isotopes of any particular element contain the same number of protons, but different numbers of neutrons.

7. 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.

8. Radioactive Decay Radioactive decay results in the emission of either:
an alpha particle (a),
a beta particle (b),
or a gamma ray(g).

9. Alpha Decay
An alpha particle is identical to that of a helium nucleus.
It contains two protons and two neutrons.

10. X Y + He Alpha Decay A Z A - 4 Z - 2 4 2 unstable atom alpha particle
more stable atom

11. Alpha Decay Rn
222 86 He 4 2 Ra
226 88

12. Alpha Decay X A Z Y
A - 4
Z - 2 + He 4 2 Ra
226 88 Rn
222 86 + He 4 2

13. Alpha Decay Rn
222 86 + Y A Z He 4 2 Rn
222 86 He 4 2 + Po
218 84

14. Alpha Decay X A Z + Th
230 90 He 4 2 He 4 2 U
234 92 + Th
230 90

15. Alpha Decay Th
230 90 + Y A Z He 4 2 He 4 2 + Ra
226 88 Th
230 90

16. Alpha Decay X A Z + Pb
214 82 He 4 2 He 4 2 + Pb
214 82 Po
218 84

17. Beta Decay
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.

18. 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.

19. Beta Decay b -1 At
218 85 Po
218 84

20. Beta Decay X A Z Y
Z + 1 + b -1 Po
218 84 Rn 85 + b -1

21. Beta Decay Th
234 90 Y A Z + b -1 Th
234 90 Pa 91 + b -1

22. Beta Decay X A Z Pb
210 82 + b -1 Tl
210 81 Pb 82 + b -1

23. Beta Decay Bi
210 83 Y A Z + b -1 Bi
210 83 Po 84 + b -1

24. Beta Decay X A Z Bi
214 83 + b -1 Pb
214 82 Bi 83 + b -1

25. Gamma Decay
Gamma rays are not charged particles like a and b particles.
Gamma rays are electromagnetic radiation with high frequency.
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 J).