1. Nuclear

2. 13.1 Structure and Property of the Nucleus

3. 13.1 Structure and Property of the Nucleus
Nucleus is composed of two
particles
Proton – positive charge
Neutron – neutral
Together they are called nucleons
13.1 Structure and Property of the Nucleus

4. 13.1 Structure and Property of the Nucleus
To present this information we use the symbol form
Z – number of protons (atomic number)
A – atomic mass (not average)
The number of Neutrons (N) is
Sometime written without the Z, as that information is redundant
13.1 Structure and Property of the Nucleus

5. 13.1 Structure and Property of the Nucleus
Isotopes – the same element, but different numbers of neutrons or mass number
These isotopes would be
Not all isotopes are equally
common
C-12 is 98.9%
C-13 is 1.1%
Called the Natural Abundance
13.1 Structure and Property of the Nucleus

6. 13.1 Structure and Property of the Nucleus
Masses of atoms are determined using a mass spectrometer
The mass is given
in unified atomic
mass units (u)
Carbon – 12 is
given the mass
of u
13.1 Structure and Property of the Nucleus

7. 13.1 Structure and Property of the Nucleus
Masses are often given in electron volts
This is derived from Einstein’s equation
Using the mass of a proton
And placing into Einstein’s equation
13.1 Structure and Property of the Nucleus

8. 13.2 Binding Energy and Nuclear Forces

9. 13.2 Binding Energy and Nuclear Forces
The total mass of a stable nucleus is always less than the sum of the masses of its separate protons and neutrons
The difference is mass is
the binding energy
So for example the mass of
Helium 4 is u
13.2 Binding Energy and Nuclear Forces

10. 13.2 Binding Energy and Nuclear Forces
This is the energy needed to break apart the nucleus
To be a stable nucleus, the mass must be less than the parts
The binding energy per
nucleon is the total
binding energy divided
by A
13.2 Binding Energy and Nuclear Forces

11. 13.2 Binding Energy and Nuclear Forces
Strong Nuclear Force – attractive force between all nucleons
Drops to essentially zero if the distance between the nucleons is greater than 10-15m
Occur by the exchange of a particle called a meson
Weak Nuclear Force – very weak, show in types of radioactive decay
13.2 Binding Energy and Nuclear Forces

12. 13.2 Binding Energy and Nuclear Forces

13. 13.3 Radioactivity

14. Henri Becquerel (1896) uranium darkens photographic plates
Radioactive decay – unstable nuclei
fall apart with
the emission of
radiation
13.3 Radioactivity

15. Rays can be classified into three catogories
Alpha (a) – barely penetrates paper
Beta (b) – penetrates up to 3mm of aluminum
Gamma (g) – penetrates several cm of lead
13.3 Radioactivity

16. An alpha particle is a helium nucleus
When an atom undergoes alpha
decay it loses 2 protons and 2
Neutrons
Reactions are written
13.3 Radioactivity

17. Parent nucleus – the original
Daughter nucleus – nucleus of new atom
Transmutation – change of one element into another
Basic form for alpha decay is
The alpha particle is ejected because it has a very large binding energy and is difficult to break apart
13.3 Radioactivity

18. Beta particle (b-) – electron Also produces an antineutrino
Antineutrino – has no
charge and almost
no mass
The result of the decay is that a neutron becomes a proton
30.5
13.3 Radioactivity

19. For Carbon – 14 decay
Or the general form which would be
The electron does not come form the electron cloud, but from the decay of a neutron into a proton
It is identical to any other electron
13.3 Radioactivity

20. Unstable isotopes with too few neutrons compared to their number of protons decay by emitting a positron
Positron – same mass as an
electron, positive charge
This is an example of an antiparticle (antimatter)
The decay pattern is
13.3 Radioactivity

21. Gamma Ray – photon of
EMR
A nucleus can be in an
excited state like an
electron
When it drops down it emits a g ray
Much larger than for electrons
For a given decay the g ray has the same energy
13.3 Radioactivity

22. The nucleus may enter an excited state by
Violent collision with another particle
The particle after a decay is often in an excited state
The equation can be
written
13.3 Radioactivity

23. 13.4 The Decay Process

24. Individual radioactive nuclei in a random process
Based on probability we can approximate the number of nuclei in a sample that will decay
Where l is the decay constant
13.4 The Decay Process

25. The greater the decay constant, the greater the rate of decay
The more radioactive it is
The equation can be solved for N using calculus and we get
Where N0 is the initial number of nuclei present
N is the number remaining after time t
The number of decays per unit time is called the activity or rate of decay
13.4 The Decay Process

26. Half-Life – the time it takes for half the original amount of parent isotope to decay (T½)
13.4 The Decay Process

27. Carbon-14 has a half-life of 5730 yr
Carbon-14 has a half-life of 5730 yr. What is the activity of a sample that contains 1022 nuclei?
1 decay/s is called a becquerel (Bq)
13.4 The Decay Process

28. Carbon-14 has a half-life of 5730 yr
Carbon-14 has a half-life of 5730 yr. What is the activity of a sample that contains 1022 nuclei?
1 decay/s is called a becquerel (Bq)
13.4 The Decay Process

29. 1.49mg of Nitrogen-13 has a half life of 600s.
How many nuclei are present?
What is the initial activity?
13.4 The Decay Process

30. 1.49mg of Nitrogen-13 has a half life of 600s.
What is the activity after 3600s?
6 half lives
If this had not been a perfect half life we would have used
13.4 The Decay Process

31. Decay Series – a successive set of decay
13.4 The Decay Process