GCE A Level Physics

1. The Atom and its Nucleus
.

2. Atomic Structure
 John Dalton revived the idea first put forward
by Demokritos that said matter was made up
of tiny indivisible spheres called atoms
 1897 J. J. Thomson discovered that all matter
contains tiny negatively-charged particles.
 He showed that these particles are smaller
than an atom.
 He had found the first subatomic particle - the
electron.

3.  Scientists then set out to find the
structure of the atom.
 Thomson thought that the atom was
a positive sphere of matter and the
negative electrons were embedded
in it as shown here
 This `model' was called the
`plum-pudding' model of the atom.

5.  Ernst Rutherford decided to probe the
atom using fast moving alpha (α)
particles.
 He got his students Geiger and Marsden
to fire the positively-charged α-particles
at very thin gold foil and observe how
they were scattered.

6. The famous Geiger-Marsden Alpha
scattering experiment
In 1909, Geiger and Marsden were
studying how alpha particles are
scattered by a thin gold foil. (Video)
Alpha
source
Thin gold foil

7.  Most α-particles are hardly deflected because they are far away from
the nucleus and the field is too weak to repel them much.
 The electrons do not deflect the α-particles because the effect of
their negative charge is spread thinly throughout the atom.

9.  In 1911 Rutherford concluded that:
 All of an atom's positive charge and most
of its mass is concentrated in a tiny core.
 Rutherford called this the nucleus.
 The electrons surround the nucleus, but
they are at relatively large distances
from it.
 The atom is mainly empty space!

10. Rutherford did the calculations!
That’s 100 000 times smaller than the size
of an atom (about 10-10 metres).

11. The atom
orbiting electrons
Nucleus (protons
and neutrons)

12. The Nuclear Model of the atom

13. Nuclide notation
Li
3
7
Proton number (Z) = number of
protons
Nucleon number (A) =
number of protons +
neutrons
Neutron number (N) = A - Z

14. Isotopes
Li
3
7
It is possible for the nuclei of the same element
to have different numbers of neutrons in the
nucleus (but it must have the same number of
protons)
Li
3
6

15. Isotopes of Hydrogen
H
1
1
The three isotopes of Hydrogen even have their
own names!
H
1
2
H
1
3
Hi! I’m
hydrogen
They call
me
deuterium
Hola! Mi
nombre es
tritium y yo
soy de
Madrid!

16. Nuclide
 One type of nucleus with a particular nucleon number
and a particular proton number

17. Rodio-activity

19. Neutrinos
 Neutrinos are bizarre particles.
 They have very little mass (much less than
an electron) and no electric charge, which
makes them very difficult to detect.
 In β+ decay, a proton decays to become a
neutron and an electron neutrino (symbol
ν) is released:
 𝒃𝒆𝒕𝒂 − 𝒑𝒍𝒖𝒔 𝜷 + 𝒅𝒆𝒄𝒂𝒚:
 𝟏
𝟏
𝒑 → 𝟎
𝟏
𝒏 + +𝟏
𝟎
𝒆 + 𝝂

20. Antineutrinos
 β-particles are emitted with a range of
speeds – some travelled more slowly
than others,
 It was deduced that some other particle
must be carrying off some of the energy
and momentum released in the decay.

21. Antineutrinos
 This particle is now known as the
electron antineutrino, with symbol 𝒗
 𝒃𝒆𝒕𝒂 − 𝒎𝒊𝒏𝒖𝒔 𝜷 − 𝒅𝒆𝒄𝒂𝒚:
 In 𝜷 − 𝒅𝒆𝒄𝒂𝒚 a nuetron changes into a
proton
 𝟎
𝟏
𝒏 → 𝟏
𝟏
𝒑 + −𝟏
𝟎
𝒆 + 𝒗

23. Summary of properties of radioactive
emissions

25. Some values you should know
 Radius of proton and radius of
a proton 10-15m
 radius of a nucleus 10-15m- 10-14m
 radius of atom 10-10m

26. The electronvolt (eV)
 When an electron (with a charge of
magnitude 1.60 .10–19 C) travels through a
potential difference, energy is transferred.
 The energy change W is given by:
 W = QV = 1.60 . 10–19 . 1 = 1.60 . 10–19 J
 So we define the electronvolt as follows:
the energy transferred when an electron
travels through a potential difference of one
volt.
 Therefore: 1 eV = 1.60 . 10–19 J

27. NB: Density of the nucleus is much larger than density of the atom:

28.  (c) A uranium.238 nucleus has a radius
of 8.9 × 10–15 m.
 Calculate, for a uranium.238 nucleus,
 (i) its mass,
 mass = ................... kg [2]
 (ii) its mean density.
 density = ..............kg m–3 [2]
U
238
92

29. What is conserved in a
nuclear decay
 nucleon number A is conserved.
 proton number Z is conserved
 mass–energy is conserved.
E= mc2

30. Fundamental Particles

31. Fundamental Particles

32. What are Leptons and Quarks?
 All matter is comprised of Leptons and
Quarks.
 They are sub-atomic particles.
 They are fundamental particles incapable of
being subdivided into smaller particles.
a. There are 6 Leptons and 6 Quarks.
b. The nucleus is made up of quarks which
manifest themselves as protons and
neutrons.
c. Each elementary particle has a
corresponding antiparticle.

33. Matter vs. Anti-Matter
 For every particle, there is an anti-particle.
 Anti-particles have the same mass as the
particle.
 Anti-particles have the same but opposite
charge.
 Anti-particles have the opposite spin.
Particle Anti-particle
Name up quark Anti-up quark
Symbol u ū
mass 7.11x10-30 kg 7.11x10-30 kg
Charge +⅔ -⅔

34. Neutrinos
 Neutrinos are three of the six leptons
 They have no electrical or strong
charge
 Neutrinos are very stable and are all
around
 Most neutrinos never interact with any
matter on Earth

35. Quarks

36. Hadrons
• Consist of particles that
interact through the strong
force.
• Hadrons are composed of
other, smaller particles
• Hadrons are in two
categories
•Baryons & Mesons

37. Baryons
 Baryons are composed of three quarks
 All but two baryons are very unstable,
they are:
The proton and neutron!!
 Most baryons are excited states of
protons and neutrons

38. Protons
 Protons are made of three quarks, two up
quarks and a down quark
Protons uud

2
3
ⅇ +
2
3
ⅇ −
1
3
ⅇ = 1ⅇ

39. Neutrons
 Neutrons are also made up of three
quarks, one up quark and two
down quarks
Neutron udd

2
3
ⅇ −
1
3
ⅇ −
1
3
ⅇ = 0

40. The Four Fundamental Forces
These forces include interactions that are attractive or repulsive, decay
and annihilation.
Strong Weak interaction
Electromagnetic Gravity

41. The Strong Force
The strongest of the 4 forces
Is only effective at distances less
than 10-15
meters (about the size
of the nucleus)
Holds quarks together
This force is carried by gluons

42. The Weak interaction Force
A very short-ranged nuclear
interaction that is involved in
beta decay
This force is carried by the W+,
W-, and the Zo boson particles.

43. The Electromagnetic Force
Causes opposite charges to
attract and like charges to
repel
Carried by a particle
called a photon

44. Gravity
 Has a negligible effect on elementary
particles
 A long-range force
 Carried by the graviton
 This is by far the weakest of the 4
fundamental forces