Document 6608048

Topic 7: Atomic and Nuclear Physics
7.1 – The atom
7.1.1 Describe a model of the atom that features a small nucleus surrounded by electrons
Students should be able to describe a simple model involving electrons k ept in orbit around the
nucleus as a result of the electrostatic attraction between the electrons and nucleus.
7.1.2 Outline the evidence that supports a nuclear model of the atom
A qualitative description of the Geiger-Marsden experiment and an interpretation of the results are
all that is required
7.1.3 Outline one limitation of the simple model of the nuclear atom
Rutherford’s Model of the Atom
After completing the Gold Foil experiment (a.k.a. – Geiger-Marsden Experiment) Rutherford was
able to conclude the existence of the nucleus and a model of the atom that consisted of a small
dense nucleus surrounded by electrons (nuclear model)
The nuclear model of the atom
Nuclear Model:
● Nucleus in the center of the atom made up of protons and neutrons (i.e. – positively
charged)
● Electrons orbited the nucleus like planets orbited the sun
● Held together by the electrostatic attraction of the electrons (negative charge) to the
protons (positive charge)
● Atom is mostly empty space (discovered from Geiger – Marsden experiment)
o Atom is about 10-10 m and the nucleus is on the order of 10-15 m
Limitations of the nuclear model:
1. Why were the electrons not pulled into the nucleus due to electrostatic
attraction?
2. What kept the nucleus bound together rather than exploding apart due to
the electrostatic repulsion?
The Geiger-Marsden Experiment:
Note: This is also referred to as the gold foil experiment.
Experimental Setup:
● A radioactive substance was used to emit alpha particles towards a very thin piece of gold
●
●
●
foil.
An alpha particle is a helium nucleus (i.e. – mass of 4 and a charge of +2) α-particle = 42He
Alpha particles were emitted toward the sheet of gold foil and were expected to travel
through relatively undeflected, instead some alpha particles were deflected at extreme
angles
o “It was quite the most incredible event that has ever happened to me in my life. It
was almost as incredible as if you fired a 15 inch shell at a piece of tissue paper
and it came back and hit you”
- E. Rutherford
They concluded that
o The deflections must have been caused by a dense positive core at the center of
the atom (they called this the nucleus)
o The nucleus must be very small compared to the size of the atom based on the
number of alpha particles that were deflected at sharp angles
o The nucleus contained protons
o They hypothesized the existence of neutrons
7.1.4 Outline Evidence for the existence of the atomic energy levels
Students should be familiar with emission and absorption spectra, but the details of atomic models
are not required.
Students should understand that light is not a continuous wave but is emitted as ‘pack ets’ or
‘photons’ of energy, each of energy hf.
The Bohr Model of the atom:
● Neils Bohr worked with Rutherford to improve the nuclear model and answer the questions
●
raised by it.
Bohr suggested:
o Electrons moved in discrete orbitals or shells around the nucleus like planets
around the Sun.
▪ The electrostatic attraction between the nucleus and the electrons caused
a centripetal force and the electrons did not get pulled in because they
were constantly moving in the shells
▪ Only a certain amount of electrons can fit in each shell
▪ The further the shell is from the nucleus the more energy is has
▪ The electrons can jump between shells if they are provided with energy
Bohr’s Model of the atom (Planetary Model)
The Photon:
It turns out that light can behave as both a wave and a particle (wave-particle duality – to be
discussed later). Light or any type of electromagnetic radiation is carried by particles known as
PHOTONS.
These photons carry the energy contained in light in particle form or as ‘packets’ of energy.
Ephoton = hf
h = Planck’s constant = 6.626 X 10-34 J/s; f = frequency of the light
Looking at this formula which type of radiation has the most energy?
Which type of visible light has the most energy? The least energy?
A beam of radiation is made up of a large number of photons each with the same frequency. The
brighter the light the more photons there are in the beam.
Emission and Absorption Spectrum:
For years scientists have known that if you pass white light through a prism you obtain the entire
spectrum of colours as shown below
They completed the same experiment using hydrogen gas by putting it in an electric discharge
tube. The gas was excited with electricity and it began to glow (i.e. – it emitted light). This light
was passed through a prism and only certain colours were found!!
Bohr’s model of the atom was able to explain and predict which lines would be formed by using the
concept that the electrons in the atom are ‘jumping’ or transitioning between different shells in the
atom. Unfortunately it only worked for hydrogen!
● Hydrogen gas is charged (i.e. – energy is inputted)
● The electrons absorb this energy and use it to jump into a higher orbital → EXCITED
STATE
● This excited state is unstable (the electrons want the lowest energy possible) so they jump
●
down to a lower orbital and when doing so release energy in the form of radiation (i.e – they
release a photon)
The energy of the photon emitted corresponds to the distance between the two states.
Ionization – occurs when one or more electrons are completely removed from the atom leaving a
charged particle or ion. This can happen when an atom absorbs a high energy photon
Absorption of light – a photon of light can ONLY be absorbed by an atom if it has exactly the right
amount of energy to excite an electron from one energy level to another.
Problems:
The diagram below shows the energy of the different orbitals in an atom.
[Note: The unit of eV = electron volt; 1 eV = 1.6 X 10-19 J]
1. How many possible energy transitions are there in this atom?
2. Calculate the maximum energy (in eV) that could be released and the frequency of the photon
3.
4.
5.
6.
7.
emitted
Calculate the minimum energy that could be released and the frequency of the associated
photon
How much energy would be required to completely remove an electron that was in the lowest
energy level? Calculate the frequency of the photon that would have enough energy to do
this.
Determine the frequency of the photon that would be required to excite an electron from the
2nd shell to the 4th shell. Where does this light fit in the electromagnetic spectrum?
When you look at an absorption spectrum there are black lines, what do this black lines
represent? What is occurring in the atom?
Using the Geiger-Marsden experiment how were they able to deduce that the size of the
nucleus is very small compared to the size of the atom?