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This week’s content explores the scale of the universe - from galaxies down to the smallest particles. We zoom in on the atom and find out about the experiments that have revealed both atomic and nuclear structure.
Course requirements for Module 1: Building Blocks of the Universe
Course requirements for Module 1: Building Blocks of the Universe
1.1: The Scale of the Universe (online simulation)
1.2: Standard Form and Orders of Magnitude (Questions)
1.3: Models of the Atom (online simulation)
1.4: The Atomic Model and Scattering (Questions)
1.5: Isotopes and Decay (Questions)
The Scale of the Universe
The Scale of the Universe
Before we delve into nuclear physics, we need to look at standard form. This is a useful way of representing very large and very small numbers. This is crucial for looking at the scale of the Universe, and particularly important in nuclear physics because the size of the nucleus is so tiny, yet the energies released are so big.
Standard form is written in the form of:
a x 10n
(where a is any number between 1 and 10 and n is any whole number)
For example, 55,000 is written as 5.5 x 104 in standard form.
In standard form, n gives us the power of 10. These powers of 10 can also be called the order of magnitude. Some powers of 10 have their own prefixes, some of which you may recognise. Have a look at the table below to find out more:
Activity 1.1
Activity 1.1
Use this interactive tool to explore the scale of the Universe - from the very smallest particles found inside the atom, all the way up to galaxies and the size of our observable universe.
You may also want to try out this "Powers of 10" activity. Notice how each picture is an image of something that is 10 times bigger or smaller than the one preceding or following it.
Activity 1.2
Activity 1.2
Log in to Isaac Physics and answer the questions:
Unit Prefixes
Standard Form
Orders of Magnitude
Using Standard Form
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The Structure of the Atom
The Structure of the Atom
The idea of the atom originated in Ancient Greece, where philosophers named the smallest building blocks of matter atoms. They believed that these were indivisible, solid particles and that differences in atomic shape and size gave rise to the different properties of matter.
Modern atomic theory then began in the early 1800s with chemist and physicist John Dalton. His idea of atoms was very similar to that of the ancient Greeks - tiny, solid balls that could not be broken down into anything simpler. However, as experiments advanced, our understanding of the atom also developed.
Following the discovery of the negative electron, and knowing that atoms were neutral overall, JJ Thompson proposed the Plum Pudding atomic model. This saw the atom as a positively charged 'pudding', with negative electrons embedded as 'plums' throughout. This model of the atom was disproved by an experiment completed by physicists Rutherford, Geiger, and Marsden.
In the video below, Kayleigh Gates and Abby-Rhian Powell from the University of Glasgow explain more about the famous Rutherford experiment, and how the same technique of scattering is still used in cutting-edge experiments today.
The plum pudding model of the atom, which was later disproved by Rutherford's experiment.
Kurzon, CC BY-SA 4.0, via Wikimedia Commons
Rutherford Model
Rutherford Model
We now understand atoms to be made of protons (positively charged particles), neutrons (neutral particles) and electrons (negatively charged particles). The protons and neutrons are at the centre of the atom in the nucleus, and the electrons surround the nucleus.
Most of the mass of an atom is concentrated in its nucleus: protons and neutrons make up most of the atomic mass. The mass of an electron is so tiny that it is often considered to be insignificant.
Activity 1.3
Activity 1.3
Use the PHET interactive simulation of Rutherford Scattering to investigate the plum pudding and Rutherford models of the atom:
Start by looking at the plum pudding model of the atom. What do you notice about the path of the alpha particles through the atom? Is this what was observed in Rutherford's experiment?
Now look at Rutherford's model of the atom. What do you notice about the paths of the alpha particles in this case? You can change between a single nucleus and multiple atoms using the icons in the top right corner of the screen.
What do you notice about the alpha particle's behaviour if you vary the number of protons in the nucleus?
What do you notice about the alpha particle's behaviour if you vary the number of neutrons in the nucleus?
Activity 1.4
Activity 1.4
Log in to Isaac Physics and answer the question:
Scattering and the Structure of the Atom
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Isotopes
Isotopes
Elements are defined by the number of protons in this nucleus. For example, anything with one proton is hydrogen, anything with two protons is helium, anything with three protons is lithium. All of these elements are listed in the periodic table.
If two atoms have the same number of protons but different numbers of neutrons, we say that they are different isotopes. For example, Hydrogen has three isotopes:
Hydrogen (hydrogen-1) has 1 proton and 0 neutrons in the nucleus (mass number 1)
Deuterium (hydrogen-2) has 1 proton and 1 neutron in the nucleus (mass number 2)
Tritium (hydrogen-3) has 1 proton and 2 neutrons in the nucleus (mass number 3)
To distinguish between isotopes, scientists use the following notation:
X is the symbol for the chemical element
A is the mass number - the total number of particles/nucleons (protons and neutrons) in the nucleus
Z is the number of protons, also referred to as the atomic number or the proton number
For example, carbon-12 is written as:
Nuclear Stability and Radioactive Decay
Nuclear Stability and Radioactive Decay
Some isotopes are stable and last for millions of years. These are the isotopes that we see in the materials around us on a daily basis. However, as we saw in the video above, most isotopes are unstable and may last for less than a second.
Isotopes that are unstable will always become more stable over time by changing the properties of their nucleus. They do this by giving out radiation, and we say that these isotopes are radioactive. When a nucleus gives out radiation, we say that it has decayed.
In this masterclass, we are focusing on three types of radioactive decay:
Alpha
Beta
Gamma
(Note that there are, in fact, two types of beta decay: beta-plus and beta-minus. In this masterclass, we will only talk about beta-minus decay, which is referred to just as beta decay.)
Other elements have lots more isotopes. The full chart of all the isotopes is called the Nuclide Chart. This is introduced by Dr Christian Diget from the University of York in the video below:
Alpha Decay
Alpha Decay
Alpha particles are helium nuclei, composed of two protons and two neutrons (24⍺).
When an alpha particle is given out by a nucleus, the mass number of the nucleus reduces by 4 and the proton number, and so the charge, reduces by 2. This means that the isotope produced is a different element to the original isotope.
In general:
For example:
Note that an alpha particle can also be written as:
Beta Decay
Beta Decay
Beta particles are high energy electrons (-10ꞵ). They are created when a neutron in the nucleus turns into a proton. When this occurs, the beta particle (electron) comes flying out of the nucleus at high speed.
For beta decay, the mass number doesn't change, because the total number of nucleons remains the same. However, the atomic number, and charge, increases by 1 (one less neutron but one more proton). This means that the isotope produced is a different element to the original isotope.
In general:
For example:
Note that a beta particle can also be written as:
Gamma Decay
Gamma Decay
Gamma radiation is a type of high energy electromagnetic wave (00𝛾). It wasn't mentioned in the video above because a gamma decay does not change the chemical element. Instead, it allows the nucleus to lose excess energy.
The emission of a gamma ray does not cause the mass or the charge of the nucleus to change.
In general:
For example:
If you want some more examples about how different isotopes decay, you can use the PHET Interactive Simulation on atoms and decays.
Activity 1.5
Activity 1.5
Log in to Isaac Physics and answer the questions:
Atomic Numbers and Nomenclature 1
Atomic Numbers and Nomenclature 2
Isotopes
Nuclear Decay Equations
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Find out more (optional)
Find out more (optional)
Investigate the Colourful Nuclide Chart
Investigate the Colourful Nuclide Chart
The Colourful Nuclide Chart, created by Dr Ed Simpson from Australian National University, can tell you a lot about the properties of the over 3000 different isotopes that exist. To get the colours to match our LEGO(R) chart, click on the menu in the top left corner, select 'Colours' and then tick the box 'Force Binding Blocks decay mode colours'.
Try investigating the chart further. For example, in the menu, select 'Data' to choose whether to display the ‘Primary Decay Mode’, 'Mass Excess', or ‘Binding Energy per Nucleon’ (among other options). Under 'Display', you can also choose to view the chart in 3D.
Explore the Nuclide Chart using Minecraft
Explore the Nuclide Chart using Minecraft
We've built our Binding Blocks Nuclide Chart in Minecraft and made it available for download. To download the world, you will need either a Java edition or Bedrock edition of Minecraft. In the world you can explore the chart from Hydrogen, through the Valley of Stability, all the way to the super heavy fissile elements. Along the way, we've placed books for you to read about some of the more interesting isotopes and in our virtual Central Hall, you can also learn about the different types of decay. Click here to download the world.
The Valley of Stability
The Valley of Stability
If you're interested in a more in depth look at the nuclide chart, and nuclear physics in general, you may want to watch "The valley of stability" - a video produced by the CEA (French Alternative Energies and Atomic Energy Commission).
Case Study: Isotopes of Carbon
Case Study: Isotopes of Carbon
At the University of York, Dr. Marina Petri, a Royal Society University Research Fellow in Nuclear Physics, studies the structure of atomic nuclei using particle accelerators to probe nuclear properties with accelerated ion beams. This includes how the protons and neutrons arrange themselves within the nucleus and how they interact with each other to form complex nuclei and more energetic states of these nuclei, known as excited states. This research has shown incredible results about excited states of neutron rich carbon isotopes, such as the first excited state of carbon-20, whose lifetime was measured as only 0.000000000010 seconds (10 ps).
By contrast, the unexcited ground state of carbon-14 is extremely long-lived, with a half life of several thousands of years before decaying to nitrogen-14. The long lifetime of carbon-14 allows us to use radiocarbon dating to determine the age of historic artefacts and paintings. Based on the observed lifetimes of excited states in carbon-14, carbon-16, carbon-18, and carbon-20, Marina and her collaborators have furthermore shown how the six protons in the nucleus of carbon isotopes change their behaviour as more neutrons are added.
References: M. Petri et al. Lifetime Measurement of the 2+1 State in 20C, Phys. Rev. Lett. 107, 10250 (2011). Phenomenological analysis of B(E2) transition strengths in neutron-rich carbon isotopes, Phys. Rev. C. 90, 067305 (2014)
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