Building Blocks of the Universe

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

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

The Scale of the Universe

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.

Image showing a screenshot from the interactive tool to explore the scale of the universe. Screenshot shows a 1mm scale, such as a grain of sand.

Activity 1.2

Standard Form and Orders of Magnitude

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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. 

A two dimensional representation of a sphere of positive charge with negative points evenly distributed within it.

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

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.

Positive protons and neutral neutrons in a cluster in the nucleus in the middle with electrons in a circular orbit around it.

Activity 1.3

Models of the Atom

Use the PHET interactive simulation of Rutherford Scattering to investigate the plum pudding and Rutherford models of the atom:

Activity 1.4

The Atomic Model and Scattering

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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:

The first three isotopes of hydrogen: hydrogen-1, where one electron is in a circular orbit around a nucleus of one proton; hydrogen-2, where one electron is in a circular orbit around a nucleus of one proton and one neutron; hydrogen-1, where one electron is in a circular orbit around a nucleus of one proton and two neutrons.

MikeRun, CC BY-SA 4.0, via Wikimedia Commons

To distinguish between isotopes, scientists use the following notation: 

The notation for an isotope X, where the element symbol X is in the middle, with a pre-subscript Z, (the atomic number) and a pre-superscript A, (the mass number).

For example, carbon-12 is written as: 

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:


(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 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 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 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:

Activity 1.5

Isotopes and Decay

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