Evidence for sub-atomic particles

Atoms are the fundamental building blocks of matter. However, even atoms are constructed of smaller, sub-atomic particles. It was originally thought that the atoms were small indivisible particles (atomos = indivisible), however a paradigm shift occured with the discovery that the atoms themselves have a sub-structure.

Evidence for this emerged though the experiments conducted in the nineteenth century on cathode rays, culminating in the experiments of Rutherford (the nuclear atom) and Chadwick (neutrons) in the early 20th century.

It is now known that even these sub atomic particles themselves have an even deeper sub-structure - the quarks!

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The sub-atomic family

The fundamental sub-atomic particles are protons, neutrons and electrons. The protons and neutrons are held together by strong nuclear binding forces in the nucleus of the atom. The electrons may be considered to be tiny particles that exist in regions of space known as orbitals around the atom.

This model of the atom is precisely that, a model. It is impossible to see atoms and, in order to be able to describe their properties, we use models representing this microscopic world that is invisible to us.

Similarly, the sub-atomic world is a strange place with unusual forces acting over infintesimally small distances. The rules of behaviour that govern the macroscopic world often break down in this strange environment, and it is important to understand that our representations and models are necessarily limited here.

The picture of the atom above is easy to discuss and is comfortably familiar. However, if you consider the actual dimensions of an atom compared to the nucleus you can see just how inaccurate even this simple picture is.

You should appreciate that the nuclear radius is much smaller than the atomic radius by a factor of about 100,000. This means that the atom is mostly empty space with a very solid and tiny nucleus. This was originally demonstrated by the scattering experiments carried out by Ernest Rutherford.

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Particle mass

The actual masses of these sub-atomic particles are very small and to the nearest whole number measured relative to the mass of a carbon-12 isotope being equal to 12 units:

Although these values suggest that the protons and neutrons are identical they do, in fact, have very slightly different masses, which is only of concern to us when considering changes in the structure in nuclear chemistry. Compared to the mass of the protons and neutrons the electrons have negligible mass and can be ignored when carrying out calculations involving mass.

Protons have a mass of 1 atomic mass unit. They are all together in the nucleus, but they cannot repel one another because of the strong nuclear force exerted by the protons and the neutrons. You could consider the neutrons to be the nuclear glue that holds the nucleus together.

This model allows us to use the atomic theory successfully to explain many observations in the microscopic world. Its use in modern particular science has been refined by the introduction of another model, which is rather more difficult to understand, called the quantum model.

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Electrical charges

The forces that hold atoms together (and to one another) are largely electrical in nature (apart from the strong nuclear charge that binds the particles in the nucleus together).

Electrically charged particles have an associated electrostatic field, rather like a magnet has a magnetic field. If another electrical charge comes into this field it will feel a force of either attraction or repulsion, depending on whether the charge carried is opposite or the same.

Like charges repel	Opposite charges attract

Overall, atoms are neutral, which means that they must have as many positive charges as negative charges.

Protons carry a single positive charge and the electrons carry a single negative charge, so in the neutral atom there are always the same number of protons and electrons.

The electrons are tiny in comparison to the protons and neutrons. The overall charge on an atom is zero, the charges of the electrons cancel out the positive charges of the protons in the nucleus.

Summary of fundamental particle charge and location

Example: A lithium atom has three protons in the nucleus along with four neutrons.

It has a total mass of 3 + 4 = 7 atomic mass units

It has 3 protons each with one positive charge so it must have three electrons each with one negative charge to cancel them out.

Therefore the atom has 3 protons, 4 neutrons and 3 electrons. Mass = 7.

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Mass number

The atomic mass number is represented by the symbol (letter) 'A'. This is not to be confused with the relative atomic mass A^r.

The mass number gives the integral number of nucleons, protons and neutrons found in the nucleus of an atom.

The relative mass is a value that is not necessarily integral that compares a mass to the mass of a carbon isotope, assigned a value of exactly 12.0000 units.

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Atomic number

This is represented by the symbol (letter) 'Z'. It shows us the number of protons in an atom (and the number of electrons in a neutral atom.)

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AZE

Any isotope of any element can be defined by using the A value, the Z value and the element symbol.

Using the values of A and Z it is possible to calculate the number of sub-atomic particles within any specific isotope of an element.

Example: Determine the number and type of sub-atomic particles in the following atom:

The atomic number is 5 therefore there are 5 protons and 5 electrons

The mass number is 11 therefore there are (11-5) = 6 neutrons

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Ions

The system can be extended to cover ions simply by adding the charge onto the element symbol. It is important to remember that a positive ion has LOST electrons.

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