Fundamental Particles

Atoms are everywhere. In fact, (almost) everything in the known universe is made up of either atoms or empty space. And they are tiny - a 2.5g US copper penny contains roughlyatoms. But what exactly is an atom?

The Ancient Greek philosopher Democritus was one of the first to believe in the existence of atoms over 2000 years ago. He believed that atoms were indestructible, had no internal structure, and that atoms of the same substance were all identical. Even the word atom itself comes from the Greek atomus, meaning indivisible. However, we know now he was only partially correct. Atoms contain smaller particles, called protons, neutrons and electrons. These particles are known as subatomic or fundamental particles, and the theory of subatomic particles and how they interact is known as the Standard Model.

As mentioned above, there are three fundamental types of subatomic particles:

• Protons.
• Neutrons.
• Electrons.

Protons

Protons are positively charged particles. They are pretty small! In actual fact, one proton is only aboukg, but we tend to measure their mass using the carbon-12 scale. On this scale, everything is compared to 1/12 of the mass of a carbon-12 atom. Here protons have a mass of approximately 1.

You’ll find protons packed together densely in the nucleus in the centre of the atom. They’re quite important, because once you know the number of protons in an atom, you know where in the periodic table you’ll find it and what element it is a part of.

Neutrons

Neutrons are neutral particles with a relative mass of about 1. They are also found alongside protons in the nucleus. Neutron numbers can vary between atoms of the same element without changing their chemical properties.

Electrons

The number of electrons in an atom determines its chemical properties and how it reacts.

Atoms have no overall charge, and this means they contain the same number of electrons as protons. If an atom gains or loses an electron to become a charged particle, it forms an ion.

The following table shows the relative masses and charges of the three fundamental particles:

A table comparing protons, neutrons and electrons.

Location of the fundamental particles

Back at the start of the twentieth century, the physicist JJ Thompson proposed that an atom contained small, negatively charged particles randomly scattered in a sea of positive charge. However, we now know that isn’t quite the case. An atom instead contains rings of electrons, known as shells, orbiting a dense mass called the nucleus.

The nucleus

Protons and neutrons are found densely packed together in the nucleus, the centre of the atom. You’ll remember that protons are positively charged whilst neutrons are neutral. This makes the nucleus positively charged too. However, particles with the same charge don’t really like hanging around together much - they tend to repel each other. An extremely powerful force called the strong nuclear force holds the protons and neutrons together in an impossibly small, dense mass.

Fig. 1: The strong nuclear force holds the protons and neutrons in the nucleus together.

Electron shells

Unlike protons and neutrons, electrons are found in shells orbiting the nucleus. These shells are also known as energy levels, and are further arranged into subshells and orbitals. We’ll look at shells in more detail in the article "Electron shells, subshells and orbitals". However, you should know a few basics:

• Electron shells are split into subshells that can each hold different numbers of orbitals and electrons.
• Shells increase in energy as they get further from the nucleus.
• Electrons are quite fussy and like being in the lowest energy state possible. When it comes to shells, they’ll always fill from the inside out if given the choice - so from the lowest energy level to the highest energy level.
• Similarly, atoms really like having full outer shells of electrons. This means that they’ll often readily gain or lose electrons in order to complete their outer shell.
• Shells don’t actually tell us where an electron is - in fact, it is impossible to know both the precise location of an electron and where it is headed. Instead, shells give us a rough approximation of where the electron will be the majority of the time.

Fig. 2: A representation of carbon’s atomic structure, showing electron shells surrounding the positively charged nucleus.

Unlike protons and neutrons, electrons are negative particles. They are attracted to the positive nucleus by a force called electrostatic attraction. However, this is much weaker than the strong nuclear force holding protons and neutrons together.