Atoms are all around us, and they're incredibly small. Even a copper penny in the US that weighs just 2.5g is made up of atoms! But what is an atom exactly? An atom is the smallest piece of an element that exists. This idea has been around for over 2000 years, with the Ancient Greek philosopher Democritus being one of the first to believe in atoms. He thought they were indestructible and had no internal structure. The word atom itself comes from the Greek word atomus, which means indivisible. But we now know that atoms have smaller parts inside them called subatomic or fundamental particles. These particles include protons, neutrons, and electrons.
Fundamental particles are particles that can't be broken down any further. Protons, neutrons, and electrons are considered fundamental particles at this level of knowledge. However, there's more to the story. Quantum scientists believe that protons and neutrons are made up of particles called quarks, which are true fundamental particles. Electrons are also a bit different. They can be split into three different particles called holons, spinons, and orbitons when found orbiting in an atom or molecule, but they can't be split up when found outside of an atom or molecule. This means that an isolated electron is a fundamental particle, but an electron that is part of an atom or molecule isn't.
Protons are tiny, positively charged particles that are found packed together in the nucleus at the center of an atom. In fact, one proton is only about 1.6 x 10^-27 kg, but we usually measure their mass using the carbon-12 scale. On this scale, protons have a mass of approximately 1. Protons are crucial in identifying elements because once you know the number of protons in an atom, you know which element it belongs to and where it is located on the periodic table.
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 negatively charged particles in an atom, are even smaller than protons. In fact, they have an actual mass of 9.11 x 10^-31 kg, or a relative mass of 1/1840 on the carbon-12 scale. Unlike protons and neutrons, electrons don't reside in the nucleus. Instead, they move around the nucleus in energy levels, also known as shells. These energy levels increase in energy as they get further from the nucleus, and electrons always try to be in the lowest energy level possible. The number of electrons in an atom is what determines its chemical properties and how it reacts.
Atoms have no overall charge because they contain the same number of electrons as protons. However, when an atom gains or loses an electron, it becomes a charged particle called an ion. Ions are atoms that have gained or lost an electron to form a charged particle.
The following table shows the relative masses and charges of the three 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.
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[^1]. This force is carried by particles called gluons that carry the strong charge between them, but they can't exist as free particles
Electrons are found in shells orbiting the nucleus and are attracted to the positive nucleus by electrostatic attraction, which is much weaker than the strong nuclear force holding protons and neutrons together. Shells are split into subshells that can each hold different numbers of orbitals and electrons, and increase in energy as they get further from the nucleus. Electrons tend to fill shells from the inside out if given the choice, and atoms tend to gain or lose electrons to complete their outer shell. Shells give us a rough approximation of where electrons will be the majority of the time, but it is impossible to know both their precise location and where they are headed.
Despite the seeming chaos of electrons rushing around the nucleus, atoms are mostly empty space. The number of protons determines an atom's element and position in the periodic table, while the combined number of protons and neutrons changes its physical properties but has little influence on its chemical properties. The number of electrons present changes an atom’s chemical properties dramatically by changing its electron configuration. The closer an atom's outer shell is to being full, the more reactive it is. For example, argon is a noble gas with a full outer shell and is relatively unreactive, while metals like sodium, lithium, and potassium must be stored in oil to prevent them from reacting with the air.
The key takeaways are:
What are fundamental particles?
Fundamental particles are particles found within an atom that aren't made up of any other particle.
What are the three fundamental particles?
The three fundamental particles are protons, neutrons and electrons.
What are fundamental particles made of?
By definition, fundamental particles aren't made up of any smaller particles.
Are electrons fundamental particles?
Yes, electrons are fundamental particles.
