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 roughly
atoms. But what exactly is an atom?
An atom is the smallest unit of an element.
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.
Fundamental particles are particles found within an atom that aren't made of any other particle. This means that they can't be split up any further.
As mentioned above, there are three fundamental types of subatomic particles:
- Protons.
- Neutrons.
- Electrons.
For this level of knowledge, we consider protons, neutrons and electrons as fundamental particles, meaning they aren't made of any other particle. But this isn't actually the case. Quantum scientists consider protons and neutrons to be made up of particles called quarks - a true fundamental particle. Electrons are a little different. When found orbiting in an atom or molecule, they can be split into three different particles called holons, spinons and orbitons, but if found outside of an atom or molecule, they can't be split up. This means that an isolated electron IS a fundamental particle, but an electron that is a part of an atom or molecule ISN'T.
Protons
Protons are positively charged particles. They are pretty small! In actual fact, one proton is only abou
kg, 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
Electrons are negative particles. They are even smaller than protons - electrons have an actual mass of
, or a relative mass of 1/1840 on the carbon-12 scale. They aren’t found alongside protons and neutrons. Instead, electrons spend their time in energy levels, also known as shells, orbiting the nucleus. Energy levels increase in energy as they get further from the nucleus, and electrons always try to be in the lowest energy level possible. We’ll look at these further later on in the article.
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.
An ion is an atom that has gained or lost an electron to form a charged particle.
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.
You should remember that although it may sound chaotic in an atom, with all the electrons rushing around the extremely dense, highly charged nucleus, the vast majority of the atom is actually empty space. Take a hydrogen atom, for example. It contains a single proton in its nucleus and is orbited by one electron. If the entire hydrogen atom was the size of the Earth, the proton itself would only be 200m across! Or to put it another way, if you blew up an atom to the size of a football stadium, its nucleus would only just be visible - it would be the size of a marble.
How do an atom’s fundamental particles influence its properties?
What makes hydrogen react so differently to helium? What causes sodium to fizz violently if placed in water whilst wetting carbon doesn’t appear to do anything? The three fundamental particles explored above determine an atom’s characteristics, the reactions it will undergo and which element it is a part of:
- The number of protons determines which element the atom is in and its position in the periodic table. For example, each carbon atom has exactly six protons. If it were to lose a proton, it would become boron.
- The combined number of protons and neutrons has little influence on the atom’s chemical properties, but increases its mass and so changes its physical properties.
- The number of electrons present changes an atom’s chemical properties dramatically by changing its electron configuration. Remember that atoms prefer to have full outer shells of electrons, and their reactivity depends on how close their outer shells are to being full. Argon is a noble gas with a full outer shell, and so is relatively unreactive, whilst metals such as sodium, lithium and potassium must be stored in oil to stop them reacting with the air!
Fundamental Particles - Key takeaways
- Atoms are made up of three fundamental particles called protons, neutrons and electrons.
- Protons and neutrons are found in the nucleus at the centre of the atom whereas electrons are found in shells orbiting the nucleus.
- Atoms are neutral and contain the same number of protons as electrons. An atom that has gained or lost an electron is called an ion.
- The electron configuration of an atom determines its chemical properties whilst its proton number determines its position in the periodic table.
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