PROTONS, NEUTRONS & ELECTRONS

As we discovered in the last lesson, protons live in the atom's centre — its nucleus. Each proton carries a positive charge of +1 and has a mass of approximately 1 atomic mass unit (u) — making it one of the heaviest subatomic particles. It is this positive charge that gives every atom its identity: the number of protons in an atom determines what element it is. Change the number of protons and you change the element entirely.

But what exactly is a positive charge?

Let's do an experiment. Grab two magnets off your fridge and push them together — one of two things will happen. They will either snap together as if pulled by an invisible force, or resist your attempts as if an invisible hand is pushing back against you.

These pushing and pulling forces are a result of charge. Charge is one of the most fundamental properties in nature — it describes whether something attracts or repels other things around it. Opposite charges attract, like charges repel. Protons carry a positive charge, and as we'll soon see, that has enormous consequences for how atoms behave.

Alongside protons in the nucleus sit the neutrons — particles of almost identical mass (approximately 1.008 atomic mass units [u]) but carrying no charge whatsoever. A neutron is electrically neutral, which is exactly where it gets its name.

This raises an obvious question: if protons all carry a positive charge and like charges repel, why doesn't the nucleus simply fly apart? The answer lies in the strong nuclear force — one of the four fundamental forces of nature. At the incredibly short distances inside a nucleus(on the order of femtometres — about 10⁻¹⁵ metres), this force is far more powerful than the electromagnetic repulsion between like charged protons, locking the nucleus together.Neutrons contribute to this binding without adding any additional repulsive charge, effectively acting as a kind of nuclear glue.

Because neutrons carry no charge, changing the number of them in a nucleus does not change what element an atom is — that is determined solely by the proton count. Atoms of the same element that differ only in their number of neutrons are called isotopes. Carbon, for example, almost always has 6 neutrons (carbon-12), but a small fraction of carbon atoms carry 7 neutrons (carbon-13) or even 8 (carbon-14) — the radioactive isotope used in archaeological dating.

Electrons are the negatively charged counterpart to protons. They carry a charge of −1 yet have only a tiny fraction of a proton's mass — about 1/1,836th — making them by far the lightest of the three subatomic particles. In a neutral atom, the number of electrons exactly matches the number of protons — one negative charge cancelling each positive charge — so the overall, or net charge, of the atom is zero. Despite their lightness, electrons are anything but insignificant.

Electrons move at extraordinary speeds around the nucleus, occupying regions of space called orbital shells rather than fixed paths — think less "planet orbiting a sun" and more "cloud of probability surrounding a core." Because electrons are negatively charged and the nucleus carries a positive charge (thanks to the protons inside it), the two are attracted to one another by the same electromagnetic force we just explored with the magnets — holding the electrons in place around the atom.

This is where things get exciting. Electrons — specifically those in the outermost orbital shell — are what do chemistry. When atoms come close to one another, it is their outer electrons that interact, overlap, and rearrange to form chemical bonds. Every reaction, every molecule, every material you have ever touched is a consequence of electrons behaving this way. We will explore bonding in much greater detail in later lessons.

One more concept worth introducing here: when an atom gains or loses electrons, its net charge is no longer zero. An atom that has lost one or more electrons has more protons than electrons and becomes positively charged. An atom that has gained electrons has more electrons than protons and becomes negatively charged. These charged atoms are called ions. Ions are everywhere in chemistry — table salt, for example, is made of sodium ions (Na⁺, one electron lost) and chloride ions (Cl⁻, one electron gained) held together by the attraction between their opposite charges. We will return to ions in much greater detail in later lessons.