Atoms are the basic building blocks of all matter, and several fundamental truths about them surprise people when they first learn them. Every solid object, liquid, and gas you encounter is made of atoms, yet atoms themselves are almost entirely empty space. There are 118 confirmed elements, each defined by a specific type of atom, and the differences between them come down to just a handful of tiny particles.
Three Particles Make Up Every Atom
Every atom contains three types of subatomic particles: protons, neutrons, and electrons. Protons carry a positive electrical charge, electrons carry a negative charge, and neutrons have no charge at all. Protons and neutrons cluster together in a dense central core called the nucleus, while electrons occupy the space surrounding it.
In a neutral atom, the number of protons equals the number of electrons, so the positive and negative charges cancel out. This balance is what keeps most of the matter around you electrically neutral. When an atom gains or loses electrons, it becomes electrically charged and is called an ion. Losing an electron creates a positively charged ion (a cation), while gaining an electron creates a negatively charged ion (an anion). This is how atoms participate in many chemical reactions and form compounds like table salt.
Protons Define Which Element an Atom Is
The number of protons in an atom’s nucleus, known as the atomic number, is what determines its identity as a chemical element. Any atom with 8 protons is oxygen. Any atom with 6 protons is carbon. Change the proton count, and you have an entirely different element with different chemical properties. This is a hard rule with no exceptions: proton number equals elemental identity.
Neutrons, on the other hand, can vary without changing the element. Atoms of the same element that have different numbers of neutrons are called isotopes. Carbon is a good example. Almost 99% of carbon on Earth is carbon-12, which has six protons and six neutrons. About 1% is carbon-13, with seven neutrons. A much rarer form, carbon-14, has eight neutrons. All three behave almost identically in chemical reactions, but they differ in mass, and that mass difference is useful for techniques like radiocarbon dating.
Atoms Are Almost Entirely Empty Space
One of the most striking facts about atoms is how little of their volume is actually “filled.” The nucleus is roughly 100,000 times smaller in radius than the atom as a whole. When you compare volumes, the nucleus occupies approximately one quadrillionth (10⁻¹⁵) of the atom’s total space. If you scaled an atom up to the size of a football stadium, the nucleus would be about the size of a marble sitting at the center.
Despite being vanishingly small, the nucleus contains almost all of an atom’s mass. Protons and neutrons are each about 1,800 times heavier than an electron, so the wispy cloud of electrons surrounding the nucleus contributes very little to the atom’s weight. This is why matter feels solid even though its building blocks are mostly empty: the electrical forces between electron clouds of neighboring atoms repel each other strongly enough that objects resist being pushed through one another.
How Small Atoms Actually Are
Individual atoms are far too small to see with the naked eye or even a standard microscope. A hydrogen atom, the smallest element, has a radius of about 120 picometers (120 trillionths of a meter). Carbon atoms are slightly larger at around 170 picometers, while sodium atoms reach about 227 picometers. To put this in perspective, a single sheet of paper is roughly a million atoms thick.
Electrons Don’t Orbit Like Planets
The classic image of electrons circling the nucleus like planets around the sun, known as the Bohr model, is a useful simplification but not an accurate picture. Electrons don’t follow neat circular paths. Instead, they exist in regions of space called orbitals, which describe where an electron is most likely to be found at any given moment. You can think of an orbital as a probability cloud: a zone where the electron spends roughly 90% of its time.
These orbitals come in different shapes. Some are spherical, others are dumbbell-shaped, and more complex ones have lobed or ring-like geometries. Each orbital holds a maximum of two electrons. The shape and arrangement of these orbitals determine how atoms bond with each other, which is ultimately why different elements have different chemical behaviors. The Bohr model still works well enough for understanding basic chemistry, but the orbital model is what physicists and chemists actually use to predict how atoms interact.
The Nucleus Is Held Together by the Strong Force
Here’s a puzzle: protons are all positively charged, and like charges repel each other. So why doesn’t the nucleus fly apart? The answer is a fundamental force of nature called the strong nuclear force. It acts only over incredibly tiny distances (less than about 2 femtometers, or 2 quadrillionths of a meter), but within that range it completely overwhelms the electrical repulsion between protons. Each proton or neutron in the nucleus is essentially glued to its nearest neighbors by this force.
The strong force also operates at an even deeper level. Protons and neutrons are not truly fundamental particles. Each one is made of smaller particles called quarks, held together by carriers of the strong force called gluons. A proton contains two “up” quarks and one “down” quark, while a neutron has one up quark and two down quarks. This internal structure accounts for more than 99% of the visible mass in the universe.
Atoms Can Gain, Lose, or Share Electrons
While the nucleus stays remarkably stable under everyday conditions, the electrons in an atom’s outer orbitals are much more dynamic. Atoms routinely gain, lose, or share electrons when they interact with other atoms, and this is the basis of all chemical bonding. When atoms transfer electrons, they form ionic bonds (like those in table salt). When they share electrons, they form covalent bonds (like those holding water molecules together).
The number of electrons in an atom’s outermost orbital layer determines how reactive it is. Elements like helium and neon have full outer orbitals, making them extremely stable and almost completely unreactive. Elements like sodium and chlorine have outer orbitals that are nearly full or nearly empty, making them highly reactive. This pattern repeats in a predictable way across the periodic table, which is exactly why the table is organized the way it is.