All substances are made up of atoms, tiny particles far too small to see with the naked eye. Every solid, liquid, and gas you encounter is built from atoms of one or more of the 118 confirmed elements on the periodic table. But atoms themselves are not the smallest pieces of matter. They contain even tinier components, and those components are built from particles smaller still.
Atoms: The Building Blocks of Every Substance
An atom is the smallest unit of an element that still behaves like that element. A pure gold ring is made entirely of gold atoms. Water is made of molecules, each containing two hydrogen atoms bonded to one oxygen atom. Whether you’re looking at iron, oxygen, diamond, or salt, the substance is ultimately a collection of atoms arranged in a specific way.
Atoms are extraordinarily small. The diameter of a typical atom ranges from about 0.6 to 6 angstroms, where one angstrom is one ten-billionth of a meter. Lined up side by side, roughly 10 million atoms would span the width of a single millimeter. Despite their size, atoms are mostly empty space, with nearly all their mass concentrated in a dense core at the center.
What’s Inside an Atom
Every atom has two main regions: a compact, positively charged nucleus at the center, and a cloud of negatively charged electrons surrounding it. The nucleus contains two types of particles, protons and neutrons, packed tightly together by an incredibly strong force. Electrons orbit this nucleus at relatively enormous distances, which is why atoms are mostly empty space.
These three particles, protons, neutrons, and electrons, have distinct roles. Protons carry a positive electrical charge. Electrons carry a negative charge of exactly the same strength but opposite sign. Neutrons carry no charge at all. In a neutral atom, the number of protons equals the number of electrons, so the charges balance out. The number of protons in the nucleus is what determines which element an atom is: one proton makes hydrogen, six protons make carbon, 79 make gold.
Protons and neutrons are nearly identical in mass and are about 1,800 times heavier than an electron. This means the nucleus, despite being tiny compared to the full atom, contains more than 99.9% of the atom’s total mass.
Going Deeper: Quarks Inside Protons and Neutrons
For a long time, protons and neutrons were considered the smallest particles in the nucleus. That changed when physicists discovered that each one is made of even smaller particles called quarks. Protons and neutrons are each built from three quarks held together by particles called gluons, which act as a kind of glue.
Only two types of quark are needed to build ordinary matter. A proton contains two “up” quarks and one “down” quark. A neutron is the mirror image: two down quarks and one up quark. That single swap is the only structural difference between the two particles, yet it changes the electrical charge from positive (proton) to neutral (neutron).
Quarks cannot exist on their own under normal conditions. They are always found bound together inside larger particles. This is why you never encounter a lone quark in everyday life.
The Fundamental Particles of Nature
Physicists have identified a set of particles that appear to have no internal structure at all. These are considered truly fundamental, meaning they are not made of anything smaller. The framework that organizes them is called the Standard Model, and it accounts for 12 types of matter particles plus a set of force-carrying particles.
The 12 matter particles fall into two families. Quarks make up one family, with six varieties: up, down, charm, strange, top, and bottom. Only the up and down quarks appear in everyday matter. The other family is called leptons, which includes the electron (the particle orbiting every atomic nucleus) along with five heavier or harder-to-detect relatives.
In addition to these matter particles, there are force carriers. Gluons carry the strong force that holds quarks together inside protons and neutrons. Photons carry the electromagnetic force responsible for light and electrical interactions. Two other particles, called the W and Z bosons, carry the weak force involved in certain types of radioactive decay.
Where Mass Comes From
One surprising discovery is that fundamental particles don’t inherently possess mass on their own. Instead, they gain mass by interacting with an invisible energy field that fills all of space, called the Higgs field. The more strongly a particle interacts with this field, the heavier it is. Photons don’t interact with the Higgs field at all, which is why light is massless. Electrons and quarks do interact with it, giving them their respective masses.
The existence of this field was confirmed in 2012 when physicists at CERN detected the Higgs boson, which is essentially a ripple in the Higgs field. That discovery validated decades of theoretical work and helped explain why matter has mass in the first place.
How Atoms Combine Into Substances
Atoms rarely exist alone. They bond together to form molecules and larger structures, and the way they bond determines what substance you end up with. Bonding happens through the behavior of electrons in the outer regions of atoms.
In some cases, atoms share electrons with each other. This is called covalent bonding and is common between nonmetal elements. Water molecules form this way: oxygen and hydrogen atoms share electrons to stay connected. In other cases, one atom essentially hands over electrons to another, creating oppositely charged particles that attract each other. This is ionic bonding, and it’s what holds together substances like table salt. Metals use a third approach, where electrons flow freely among a sea of metal atoms, giving metals their ability to conduct electricity and heat.
These bonding interactions explain why the same basic set of atoms can produce such wildly different substances. Carbon atoms bonded in one arrangement produce diamond. The same carbon atoms bonded differently produce graphite, the soft material in pencil lead.
How Scientists Discovered Atoms Are Divisible
For roughly 2,000 years, atoms were assumed to be solid, indivisible spheres. That idea held until 1897, when J.J. Thomson discovered the electron, proving that atoms have internal structure. Thomson proposed a model where negative electrons were scattered throughout a ball of positive charge.
In 1911, Ernest Rutherford overturned that picture with a famous experiment. His team fired tiny, fast-moving particles at a thin sheet of gold foil. Most passed straight through, but a few bounced back at sharp angles. This could only happen if the atom’s positive charge and nearly all its mass were concentrated in a tiny central nucleus, with electrons orbiting far away. In 1932, James Chadwick completed the picture by discovering the neutron, a neutral particle sitting alongside protons in the nucleus. The discovery of quarks inside protons and neutrons came later in the 20th century, revealing one more layer of structure beneath what had seemed like the bottom.