Every element is made up of atoms, and every atom is built from three smaller particles: protons, neutrons, and electrons. These three subatomic particles, arranged in different combinations, account for all 118 known elements on the periodic table. The number of protons in an atom is what determines which element it is, while variations in neutrons and electrons change its mass and behavior without changing its identity.
The Three Subatomic Particles
An atom has two distinct regions. The nucleus sits at the center and contains protons (positively charged) and neutrons (no charge). Surrounding the nucleus is a cloud of electrons, which carry a negative charge. In a neutral atom, the number of electrons equals the number of protons, so the charges balance out.
Protons and neutrons account for nearly all of an atom’s mass. Electrons are roughly 1,800 times lighter than a proton, so they contribute almost nothing to the overall weight. Despite being so light, electrons play an outsized role in how elements interact with each other, because chemical reactions are essentially about electrons being shared, gained, or lost between atoms.
Why Protons Define the Element
The number of protons in an atom’s nucleus is called the atomic number, and it works like a fingerprint. Hydrogen always has 1 proton. Carbon always has 6. Gold always has 79. Change the proton count and you have a completely different element. This is what separates one element from another on the periodic table, where elements are ordered from 1 (hydrogen) to 118 (oganesson) by their atomic number.
You can add or remove neutrons from an atom and it remains the same element. You can strip away electrons and it’s still the same element. But if you change the number of protons, you’ve transformed it into something else entirely.
Neutrons and Isotopes
Neutrons sit alongside protons in the nucleus and add mass without affecting an element’s chemical identity. Atoms of the same element can have different numbers of neutrons, and these variations are called isotopes. Carbon, for example, most commonly has 6 neutrons (carbon-12), but some carbon atoms have 7 neutrons (carbon-13) or 8 neutrons (carbon-14). All three are carbon. All three react the same way chemically. They just have slightly different masses.
Scientists measure atomic mass in atomic mass units, a scale based on carbon-12. One atom of carbon-12 is defined as exactly 12 atomic mass units, and every other element’s mass is measured relative to that standard. One atom of helium-4, for instance, has a mass of about 4.003 atomic mass units, while sulfur-32 comes in at 31.972.
Electrons and Chemical Behavior
Electrons occupy energy levels (sometimes called shells) around the nucleus. The electrons in the outermost level, known as valence electrons, determine how an element reacts with other elements. Every chemical reaction involves electrons being removed, added, or shared between atoms. Whether an element is highly reactive like sodium or nearly inert like neon comes down to how many valence electrons it has and how tightly they’re held.
This is why elements in the same column of the periodic table tend to behave similarly. They share the same number of valence electrons, even though they differ in total proton and neutron count.
What Holds the Nucleus Together
Protons are all positively charged, and like charges repel each other. So what keeps the nucleus from flying apart? A fundamental force called the strong force, which is carried by particles known as gluons. At close range, the strong force is far more powerful than the electrical repulsion between protons, and it binds protons and neutrons tightly together into a stable nucleus. Without it, atoms heavier than hydrogen could not exist.
Going Deeper: Quarks and Gluons
Protons and neutrons are not the smallest particles in the story. Each one is made of even tinier components called quarks, held together by gluons. A proton contains two “up” quarks and one “down” quark. A neutron has two “down” quarks and one “up” quark. For most practical purposes, thinking in terms of protons, neutrons, and electrons is enough. But at the deepest level, quarks and gluons are the true building blocks of atomic nuclei.
Same Element, Different Forms
Even when you’re dealing with a single element, the way its atoms arrange themselves can produce dramatically different materials. Carbon is the classic example. When carbon atoms bond in a three-dimensional lattice where each atom connects to four neighbors, you get diamond, the hardest known natural material. When those same carbon atoms arrange in flat hexagonal sheets that slide over one another, you get graphite, the soft material in pencil lead.
These different structural forms of the same element are called allotropes, and they show up across the periodic table. Phosphorus exists as white phosphorus (highly reactive and toxic), red phosphorus (stable enough for matchstick tips), and black phosphorus (a layered semiconductor). Sulfur has multiple crystal forms depending on temperature. The atoms are identical in each case. Only their arrangement changes, and that’s enough to create entirely different physical properties.
Natural and Synthetic Elements
Of the 118 recognized elements, about 90 occur naturally on Earth. The rest have been created in laboratories by smashing atoms together in particle accelerators. Scientists fire beams of lighter atoms at targets made of heavy elements, and when nuclei collide and fuse, they can briefly form a new, heavier element. Researchers at facilities in Japan, the United States, and Russia are currently working to synthesize elements 119 and 120 using this approach.
These superheavy elements typically exist for only fractions of a second before their oversized nuclei break apart. But their creation confirms the same principle that governs every element: add one more proton to the nucleus, and you’ve made something the universe has never seen before.