An element is a pure substance made of only one kind of atom, defined by the number of protons in its nucleus. Hydrogen has 1 proton, helium has 2, carbon has 6, gold has 79. That proton count, called the atomic number, is what makes each element unique. No chemical or physical process can break an element down into something simpler.
There are 118 recognized elements in total. Of those, 92 occur naturally on Earth, two more are byproducts of nuclear reactors, and over 20 have been created artificially by smashing atoms together in particle accelerators.
What Makes One Element Different From Another
Every atom has a nucleus containing protons and neutrons, surrounded by electrons. The number of protons is the single thing that determines which element you’re looking at. A nucleus with 17 protons is always chlorine. A nucleus with 38 protons is always strontium. Change the proton count by even one, and you have a completely different element with different properties.
Neutrons can vary without changing the element’s identity. Strontium, for instance, most commonly has 50 neutrons, but strontium atoms found in nature can have anywhere from 44 to 52 neutrons. These variations are called isotopes. They’re still the same element, still behave the same way chemically, but they differ slightly in mass. This is why the atomic mass listed on the periodic table is usually a decimal rather than a whole number: it’s an average across all the naturally occurring isotopes.
Elements vs. Compounds vs. Mixtures
An element contains only one kind of atom. It cannot be simplified further by any chemical reaction. Some elements exist as lone atoms (like argon), while others naturally pair up into molecules (like oxygen, which forms O₂). Either way, only one type of atom is involved.
A compound is two or more different elements bonded together chemically, always in the same ratio. Water is always two hydrogen atoms to one oxygen atom. You can break a compound back into its component elements through chemical reactions, but not by physically pulling it apart. Importantly, a compound’s properties differ from the elements that make it up. Sodium is a reactive metal, chlorine is a toxic gas, but combined they form table salt.
A mixture is two or more elements or compounds sitting together without chemically bonding. Trail mix, saltwater, air. You can separate a mixture through physical means like filtering, evaporating, or sorting. The individual components typically keep their original properties.
How the Periodic Table Is Organized
The periodic table arranges all 118 elements by atomic number, left to right and top to bottom. But the layout does more than just count protons. Its rows and columns reveal patterns in how elements behave.
The seven horizontal rows are called periods. Each period represents a new layer of electrons around the nucleus. Elements in the first period have one electron shell, elements in the second have two, and so on. As you move across a period from left to right, the elements shift from highly reactive metals on the left to nonmetals and eventually to nearly inert noble gases on the far right.
The vertical columns are called groups, and these are where the most useful patterns appear. Elements in the same group share the same number of electrons in their outermost shell, which means they behave similarly in chemical reactions. Group 1 elements (lithium, sodium, potassium) are all soft, highly reactive metals. Group 17 elements (fluorine, chlorine, bromine) are all reactive nonmetals. Group 18 elements (helium, neon, argon) barely react with anything at all.
Metals, Nonmetals, and Metalloids
The periodic table divides roughly into three categories based on physical and chemical properties.
- Metals make up the majority of elements. They’re shiny, conduct heat and electricity well, and can be hammered into sheets or drawn into wires. All metals are solid at room temperature except mercury. In chemical reactions, metals tend to lose electrons. Common examples include iron, copper, aluminum, and gold.
- Nonmetals sit on the upper right side of the table. They’re generally poor conductors, often brittle when solid, and can exist as gases, liquids, or solids at room temperature. In reactions with metals, nonmetals tend to gain electrons. Oxygen, nitrogen, sulfur, and carbon are all nonmetals.
- Metalloids straddle the boundary between metals and nonmetals and share characteristics of both. Silicon, the most well-known metalloid, looks shiny like a metal but is brittle and conducts electricity only under certain conditions, making it a semiconductor. This property is what makes silicon the foundation of computer chips and modern electronics.
Elements That Make Up Almost Everything
Despite 118 elements existing, a handful dominate the physical world. Hydrogen and helium account for the vast majority of the visible universe, forged in the moments after the Big Bang and in the cores of stars. Heavier elements like carbon, oxygen, and iron were produced later through stellar fusion and supernova explosions.
On Earth, the crust is dominated by oxygen (bound up in rocks and minerals), silicon, and aluminum. The oceans are primarily hydrogen and oxygen in the form of water, with dissolved sodium and chlorine making up most of the salt content.
The human body runs on a surprisingly small set of elements. By mass, about 96 percent of your body is just four elements: oxygen at 65 percent, carbon at 18.5 percent, hydrogen at 9.5 percent, and nitrogen at 3.3 percent. The remaining 4 percent includes calcium in your bones, iron in your blood, phosphorus in your DNA, and trace amounts of elements like zinc and iodine that keep essential processes running.
How New Elements Get Their Names
When physicists create a new element in a lab, the discovery has to be confirmed by a joint working group from IUPAC (the International Union of Pure and Applied Chemistry) and IUPAP (its counterpart in physics). Once confirmed, the discoverers propose a name and symbol. Elements can be named after a mythological figure, a mineral, a place, a property, or a scientist.
The name must follow a specific pattern based on where the element falls on the periodic table. Elements in groups 1 through 16 end in “-ium” (like oganesson’s neighbor flerovium). Group 17 elements end in “-ine” (like chlorine and fluorine). Group 18 elements end in “-on” (like neon and argon). The last four elements to receive official names were elements 113, 115, 117, and 118, all approved in November 2016: nihonium, moscovium, tennessine, and oganesson.
These synthetic, superheavy elements exist for only fractions of a second before decaying into lighter elements. They’re created by firing beams of atoms at target materials in particle accelerators, and even a single confirmed atom can be enough to claim discovery. Their practical applications are essentially nonexistent for now, but they help physicists test theories about nuclear structure and the theoretical limits of matter.