Everything you can see and touch, from the air you breathe to the screen you are reading, your own body and the distant stars, is made of atoms. These tiny building blocks are so small that a single grain of sand contains more atoms than there are grains of sand on all the world's beaches. Yet there are only a little over a hundred different kinds of atoms, called elements, and they combine in countless ways to make everything in the universe. The periodic table organizes these elements in a way that reveals deep patterns in their behavior. This explainer describes what atoms are made of and how to read the periodic table.
What is an atom?
An atom is the smallest unit of an element that keeps the chemical properties of that element. The idea is ancient: the Greek philosopher Democritus, around 400 BCE, proposed that matter is made of tiny, indivisible particles he called "atomos," meaning uncuttable. But it was only in the nineteenth and twentieth centuries that scientists confirmed atoms exist and discovered their structure.
Atoms are incredibly small, typically about 0.1 to 0.5 nanometers across. It would take about 10 million atoms lined up side by side to span a millimeter.
Inside the atom
Atoms are made of three main kinds of particles:
| Particle | Charge | Location | Relative mass |
|---|---|---|---|
| Proton | positive (+1) | nucleus | about 1 |
| Neutron | none (0) | nucleus | about 1 |
| Electron | negative (−1) | around the nucleus | about 1/1,836 |
The nucleus, at the center of the atom, contains protons and neutrons. It is extremely small but contains almost all of the atom's mass. If an atom were the size of a football stadium, its nucleus would be about the size of a small marble at the center. Most of an atom is empty space.
Electrons move around the nucleus in regions called orbitals. They are held in place by the attraction between their negative charge and the positive charge of the protons. In a neutral atom, the number of electrons equals the number of protons.
What makes an element?
An element is a substance made of atoms that all have the same number of protons. This number is called the atomic number. It defines the element:
- Every atom with 1 proton is hydrogen.
- Every atom with 6 protons is carbon.
- Every atom with 8 protons is oxygen.
- Every atom with 79 protons is gold.
If you could add a proton to an atom's nucleus, you would turn it into a different element. This happens naturally in stars and in nuclear reactions.
Isotopes
Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons. These different versions are called isotopes. For example:
- Carbon-12 has 6 protons and 6 neutrons. It is the most common form of carbon.
- Carbon-13 has 6 protons and 7 neutrons.
- Carbon-14 has 6 protons and 8 neutrons. It is radioactive and used for radiocarbon dating.
Isotopes of an element behave almost identically in chemical reactions, because chemistry depends mainly on electrons. Some isotopes are unstable, or radioactive, and break down over time, releasing radiation. Their predictable decay rates allow scientists to date rocks, fossils and archaeological artifacts.
Ions
When an atom gains or loses electrons, it becomes an ion, an atom with an electric charge. Losing electrons creates a positive ion, such as the sodium ion, Na⁺. Gaining electrons creates a negative ion, such as the chloride ion, Cl⁻. Table salt, sodium chloride, is made of sodium and chloride ions held together by their opposite charges.
Electron shells and chemical behavior
Electrons are arranged around the nucleus in energy levels, or shells. The first shell can hold up to 2 electrons, the second up to 8, and the third, in simple models, up to 8 for the lighter elements, with more complex patterns for heavier ones.
The electrons in the outermost shell, called valence electrons, largely determine how an atom behaves chemically. Atoms tend to react in ways that give them a full outer shell, by gaining, losing or sharing electrons:
- Atoms with one or two valence electrons, like sodium, tend to lose them.
- Atoms that need one or two more electrons to fill their outer shell, like chlorine and oxygen, tend to gain them.
- Atoms with full outer shells, like neon and argon, are very stable and rarely react.
The periodic table
In 1869, the Russian chemist Dmitri Mendeleev arranged the known elements in a table by their atomic masses and properties. He noticed that properties repeated at regular intervals, or periodically, which is why it is called the periodic table. Remarkably, Mendeleev left gaps in his table for elements that had not yet been discovered and predicted their properties. When gallium, scandium and germanium were later discovered, they matched his predictions closely.
Today, the periodic table is arranged by atomic number, a refinement made after scientists discovered protons. It contains 118 confirmed elements. About 90 occur naturally on Earth; the rest have been created in laboratories.
Periods (rows)
The horizontal rows are called periods. There are seven. Each period corresponds to a new electron shell being filled. As you move from left to right across a period, the atomic number increases by one for each element, and the elements change from metals to nonmetals.
Groups (columns)
The vertical columns are called groups. There are 18. Elements in the same group have the same number of valence electrons and therefore similar chemical properties. Some groups have special names:
| Group | Name | Examples | Properties |
|---|---|---|---|
| 1 | alkali metals | lithium, sodium, potassium | soft, very reactive metals that react with water |
| 2 | alkaline earth metals | magnesium, calcium | reactive metals, less so than group 1 |
| 3 to 12 | transition metals | iron, copper, gold, silver | hard, shiny, good conductors, form colored compounds |
| 17 | halogens | fluorine, chlorine, iodine | very reactive nonmetals |
| 18 | noble gases | helium, neon, argon | very unreactive gases |
Hydrogen sits at the top of group 1 but is a nonmetal gas with unique properties.
Two rows usually shown separately at the bottom of the table are the lanthanides and actinides. The lanthanides include many rare earth elements used in magnets, electronics and lighting. The actinides include uranium and plutonium.
Metals, nonmetals and metalloids
The periodic table can also be divided into broad categories:
- Metals, on the left and in the center, make up most elements. They are usually shiny, conduct heat and electricity and can be shaped.
- Nonmetals, on the upper right, include gases like oxygen and nitrogen and solids like carbon and sulfur. They are poor conductors.
- Metalloids, along a zigzag line between metals and nonmetals, such as silicon and germanium, have intermediate properties and are important in electronics.
Reading an element's box
Each box in the periodic table typically shows:
- The atomic number, the number of protons.
- The chemical symbol, one or two letters, such as H for hydrogen, Fe for iron (from the Latin "ferrum") and Au for gold (from the Latin "aurum").
- The name of the element.
- The atomic mass, the average mass of the element's atoms, accounting for its natural isotopes.
Where the elements came from
The lightest elements, hydrogen, helium and a little lithium, formed in the first few minutes after the Big Bang. Most heavier elements were forged inside stars through nuclear fusion, and in supernova explosions and collisions of neutron stars. The carbon in our bodies, the oxygen we breathe and the iron in our blood were all made in stars.
Molecules and compounds
Atoms rarely exist alone. They bond together to form molecules and compounds. A molecule is two or more atoms joined together, such as oxygen gas (O₂). A compound is a substance made of two or more different elements chemically bonded, such as water (H₂O), carbon dioxide (CO₂) and table salt (NaCl). The properties of compounds can be very different from those of their elements: sodium is a reactive metal and chlorine a toxic gas, but together they form harmless table salt.
Questions readers ask
What is an atom made of? A nucleus of protons and neutrons, surrounded by electrons.
What determines which element an atom is? The number of protons in its nucleus, called the atomic number.
What are isotopes? Atoms of the same element with different numbers of neutrons.
Why are elements in the same column similar? They have the same number of valence electrons, which largely determines chemical behavior.
How many elements are there? 118 elements have been confirmed, about 90 of which occur naturally.
In short
Atoms are the building blocks of matter, made of a tiny nucleus of protons and neutrons surrounded by electrons. The number of protons defines each element, while different numbers of neutrons create isotopes. Electrons, especially those in the outer shell, determine how atoms bond and react. The periodic table organizes the 118 known elements by atomic number into periods and groups, revealing patterns in their properties. From the hydrogen in water to the carbon in our cells, these few kinds of atoms combine to build the entire material world.