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Model of the atom - 3-D Helium atom - ground state
Helium atom model
Showing nucleus with two protons (blue)
and two neutrons (red),
orbited by two electrons (waves).
Smallest recognised division of a chemical element
Mass: 1.66 x 10(−27) to 4.52 x 10(−25) kg
Electric charge: zero
Diameter: 10 pm to 100 pm

An atom is the basic type of particle that makes up matter. It is the smallest part of a chemical element, which has the properties of that element.

Atoms are very tiny particles. If atoms are taken to be ball-shaped, they have diameters of about 10−8 meters. They have masses around 10−23g.

Atoms are made up of 3 sub-atomic particles - protons, neutrons and electrons.

At the center of the atom is a tiny dense region called the nucleus. The nucleus contains protons and neutrons, which are closely packed together. A proton and a neutron have about the same mass. Protons are positively charged, but neutron have no charge. This means the nucleus is positively charged.

Electrons are negatively charged. They are much lighter than the protons and neutrons in the nucleus. Electrons move very fast around the nucleus.

In a neutral atom, the numbers of protons is equal to the number of electrons. In other atoms, the number of neutrons may not be equal to the number of protons.

Charged atoms are called ions.

Atoms with fewer electrons than protons have a positive charge. These atoms are called cations.

Atoms with more electrons than protons have a negative charge. These atoms are called anions.

Electrons orbit at different distances from the nucleus. We say those closest to the nucleus are in one "shell," and those a little farther away are in another "shell." Each shell is given a number. The one closest to the nucleus is shell 1, the next one out is shell 2, and so on.

The nucleus is very small, even compared with the atom. The nucleus is more than 100,000 times smaller than the atom - the size of the atom is determined by the size of the outer electron shell. To give people another idea of the size difference, if the atom were the size of a football stadium, the atom's nucleus would be the size of a pea somewhere in the center. Most of the atom is empty space, occupied by electrons spinning around somewhere inside.

Atoms are generally classified by their atomic number. The atomic number of an atom is the number of protons in the atom (in atoms that do not have an electric charge, this is the same as the number of electrons). The atomic number decides what family or element the atom belongs to. For example, carbon atoms are those atoms containing 6 protons. All atoms with the same atomic number share a wide variety of physical properties and exhibit the same chemical behavior. The various kinds of atoms are listed in the Periodic table. The mass number or nucleon number is the total of protons and neutrons. The number of neutrons does not have any effect on the element of the atom - within an element family are several members, each with the same atomic number but different mass numbers. These are called isotopes. To write the name of an isotope, we write the name of an element followed by its mass number, e.g. Carbon 14 (which contains 6 protons and 8 neutrons in each atom). Some isotopes are radioactive, meaning they give off radiation, while others are known as stable, meaning they do not give off any radiation, and any atom that has an atomic number of 83 or higher is also radioactive, regardless of what isotope it is. Radioactive atoms and isotopes are dangerous to anything living thing except in the smallest possible amounts, and should only handled by experts, since they can make people very sick if not handled correctly. Doctors sometimes use isotopes, like the isotopes of Barium and Iodine to help find illnesses, but should still be handled with care, even though they use very small amounts, not enough to make a person sick.

There are only 94 different types of atoms that occur naturally in the world. (A few more have been made artificially in a laboratory). Every different kind of atom has a name and every name has an abbreviation. The 94 kinds of atoms are usually listed in a table. The table is called the periodic table. A scientist named Dmitri Mendeleev was the first to think of the table. In recognition of this, and other accomplishments, element 101 was named Mendelevium when it was discovered. Many atoms that have higher numbers are radioactive, like Radium and Uranium, and should never be handled without a lot of care and protection, since they give off so much radiation.

The simplest atom is the hydrogen atom, having atomic number 1. The commonest type of hydrogen atoms consists of 1 proton and 1 electron (with no neutron). The hydrogen isotope containing 1 additional neutron is called deuterium; the hydrogen isotope with 2 additional neutrons is called tritium. It has been the subject of much interest in science, particularly in the early development of quantum theory.

The first 11 kinds of atoms (also called elements) are called hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon and sodium. Their abbreviations are H, He, Li, Be, B, C, N, O, F, Ne, Na. You may notice that Sodium does not fit with Na. This is because sodium was most commonly called natrium in the past. This has carried over into other fields of science as well. Hyponatremia is a medical condition where the body lacks enough salt to function properly.

All matter and all substances are made up of these 94 kinds of atoms. Most things contain far fewer types of atoms. Water, for example, contains only atoms of hydrogen and oxygen. In water these atoms form combinations called molecules.

The word "atom" comes from the Greek ἀτόμος, indivisible, from ἀ-, not, and τόμος, a cut. Until the end of the 19th century, it was believed that atoms were very tiny 'solid' spheres which could never be divided. This idea dates back to Democritus and Leucippus, Greek philosophers in the 5th century BC.