What is formed when two or more different kinds of atoms or elements join together chemically?

Updated April 26, 2018

By Chris Deziel

The periodic table is a catalogue of all the known elements, and it's safe to say that this universe wouldn't exist if these elements didn't combine. Each element is characterized by an atom with a certain number of protons and neutrons in its nucleus and a certain number of electrons surrounding them. When atoms combine, they share their outermost electrons to create more sustainable energy states. This sharing bonds the atoms into an ionic structure or a molecule.

Atoms can combine into ionic lattice structures or into covalent molecules. When different types of atoms combine, the result is called a compound.

The propensity for an atom to combine depends on the number of electrons it has in its outer shell. Every shell has eight spaces for electrons except the first shell, which has only two spaces. If a few of the spaces aren't occupied, an atom seeks to acquire or share electrons to fill it to achieve a stable outer shell with eight electrons. On the other hand, it's easier for an atom with just a few extra electrons to get rid of them to achieve stability. The noble gases, which include helium, argon and neon, already have stable outer shells filled with electrons, so these elements do not form combinations with each other or with other atoms.

Ionic Compound: An atom with only one electron in its outer shell seeks to donate the electron to another atom, while one with a single space will readily accept it. The atom that donates this electron becomes positively charged as a result, and the atom that accepts it becomes negatively charged. Electrostatic attraction then bonds the atoms into a lattice structure. This isn't a molecule, because the pairs of atoms aren't independent, but it's a compound, because it's formed from two different elements. Common table salt, sodium chloride (NaCl), is the classic example of an ionic compound.

Covalent Bonding: An atom with one, two, three or four extra electrons in its outer shell, or one missing one, two or three electrons, seeks to share electrons to achieve stability. When this sharing happens in pairs, the bond is called a covalent bond, and it can be very strong. The water molecule, which is formed when an oxygen molecule fills its outer shells with electrons from two hydrogen atoms, is an example. Atoms can share one, two or three electron pairs, and the compounds they form tend to have lower melting and boiling points than ionic compounds.

All elements except metals form covalent bonds. Part of what makes a metal what it is is its propensity to lose the electrons in its outer shell and become an ion, which is a charged particle. Ions prefer to coalesce into solid lattice structures. Covalent molecules, on the other hand, more often form liquids or gases.

Atoms can combine to form simple molecules, such as water, or they can combine in large strings to form complex ones, such as sucrose (C12H22O11). Because carbon has four electrons in its outer shell, it donates and accepts electrons equally well, and it's the building block of all organic molecules on which life depends. All inorganic and organic molecules composed of more than one element are compounds. Examples are hydrogen chloride (HCl), methane (CH4), carbon dioxide (CO2) and sucrose.

It's also common for atoms of the same element to share electrons to achieve stability. The two most abundant gases in the atmosphere, nitrogen (N2) and oxygen (O2), are composed of molecules formed from a single element. Nitrogen and oxygen molecules are not compounds, because they are not composed of different elements. Even ozone (O3), a less stable and more reactive combination of oxygen molecules, fails to qualify as a compound, because it consists of only a single element.

Updated March 31, 2020

By Chris Deziel

Reviewed by: Lana Bandoim, B.S.

Since John Dalton established the existence of atoms in the early 1800s, they have been considered the building blocks of matter. Scientists now know that atoms themselves are built from smaller particles, which are in turn composed of even smaller ones, and nobody really knows how far the regression goes.

On the constructive side, however, atoms combine to form all the chemical compounds that comprise the physical world and everything in it.

Fill in the blank: "A __ is composed of two or more atoms held together by chemical bonds." There is more than one answer, but the one that probably crossed your mind first is "molecule." Every grouping of two or more atoms is a molecule. Some are very simple, such as the oxygen molecules you breathe, which are formed by two oxygen atoms (O2), and some are huge, like the TTN genes in the human body. Composed of carbon, hydrogen, oxygen, nitrogen and sulfur, the so-called Titan molecule is composed of a whopping 539,030 atoms.

The other word you can use to fill in the blank is "compound." A compound is a molecule that is a substance made of two or more elements chemically combined in a set ratio. It contains more than one element, or type of atom. A compound is always a molecule, but a molecule isn't always a compound. The Titan molecule is a compound, and simpler examples of compounds include include sodium chloride (NaCl), or table salt, and dihydrogen oxide (H2O), or water.

To understand how atoms combine, remember that they are composed of smaller particles. They are called electrons, protons and neutrons. Electrons have a negative charge, protons have an equal positive charge, and neutrons have no charge. An atom has an equal number of electrons and protons, which makes it electrostatically neutral, but if electrostatic neutrality was all that mattered, atoms would never combine.

Electrons circle the nucleus in discrete orbits, or shells, that can hold a fixed number of them, and that number increases with the radius of the orbit.

If an atom is missing electrons to fill a shell, usually its outer one, it is unbalanced, and to gain stability, it seeks to get them from another atom in one of two ways. It can "steal' an electron (more generously, the other atom can "donate" it), or the two atoms can share electrons. Either way, the atoms become bonded chemically to form a molecule.

When an atom donates an electron to the other, both atoms become ions, each with an opposite charge, and they are bonded by electrostatic attraction. This is called an ionic bond. When atoms share electrons to complete each other's outer shells, they form a covalent bond, which isn't as strong as an ionic bond, but it's much more common.

If you don't like to call molecules compounds, you can distinguish them as homonuclear, which means made of one element, or heteronuclear, which means made of more than one element. H2, O2 and P4 are examples of the former, while CO2, HCl and CH4 are examples of the latter. The list of heteronuclear molecules is obviously much longer than that of homonuclear ones, as most molecules are compounds.

Molecules aren't always electrostatically neutral. Ionic molecules combine in a way that leaves them with a charge, and they can form ionic bonds with other molecules. Some molecules, such as the water molecule, are polar, because the way the atoms combine creates a net positive charge on one side and a negative charge on the other. This charge isn't as strong as the one that forms a chemical bond, but it's strong enough to produce some strange and important behavior.

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Today’s Wonder of the Day was inspired by Zachary. Zachary Wonders, “how do atoms make you?” Thanks for WONDERing with us, Zachary!

The universe really is an amazing place. We have the planets and the Sun in our solar system, which is part of the larger Milky Way galaxy.

Here on Earth, we have tall mountains, deep seas, and miles upon miles of all sorts of land. Over seven billion human beings and an untold number of animals inhabit Earth. And yet all of these things are composed of variations of the same small set of materials we know as elements.

On the smallest scale, we are made up of countless tiny atoms. Too small to see, those atoms combine in an infinite number of ways to make us who and what we are. Likewise, they make up all the matter around us, from mountains and seas to planets and stars.

Why exactly do atoms tend to combine together? Why aren't they happy just hanging out by themselves? As it turns out, it's purely a matter of chemistry.

Atoms form chemical bonds with other atoms when there's an electrostatic attraction between them. This attraction results from the properties and characteristics of the atoms' outermost electrons, which are known as valence electrons.

When two or more atoms chemically bond together, they form a molecule. Sometimes the atoms are all from the same element. For example, when three oxygen atoms bond together, they form a molecule of ozone (O3).

If a molecule forms from atoms of two or more different elements, we call it a compound. A common compound everyone is familiar with results from the chemical bonding of two atoms of hydrogen with one atom of oxygen to form a molecule of water (H2O).

Atoms can chemically bond in many ways. However, there are three primary types of chemical bonds that you'll see most often: covalent, metallic, and ionic.

Covalent bonds form between materials. In a covalent bond, electrons are shared between atoms. The bonds between the two hydrogen atoms and the oxygen atom in a molecule of water are covalent bonds.

As its name implies, a metallic bond occurs between metallic substances. The valence atoms in metals move freely and thus form bonds easily. This is what makes metals good conductors of heat and electricity.

Ionic bonds form between a metal and a nonmetal substance. In an ionic bond, electrons get transferred from the metal to the nonmetal substance. An example of a compound formed with ionic bonds is sodium chloride (NaCl), otherwise known as table salt!