What Is a Group of Atoms That Are Joined Together and Act as a Single Unit?


Atoms are the fundamental building blocks of everything in the universe. Their different properties divide them into 118 elements, which can combine in millions of ways. Scientists call these combinations of atoms molecules and compounds. Molecules make up every familiar object that you know, from the air you breathe to your lungs that take it in. Scientists work extensively with substances made of molecules, so it is important to know what a molecule is and what properties it has.

What Are Molecules and Compounds?

  • A molecule is two or more atoms chemically joined together. If at least two different elements make up the molecule, it is called a compound. For example, hydrogen gas (H2) and water (H2O) are molecules, but water is also a compound because it is made up of hydrogen and oxygen. A molecule acts like a single unit and is the smallest piece of a substance that will retain all the properties of that substance. For instance, if you decomposed sugar (C12H22O11) to anything smaller than its molecular level, it would no longer be sugar. It would be individual carbon, hydrogen and oxygen atoms.

How Do Molecules Form?

  • Every atom is made up of positively charged protons and negatively charged electrons. These electrons are arranged in levels called orbitals or shells. The most energetic electrons reside in the outermost orbital, called the valence shell, and can be shared with other atoms to form a molecule. The number of electrons an orbital can hold determines what kind of molecules will form. For example, chlorine (Na) can accept only one electron before its valence shell is full. Therefore it can combine with one sodium atom to make table salt (NaCl) but not two to form Na2Cl.

Types of Molecules

  • Molecules can be covalent, polar covalent, ionic or metallic. Covalent compounds form when two atoms share their electrons equally. For this to occur, both atoms must have the same electronegativity, or pull on the electrons. Only identical atoms have exactly the same electronegativity, so true covalent bonds form only between elements that bond with themselves, such as hydrogen gas (H2). Atoms that share their electrons slightly unevenly are called polar covalent molecules. In this type of compound, one atom has a slightly stronger pull on the electron than the other; therefore the electron spends more time around the stronger atom, creating a temporary positive and negative end. Ionic compounds form when one atom has a much stronger pull on the electron than the other atom, causing it to control it most of the time. Metallic atoms share their electrons freely with many atoms, causing an electron flow, which makes them good conductors of electricity.

Compound Vs. Mixture

  • Two elements must chemically bond to form a molecule; that is, they must share electrons. If they are combined in such a way that they appear to be a single substance, but have not bonded chemically, it is a mixture. For example, water is a compound because hydrogen and oxygen share electrons. Sugar water is a mixture; although its components are physically mingled, they have not chemically bonded. A mixture will usually resemble its components, just as the sugar water tastes sweet, like sugar, and remains a liquid, like water. Compounds will not retain the properties of their components. For example, table salt (NaCl) is made of sodium, which bursts into flames when it touches water, and chlorine, which is a disinfectant. However, when you put them together, they form a stable, edible substance.


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