Molecules are groups of atoms held together by 2 types of chemical bonds. Molecules can either consist of 1 type of atom or different types of atoms. Molecular studies is a recent field; the first molecules studied were air molecules in 1865.

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Model Of Hemoglobin Molecule - Dr. Makie Murayama by Charles PhillipsLIFE Photo Collection

The invention of the Scanning Tunneling Microscope in 1981 allowed scientists to directly observe molecules for the first time.

What are molecules?

Molecules are groups of atoms held together by chemical bonds. These groups of atoms form all matter; everything you see and touch is composed of molecules. Even you are made up of billions of billions of different types of molecules!


Scientists once thought atoms were the smallest units of matter in the universe. We now know that atoms consist of smaller particles: electrons, protons, and neutrons. It takes a whole lot of atoms to make up any type of matter.


Atoms of the same type are called elements. 

Elements cannot be broken down further; for example, water (a non–element) can be broken down into hydrogen and oxygen. Water is made up of 2 elements that form a molecule: hydrogen and oxygen.


Molecules are groups of atoms that are bonded together. They take a specific form; some look like a spiral and some look like a pyramid. Molecules make up everything we see and even things we don’t see, like the air we breathe. 


Every organism on Earth is also made up of molecules. Molecules that are part of the human body include nucleotides found in DNA, proteins, carbohydrates, and lipids. The biggest molecule in nature, Chromosome 1, contains about 10 billion atoms.

Chemical compounds

Compounds are types of molecules that contain 2 or more different types of elements. Chemical bonds join the different elements together. A pure substance, if we apply various chemical reactions to a compound, we can separate it into its components.


Elements are composed of molecules of 1 type of atom. An atom changes type depending on how many electrons, protons, and neutrons it has. This atom is boron, the fifth element in the periodic table. It has 5 electrons.


Water is a chemical compound that consists of 1 oxygen atom and 2 hydrogen atoms, written like this: H2O. Also known as dihydrogen monoxide, the composition of water remains the same in all its states of matter.


Glucose is one of the most important sources of energy in the human body. This chemical compound is comprised of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Our blood carries the glucose we consume to all the organs in our bodies.


Ozone, found in Earth’s atmosphere isn’t a chemical compound, as it consists only of oxygen atoms. As it’s a molecule, these atoms bond together chemically. A gas, ozone is composed of 3 oxygen atoms, and we write its formula like this: O3.

Ionic bonding

Atoms can bond together to form molecules in 2 different ways. These are through ionic bonding and covalent bonding. Ionic bonding involves the transfer of electrons between atoms. Compounds formed by ionic bonding are hard, brittle,  and difficult to separate. 

Often they present in crystal form.

Valence electrons

Electrons are located in different shells, or spheres within an atom. Found in the very outer shell of an atom, they are involved in ionic bonding. They act as a protective barrier for the atom, helping to keep it stable.

Full shells

Ionic bonding occurs when atoms transfer electrons to create full outer shells. A positively charged sodium atom has 1 valence electron. A negatively charged chlorine atom has 7 valence electrons, 6 in pairs and 1 on its own.

Electrostatic attraction

Charged atoms, also called ions, attract atoms that have an opposite charge. The chlorine atom reacts with the sodium atom. The single valence electron in the sodium atom transfers across to the negatively charged chlorine atom.

Valence shells

Once the transfer occurs, both the sodium and the chlorine atoms become neutrally charged and form the harmless compound NaCl, or table salt. If an atom doesn’t have a spare electron to create a shell, one will act as 2, making a figure 8 around both atoms.

Covalent bonding

Unlike ionic bonding, which involves the complete transfer of electrons between 1 type of atom to another, covalent bonding involves 2 nonmetal atoms sharing their valence electrons. Ideally, the full outer shell of this bond will have 8 valence electrons. 

Polar and nonpolar

Two types of covalent bonds exist: polar and nonpolar. Nonpolar bonds occur when atoms share electrons equally. Polar bonds occur when atoms don’t share the electrons equally, and 1 atom pulls more electrons in than the other.

Hydrogen and oxygen

Hydrogen and oxygen, 2 nonmetal atoms, form a covalent bond very important to life. While hydrogen only has 1 electron, oxygen has 8 electrons, 6 in its inner shell and 2 valence electrons located in its outermost shell. 


Water is formed when hydrogen and oxygen react and polar covalent bonding occurs. Water is considered a polar bond as the electrons that form the bond spend more time closer to the oxygen atom than the 2 hydrogen atoms. 


All the atoms within a water molecule have full valence shells, resulting in an overall charge of neutral for the molecule. In general, covalent bonds are weaker than ionic bonds, and are considered poor conductors of electricity and heat.


Polymers are lots of the same molecules strung together to form long chains. Produced by organisms, polymers occur naturally. Humans can also easily manufacture and create them. Polymers can either be stretchy and flexible, or hard and unyielding.


The polymer wood occurs naturally. The primary component of wood is a polymer called cellulose, which are glucose molecules connected together. For every glucose molecule added to the cellulose chain, 1 molecule of water is released.


Rubber is composed of polymers called elastomers. After these polymers are stretched, they return back to their original shape. Rubber molecules naturally bunch together. When pulled, the molecules straighten and then return to their bunched shape when the pressure is released.


Plastics are man–made polymers, usually produced from carbon in oil or natural gas. Polyethylene is the world's most common type of plastic. It consists of a long string of carbon atoms, with 2 hydrogen atoms connected to each carbon atom.


DNA is a naturally occuring polymer. It consists of the nucleotides adenine, guanine, thymine, and cytosine. These nucleotides are made up of a complex mix of sugar, nitrogens, and at least 1 molecule from the phosphate group. 


Enzymes are made out of proteins that are produced by living organisms. They act as catalysts for many different chemical reactions. Each enzyme has a very specific job, and will only react with a specific type of substance. 


Enzymes are necessary for life as they help along many different biochemical processes such as digestion and metabolism. The activity of an enzyme can be affected by temperature, acidity, concentration, and the presence of inhibitor molecules that work against the enzyme.


Most enzymes are made up of long protein chains. Essential to life, proteins make up about 20% of the human body. Every cell in our bodies uses proteins to perform functions, and, luckily, our cells produce the proteins that we need.


Amylase is a digestive enzyme that breaks down starch in food. A family of similar enzymes works in a wide range of animals and plants. As we age, our bodies produce less amylase so that digesting carbohydrates may be difficult as we get older.


The digestive enzyme lipase breaks down fat. Found in the stomach, the body also uses it to ward off allergic conditions and infectious viruses. The pancreas releases lipase into the small intestine to split the fats and absorb them.

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