What Is a Molecule?
A molecule is a group of two or more atoms that are bonded together. Atoms bond by sharing electrons — this type of bond is called a covalent bond.
Molecules can be made of:
- Atoms of the same element — for example, an oxygen molecule (O₂) is two oxygen atoms bonded together.
- Atoms of different elements — for example, a water molecule (H₂O) is two hydrogen atoms bonded to one oxygen atom.
Molecules of Elements
Some elements exist naturally as molecules, not as single atoms. These are called diatomic molecules (two atoms bonded together). The most important diatomic elements are:
| Molecule | Formula | Found in |
|---|---|---|
| Hydrogen | H₂ | Stars, some fuels |
| Oxygen | O₂ | Air (21%) |
| Nitrogen | N₂ | Air (78%) |
| Chlorine | Cl₂ | Disinfectants, swimming pools |
A useful memory trick: the diatomic elements are H, He, N, O, F, Cl, Br, I — "Have No Fear Of Cold, Brutal Ice" (though helium is actually monatomic, not diatomic). The diatomic ones are H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂.
Molecules of Compounds
When atoms of different elements bond together, the result is a compound. Unlike a mixture, the atoms in a compound are chemically joined and cannot be separated by physical means.
Some important molecular compounds:
| Name | Formula | Description |
|---|---|---|
| Water | H₂O | 2 hydrogen, 1 oxygen atom |
| Carbon dioxide | CO₂ | 1 carbon, 2 oxygen atoms |
| Ammonia | NH₃ | 1 nitrogen, 3 hydrogen atoms |
| Methane | CH₄ | 1 carbon, 4 hydrogen atoms |
How Molecules Are Represented
Chemical formulas show the types and numbers of atoms in a molecule:
- The letter(s) represent the element.
- A subscript number after a symbol shows how many atoms of that element are present.
- No subscript means only 1 atom.
For example, CO₂ means one carbon atom and two oxygen atoms.
Displayed (structural) formulas show how the atoms are connected using lines for bonds. In H₂O, the two hydrogen atoms are connected to the central oxygen atom at an angle of about 104.5°.
The properties of a molecule are completely different from the properties of its individual atoms. Sodium (Na) is a soft reactive metal. Chlorine (Cl₂) is a poisonous yellow-green gas. But together they form sodium chloride (NaCl) — ordinary table salt — which is safe to eat!
Molecular Mass
The molecular mass is the total mass of all the atoms in a molecule. You calculate it by adding up the atomic masses from the periodic table. For example, water (H₂O) has a molecular mass of 2 × 1 + 16 = 18 (in atomic mass units, u).
Real world: Water molecules in everyday life
Water (H₂O) is made of two hydrogen atoms and one oxygen atom per molecule. Its bent shape (at ~104.5°) gives it polarity, allowing it to dissolve salts and sugars. This is why water is called the "universal solvent" — the molecular structure of H₂O makes it vital for cooking, cleaning, and all life processes.
Real world: Oxygen we breathe
The oxygen you breathe is O₂ — two oxygen atoms bonded together. Single oxygen atoms (O) are far too reactive to exist freely in air. It is the O₂ molecule that reacts in your cells during respiration to release energy from food. Doctors and hospitals use pure O₂ in oxygen tanks for patients who cannot breathe normally.
Real world: CO₂ in fizzy drinks
Carbon dioxide (CO₂) — 1 carbon + 2 oxygen atoms — is dissolved under pressure into fizzy drinks. When you open the bottle, pressure drops and the CO₂ comes out of solution as bubbles. CO₂ is also produced when you breathe out and when fuels burn. Its molecular structure makes it a greenhouse gas that traps heat in the atmosphere.
Where do you see this in real life? H₂O in your glass, O₂ in the air, CO₂ in fizzy drinks, N₂ in the atmosphere — molecules are everywhere. The properties of each substance come entirely from which atoms are bonded together and how.
Practice Problems
How many atoms are in one molecule of glucose, C₆H₁₂O₆?
Show answer
What is the difference between O (one oxygen atom) and O₂ (an oxygen molecule)?