Pure Substances: Elements vs Compounds
An element is a pure substance made of only one kind of atom that cannot be broken down into simpler substances by chemical means, while a compound is a pure substance made of two or more elements chemically combined in a fixed ratio. In brief: oxygen (O2) and iron (Fe) are elements, water (H2O) and table salt (NaCl) are compounds, and a compound has properties that are different from those of the elements it contains.
All pure substances fall into these two groups. Elements are the basic building blocks listed in the periodic table, and compounds are formed when atoms of different elements bond together. This article explains the two types, shows how to tell them apart, lists examples in tables, works through the law of definite proportions with numbers and gives the history of how chemists identified elements.
What is an element?
An element consists of atoms that all have the same number of protons, which is the atomic number. Chemists have identified 118 elements, of which about 90 occur naturally on Earth in significant amounts. The rest were made artificially in laboratories. Each element has a one- or two-letter symbol, such as H for hydrogen, C for carbon and Fe for iron.
Some elements exist as single atoms, such as the noble gas helium. Others form molecules made of the same atoms. Oxygen is found as O2, ozone as O3, nitrogen as N2 and sulfur as S8. A molecule of oxygen contains two atoms, yet oxygen is still an element since both atoms are of the same kind.
What is a compound?
A compound contains atoms of two or more different elements joined by chemical bonds in a fixed ratio. The formula shows the ratio: H2O has two hydrogen atoms for every oxygen atom, and CO2 has one carbon atom for every two oxygen atoms. Compounds can only be separated into elements by chemical reactions, for example by electrolysis or by heating.
The properties of a compound differ from those of its elements. Sodium is a soft, reactive metal and chlorine is a poisonous green gas, but sodium chloride is a harmless white crystal that we eat on food.
Elements and compounds compared
| Feature | Element | Compound |
|---|---|---|
| Kinds of atoms | One | Two or more |
| Can be broken down chemically? | No | Yes, into elements or simpler compounds |
| Represented by | Symbol (Fe, O2) | Formula (H2O, NaCl) |
| Number known | 118 | Many millions |
| Examples | Hydrogen, copper, neon | Water, carbon dioxide, ammonia |
What is the law of definite proportions?
Joseph Proust showed around 1800 that a given compound always contains the same elements in the same proportions by mass, regardless of its source. Pure water always has about 1 part hydrogen to 8 parts oxygen by mass. The law distinguishes compounds from mixtures, whose composition can vary.
| Compound | Formula | Elements | Mass ratio |
|---|---|---|---|
| Water | H2O | H : O | 1 : 8 |
| Carbon dioxide | CO2 | C : O | 3 : 8 |
| Sodium chloride | NaCl | Na : Cl | about 23 : 35.5 |
| Iron(II) sulfide | FeS | Fe : S | about 7 : 4 |
Worked example: composition by mass
- Water. The atomic masses are H = 1.008 and O = 16.00. In H2O the hydrogen mass is 2 × 1.008 = 2.016 and oxygen is 16.00. The ratio H : O = 2.016 : 16.00 = 1 : 7.94, or about 1 : 8.
- Percentage of oxygen in water. The molar mass is 18.016 g/mol, so oxygen is 16.00 ÷ 18.016 × 100% = 88.8%.
- Mass from a sample. In 36.0 g of water, the mass of hydrogen is 36.0 × 2.016 ÷ 18.016 = 4.03 g, and the mass of oxygen is 36.0 − 4.03 = 31.97 g, or about 32.0 g.
- Carbon dioxide. The molar mass is 12.01 + 2 × 16.00 = 44.01 g/mol. Carbon is 12.01 ÷ 44.01 × 100% = 27.3% by mass, and oxygen is 72.7%.
If an analysis of an unknown sample shows 11.2% hydrogen and 88.8% oxygen, it matches water. Any other ratio would indicate a different compound or a mixture.
How can you tell an element from a compound?
- Chemical test: if a pure substance can be decomposed into simpler substances, it is a compound. Passing an electric current through water gives hydrogen and oxygen.
- Formula: a single symbol or a molecule of identical atoms, such as N2, is an element. Different symbols in the formula, such as NH3, show a compound.
- Periodic table: every element appears there. A substance that is not in the periodic table as a whole is a compound or a mixture.
How were elements identified?
Ancient thinkers proposed that everything is made of a few elements such as earth, water, air and fire. In 1661 Robert Boyle defined an element as a substance that cannot be broken down into simpler ones, in his book The Sceptical Chymist. Antoine Lavoisier published a list of about 33 substances that he regarded as elements in 1789, although some of them, such as lime, were later found to be compounds. Mendeleev’s periodic table of 1869 organized the known elements, and elements were discovered steadily after that.
Which mistakes are common?
Mistake 1: Thinking that O2 is a compound
It contains only one kind of atom, so it is an element.
Mistake 2: Calling a mixture of gases a compound
Air contains several elements and compounds that are not chemically bonded, so it is a mixture.
Mistake 3: Expecting a compound to look like its elements
Compounds have new properties. Water does not resemble either hydrogen or oxygen.
Mistake 4: Writing a molecule of an element as a compound
The words “molecule” and “compound” are different. All compounds with molecules consist of molecules, but not all molecules are compounds.