An element is a substance made of only one type of atom, such as oxygen (O) or iron (Fe). Each element has a symbol of one or two letters; the first is a capital. A compound is made of two or more different elements chemically joined, such as water, H₂O, or carbon dioxide, CO₂. A compound has different properties from its elements: sodium is a reactive metal and chlorine a poisonous gas, but sodium chloride is table salt.
A molecule is two or more atoms bonded together. A formula shows the atoms in it: H₂O has 2 hydrogen atoms and 1 oxygen atom. A mixture contains substances that are not chemically joined, like air or sea water, and it can be separated by physical methods such as filtering or distillation.
Worked example
How many atoms are in one molecule of CO₂, and of which elements?
- C means one carbon atom.
- O₂ means two oxygen atoms.
Answer:So there are 3 atoms: 1 carbon and 2 oxygen.
Key words
- element
- made of one type of atom
- compound
- two or more elements chemically joined
- molecule
- atoms bonded together
- formula
- symbols and numbers showing the atoms in a substance
New questions every time.
NGSS-referenced: MS-PS1-1