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Atoms, elements and compounds
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Everything is made from atoms. There are only about 100 or so kinds of atom, but they join up to make millions of substances.
Atoms and elements
An atom is the smallest particle of an element. In the simple model of John Dalton, atoms are tiny solid spheres, and each element has its own type of atom.
An element contains only one type of atom. 118 elements are known, and they are listed in the periodic table.
Each element has a symbol: O (oxygen), C (carbon), H (hydrogen), Na (sodium), Fe (iron), Cu (copper).
An element contains only one type of atom. 118 elements are known, and they are listed in the periodic table.
Each element has a symbol: O (oxygen), C (carbon), H (hydrogen), Na (sodium), Fe (iron), Cu (copper).
Molecules and compounds
A molecule is two or more atoms joined together, such as O2.
A compound contains atoms of two or more different elements chemically joined, such as water, H2O.
A compound has different properties from the elements in it. Sodium (a reactive metal) and chlorine (a toxic gas) make sodium chloride: table salt.
In a mixture, the substances are not chemically joined, so they are easy to separate.
A compound contains atoms of two or more different elements chemically joined, such as water, H2O.
A compound has different properties from the elements in it. Sodium (a reactive metal) and chlorine (a toxic gas) make sodium chloride: table salt.
In a mixture, the substances are not chemically joined, so they are easy to separate.
Formulae and names
A formula shows which atoms are in a substance and how many. A small number after a symbol counts the atoms just before it.
CO2 has 1 carbon atom and 2 oxygen atoms. H2SO4 has 2 H, 1 S and 4 O: 7 atoms in total.
Names: a compound ending in -ide usually has two elements (sodium chloride, NaCl). Ending in -ate means oxygen is also there (calcium carbonate, CaCO3).
CO2 has 1 carbon atom and 2 oxygen atoms. H2SO4 has 2 H, 1 S and 4 O: 7 atoms in total.
Names: a compound ending in -ide usually has two elements (sodium chloride, NaCl). Ending in -ate means oxygen is also there (calcium carbonate, CaCO3).
How many atoms of each element are in CaCO3, and how many atoms in total?
- Ca: 1 calcium atom.
- C: 1 carbon atom.
- O3: 3 oxygen atoms.
- Total: 1 + 1 + 3
Answer: 1 Ca, 1 C, 3 O: 5 atoms
Element: one type of atom. Compound: two or more elements chemically joined. Mixture: not chemically joined. A formula counts the atoms, e.g. H2O has 2 H and 1 O. -ide: two elements; -ate: oxygen too.
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