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8F The Periodic Table, Atoms, elements and compounds, Group 1 elements -…
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Atoms, elements and compounds
An element is a simple substance. It is made up of identical particles called atoms. *Atoms are the building block.
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Compounds are made from two or more atoms chemically joined together. For example, water is a compound of hydrogen and oxygen. Each water molecule contains two hydrogen atoms and one oxygen atom. We show this in its chemical formula, H2O.
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CO2 - One carbon atom and 2 oxygen atoms
Sulfur (S8) has eight atoms
Methane (CH4) has five atoms, one of carbon (C) and four of hydrogen
Elements, mixtures and compounds
Mixture - An impure substance made from different elements or compounds mixed together that are not chemically joined.
Air is a mixture that contains the elements nitrogen, oxygen and argon, and also the compound carbon dioxide.
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Group 1: Alkali metals
Properties - soft, low melting/boiling points, low density
The alkali metals are so reactive that, as elements, they have to be stored in
oil. This stops them reacting with oxygen in the air.
Alkali metals are soft enough to be cut with a knife, and the most common alkali metals, lithium, sodium and potassium, all float on water.
The elements in group 1 also react with water and form alkaline compounds. This is why they are called alkali metals.
