Page 57 - 新思维科学学生用书7 样章

2 Materials and their structure

What is a compound?

You have looked at elements in the Periodic Table. An element is made up of only one type of atom.
Many substances are made up of more than one type of atom. If the different types of atom are joined
tightly together, then the substance is a compound.

The chemical term for two atoms joining tightly together is bonding. In a compound, two or more
different kinds of atom are bonded. For example, when sodium atoms bond with chlorine atoms, they
form the compound sodium chloride.

Properties of elements and compounds

A compound is very different from the elements from which it is made. When two different elements
are bonded, they completely lose the properties of the individual elements. The compound has totally
new properties.

The first two photographs show the two elements sodium and chlorine. The third photograph shows
the compound that is made when sodium and chlorine atoms bond together. This compound –

sodium chloride – is not at all like either sodium or chlorine.

+

Sodium, an element Sodium chloride, a
Chlorine, an
element compound

You may have eaten some sodium chloride today. Sodium chloride is common salt. You would not
want to eat any sodium or chlorine, though.

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