Silicon dioxide (often called silica) is the main compound found in sand. It is an example of a giant covalent substance. It contains many silicon and oxygen atoms. These are joined together by strong covalent bonds in a regular arrangement, forming a giant covalent network or lattice structure. There is no fixed number of atoms joined together in this type of structure, so lattices should not be described as molecules. However, the atoms in the compound will be present in the ratio indicated by the chemical formula.
Giant covalent substances are solids at room temperature and have very high melting points and boiling points. Simple covalent substances have low melting and boiling points because melting or boiling these substances overcomes weak forces between the molecules. However, giant covalent substances do not contain small molecules - for example, a crystal of diamond is a single covalent lattice. Melting or boiling giant covalent lattices requires covalent bonds to break. Covalent bonds are strong, so a lot of energy is needed to break up giant covalent substances during melting and boiling.
Giant covalent substances have no overall charge, nor do they contain free-moving ions or electrons, so most cannot conduct electricity. Graphite, a form of carbon, which can conduct electricity, is an exception.
Non-metalelements can form a covalent bond with another atom of the same element. This happens in some non-metal elements, for example hydrogen or oxygen, when the atoms that join together are of the same element. There are also some compounds, for example hydrazine N2H4, where molecules contain a bond between two atoms of the same element.
Carbon has an unusual ability to bond to itself. Carbon atoms can join together to make molecules with very long chains of carbon atoms, and there are also compounds where carbon forms rings. As an element, carbon can bond to itself to make even more complex shapes.
Molecules containing carbon chains have other non-metal elements, usually hydrogen, oxygen and nitrogen, attached to the carbon chain. There are huge numbers of compounds that contain rings or chains of carbon atoms. There is even a whole area of chemistry that studies these compounds - this is called organic chemistry.
Humans are carbon-based organisms. Many of the molecules that are essential for human life - carbohydrates, proteins and DNA - contain carbon chains and rings.
Other carbon-based molecules are of huge importance to humans. One example is crude oil. Oil is a complex mixture of molecules, mostly compounds of hydrogen and carbon, that contain carbon chains and carbon rings. Oil is used as a fuel, but is also an important raw material for the chemical industry. It is used to make plastics, pharmaceutical drugs and a number of other synthetic molecules that we rely on.