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1. A molecule is a group of at least two atoms chemically combined in definite

proportions.

For example, a water molecule (H2O) is always composed of two atoms of hydrogen chemically combined with one of oxygen. (a 2 hydrogen :1 oxygen proportion)

If you have a combination of three hydrogen atoms with one oxygen atom

(3 hydrogen:1 oxygen ) , the combination is notwater;this is what is meant by defininte proportions.

2. Molecules are held together by covalent bonds;i.e., the sharing of pairs of electrons holds the atoms together. Also, molecules are held together by columbic forces or forces of electrostatic attraction in case of ionic compounds.

Additional Note : * If different kinds of atoms combine to form a molecule, the

substance formed is called a compound.

An example of a compound is water.

* If atoms of the same kind combine to form a molecule, the substance is

called an element.

Example , oxygen (O2)

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14y ago

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