As previously stated, all matter is composed of atoms that which are capable of uniting to form chemical compounds. The various forms of matter can be summarized from a chemical point of view as follows.

1.Molecules are groups or clusters of atoms held together firmly by means of chemical bonding. There are two general types of molecules.

a.Molecule of an element - Two single atoms of the same element, in certain cases, can become fastened to one another by a chemical bond to form a molecule. Examples of this are hydrogen (H₂), oxygen (O₂), and bromine (Br₂). Most gaseous elements exist as molecules of two atoms.

b.Molecules of a compound - A compound contains at least two different kinds of atoms. Molecules are the ultimate particles of chemical compounds. Examples of compounds are hydrogen chloride (HCI), water (H₂O), methane (CH₄), and ammonia (NH₃).

2.Elements are substances that cannot be decomposed by ordinary types of chemical change nor made by chemical union.

3.Compounds are substances containing more than one constituent element and having properties different from those of the individual elements. The composition of a specific compound is always definite.

4.Mixtures consist of two or more substances intermingled with no constant percentage composition. Each component retains its original properties.