Matter exists in different forms. The material in a door or wall is solid and impenetrable. Water has a fluid texture, and a person can move through it with some difficulty. Air is an even more tenuous fluid and does not offer much resistance as people, animals or such objects as cars move through it. Plasma, a heated form of matter found in stars, resembles air in certain respects, but has electrical properties that set it apart from such gases as nitrogen and oxygen. Despite these differences, all forms of matter take up space, even gases and plasma. In the case of air, this becomes evident when atmospheric gases take control of a funnel-shaped area known as the funnel cloud of a tornado. Moreover, all forms of matter have a property variously called weight or mass. It is easy to weigh solids or liquids, and experience reveals that gases have mass or weight. It is harder to lift a tank filled with LPG gas than an empty tank.
At one time the tiny atom was regarded as the basic indivisible constituent of matter. It is indeed a fundamental unit that enters into the composition of solids, liquids and gases, but it is not indivisible. It contains smaller constituents: positively charged particles called protons, negatively charged electrons and uncharged neutrons. Evidence has accumulated that even these three particles are not indivisible. Three quarks may be the building blocks that form protons and neutrons, and string theory postulates that even smaller strings are the basic constituents of all matter. Such particles as electrons exhibit wave-like properties, and nuclear reactions convert matter into energy. These factors must be taken into account when considering the basic composition of atomic particles.
Elements are homogeneous masses formed by the confluence of many identical atoms. Consequently, elements are simple substances incapable of chemical decomposition. Typical examples are helium, mercury and carbon. Teams of scientists in the United States, Russia and Germany have synthesized further elements, such as berkelium, hassium and dubnium.
Like elements, compounds are homogeneous forms of matter. However, whereas elements consist of a single type of atom, compounds consist of a single type of molecule. A molecule is a number of atoms chemically united to form a single unit. If all atoms that enter into the composition of a molecule are the same, such as the two oxygen atoms in molecular oxygen, the resultant substance is still an element, but if two or more kinds of atoms form a molecule, such as the two hydrogen atoms united with a single oxygen atom in the water molecule, the aggregation of such molecules results in a compound. Natural or laboratory decomposition of compounds occurs through the cleavage of the bonds that unite the disparate atoms into molecules.