Matter is commonly defined as anything which has mass and occupies space. However, diverse fields use the word in diverse and occasionally
incompatible ways; there is no single agreed scientific meaning of the term "matter".
All matter is composed of units called atoms and other particles which have mass. These atoms and particles have weight and occupy volume. Basically, atoms are made of electrically-charged particles —protons, found in the nucleus, and electrons, traveling about the nucleus. In addition, there are neutrons in the nucleus with a weight similar to protons, but lacking charge. Hydrogen is the only element which, in simple form, has no neutrons in the nucleus.
The relations of matter to energy are explored by physical scientists. Large pieces of matter, when moving, have mechanical energy. Streams of electrons from atoms carry electrical energy. Chemical changes either set free or store up energy as new groups of atoms (molecules) are formed. Some atoms such as uranium and radium aotomatically break down, letting loose certain particles along with freed energy from their nuclei.
Matter may exist in four states called solid, liquid, gas or plasma. Whether a substance exists in any of these four states or conditions depends upon the amount of heat energy possessed by it. For example, if heat energy is added to solid ice, it melts. This energy gives the molecules of water greater freedom of motion than the ice molecules had. Heat added to water at the boiling point changes water to a gas (water vapor).
The "Law of Conservation of Energy" states that "matter cannot be created or destroyed." This is amended with ". . . by ordinary means." Another version states that there exists a given amount of energy and matter, known to be interchangeable.
In physics and chemistry, matter displays both wave-like and particle-like properties, the so-called wave–particle duality.
