Matter is anything that has mass and occupies space. Atoms and molecules are the particles of which matter is made. An atom is the smallest particle of an element that has the properties of that element. Atoms are the basic building blocks of matter. An atom consists of three types of subatomic particles: protons, neutrons, and electrons. Molecules are groups of atoms held together by chemical bonds. Some substances exist in atomic form, and others exist in molecular form. Solids, liquids, gases, and plasma are different states of matter. Note that one form of matter can be converted into another by addition or removal of energy or changes in pressure.
A solid is a state of matter that has a definite volume and a definite shape, consisting of particles that are closely packed and move only by vibrating. Dimensions can be seen clearly in a solid, and a solid is rigid in nature. Examples of solids at room temperature include sugar, salt, wood, and rocks. The atoms and molecules of solids vibrate in their respective positions. The movement is small compared to that of other states of matter.
A liquid is a state of matter that has a definite volume but not a definite shape, consisting of particles that are close together and flow freely. The atoms and molecules of a liquid have greater freedom than those of solids and continually move. Examples of liquids at room temperature are water, gasoline, and mercury.
A gas is a state of matter that has neither a definite volume nor a definite shape, consisting of particles that are far apart and move randomly to fill their container. Atoms and molecules of a gas have more freedom than those of liquids. Atoms and molecules of a gas are much farther apart than in solids and liquids, and they collide with the walls of their container. Examples of gases at room temperature are hydrogen, oxygen, and carbon dioxide.Plasma is a state of matter consisting of charged particles in a diffuse gaseous state with equal numbers of positive ions and free electrons. Particles in a plasma are in an excited state due to an applied electric field or energy addition. Plasma is similar to a gas, but instead of neutral atoms or molecules, plasma has free positively charged ions, called cations, and negatively charged electrons. Hence plasma is actually ionized gas, and it does not have a specific shape or size. Unlike gases, plasmas are electrically conductive. Examples of plasma are stars, lightning, and ionized gases, such as neon (Ne) in neon lights and mercury (Hg) in fluorescent lamps.