Smallest configuration of an element or compound that can exist independently. One molecule is made up of a group of atoms held together by covalent or ionic bonds. Several non-metal elements exist as molecules. For example, hydrogen atoms, at room temperature, do not exist independently. They are bonded in pairs to form hydrogen molecules. A molecule of a compound consists of two or more different atoms bonded together. For example, carbon dioxide is made up of molecules, each containing one carbon and two oxygen atoms bonded together. The molecular formula is made up of the chemical symbols representing each element in the molecule and numbers showing how many atoms of each element are present. For example, the formula for hydrogen is H2, and for carbon dioxide is CO2. Molecules vary in size and complexity from the hydrogen molecule to the large macromolecules of proteins. In general, elements and compounds with molecular structures have similar properties. They have low melting and boiling points, so that many molecular substances are gases or liquids at room temperatures. They are usually insoluble in water and do not conduct electricity even when melted.
Molecules may be held together by ionic bonds, in which the atoms gain or lose electrons to form ions, or by covalent bonds, where electrons from each atom are shared to form the bond.
Kinetic theory of matter
According to the molecular or kinetic theory of matter, matter is made up of molecules that are in a state of constant motion, the extent of which depends on their temperature. Molecules also exert forces on one another. The nature and strength of these forces depends on the temperature and state of the matter (solid, liquid, or gas).
The existence of molecules was first inferred from the Italian physicist Amedeo Avogadro's hypothesis in 1811. He observed that when gases combine, they do so in simple proportions. For example, exactly one volume of oxygen and two volumes of hydrogen combine to produce water. He hypothesized that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. Avogadro's hypothesis only became generally accepted in 1860 when proposed by the Italian chemist Stanislao Cannizzaro.
The movement of some molecules can be observed with a microscope. As early as 1827, Robert Brown observed that very fine pollen grains suspended in water move about in a continuously agitated manner. This continuous, random motion of particles in a fluid medium (gas or liquid) as they are subjected to impact from the molecules of the medium is known as Brownian movement. The spontaneous and random movement of molecules or particles in a fluid can also be observed as diffusion, occurring from a region in which they are at a high concentration to a region of lower concentration, until a uniform concentration is achieved throughout. No mechanical mixing or stirring is involved. For example, if a drop of ink is added to water, its molecules will diffuse until the colour becomes evenly distributed.
Kinetic theory of gases
The effects of pressure, temperature, and volume on a gas were investigated during the 17th and 18th centuries. Boyle's law states that for a fixed mass of gas the volume of the gas is inversely proportional to the pressure at constant temperature. Charles's law states that for a fixed mass of gas the volume of the gas is proportional to the absolute temperature at constant pressure. The pressure law states that the pressure of a fixed mass of gas at constant volume is directly proportional to its absolute temperature. These statements together give the gas laws which can be expressed as: (pressure × volume)/temperature = constant . Gases behave in good agreement with the gas laws over a wide range of temperatures and pressures. A plot of the volume of a gas against its temperature gives a straight line, showing that the two are proportional. The line intercepts the temperature axis at −273°C/−459°F. This suggests that, if the gas did not liquefy first, it would occupy zero volume at a temperature of −273°C. This temperature is referred to as absolute zero, or zero kelvin (0 K) on the Kelvin scale, and is the lowest temperature theoretically possible. The gas laws are an idealization applying strictly only to ideal gases, the molecules of which are assumed to occupy negligible volume and to exert negligible forces on each other. A real gas behaves rather differently, and van der Waals' law contains a correction to the gas laws to account for the non-ideal behaviour of real gases.
Change of state
As matter is heated its temperature may rise or it may cause a change of state. As the internal energy of matter increases the energy possessed by each particle increases too. This can be visualized as the kinetic energy of the molecules increasing, causing them to move more quickly. This movement includes both vibration within the molecule (assuming the substance is made of more than one atom) and rotation. A solid is made of particles that are held together by forces. As a solid is heated, the particles vibrate more vigorously, taking up more space, and causing the material to expand. As the temperature of the solid increases, it reaches its melting point and turns into a liquid. The particles in a liquid can move around more freely but there are still forces between them. As further energy is added, the particles move faster until they are able to overcome the forces between them. When the boiling point is reached the liquid boils and becomes a gas. Gas particles move around independently of one another except when they collide. Different objects require different amounts of heat energy to change their temperatures by the same amount. The heat capacity of an object is the quantity of heat required to raise its temperature by one degree. The specific heat capacity of a substance is the heat capacity per unit of mass, measured in joules per kilogram per kelvin. As a substance is changing state while being heated, its temperature remains constant. For example, water boils at a constant temperature as it turns to steam. The energy required to cause the change of state is called latent heat. This energy is used to break down the forces holding the particles together so that the change in state can occur. Specific latent heat is the thermal energy required to change the state of a certain mass of that substance without any temperature change. Evaporation causes cooling as a liquid vaporizes. Heat is transferred by the movement of particles (that possess kinetic energy) by conduction, convection, and radiation. Conduction involves the movement of heat through a solid material by the movement of
free electrons. Convection involves the transfer of energy by the movement of fluid particles. Convection currents are caused by the expansion of a liquid or gas as its temperature rises. The expanded material, being less dense, rises above colder and therefore denser material.
The size and shape of molecules
The shape of a molecule profoundly affects its chemical, physical, and biological properties. Optical isomers (molecules that are mirror images of each other) rotate plane polarized light in opposite directions; isomers of drug molecules may have different biological effects; and enzyme reactions are crucially dependent on the shape of the enzyme and the substrate on which it acts.
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