In addition to the 92 naturally occurring elements, scientists have synthesized more than a dozen others that are not found in nature. These are called the transuranium elements because they are all made of atoms that have more mass than the uranium atom. All transuranium elements, therefore, have atomic numbers greater than 92. Transuranium elements through atomic number 107 have been synthesized, though many of them live only a fraction of a second. Soviet scientists have reported evidence of the elements through number 110 but this remains unconfirmed. British scientists believe they have observed element number 112. If this is so, the atom of this new element would be the most massive ever observed.
Substances that are composed of more than one kind of atom are either compounds or mixtures. The atoms in compounds join together chemically to form molecules. Molecules are held together by electrical forces between one or more electrons of one atom and the nucleus of another atom.
For example, two atoms of hydrogen and one atom of oxygen share electrons and form a water molecule. The chemical symbol for water, H2O, denotes this combination.
The atoms or molecules in a mixture intermingle with one another but are not joined chemically. Salt water is a kind of mixture called a solution. Salt ions (electrically charged fragments of molecules) spread throughout the water.
Regardless of whether water is in the solid, liquid, or gaseous state, its molecules always consist of one atom of oxygen and two atoms of hydrogen. The difference between solid water, liquid water, and gaseous water has nothing to do with the chemistry of water. Instead, this difference depends on which of two kinds of energy is larger: the binding energy associated with the attraction between molecules or the disruptive thermal energy.
Atomic Theory and the States of Matter
A certain amount of attraction exists between all molecules. If repulsive forces are weaker than these intermolecular attractive forces, the molecules stick together. However, molecules are in constant motion because of their thermal, or heat, energy. As the temperature of a substance increases, this molecular motion becomes greater. As the temperature decreases, it becomes smaller.
In a solid, the intermolecular attractive forces overcome the disruptive thermal energies of the molecules. The molecules are bound together in a rigid, orderly arrangement called a crystal. Many physicists do not regard amorphous substances, such as glass, cold butter, and certain plastics, as true solids but as extremely viscous liquids.
Although the molecules in a crystal are held rigidly in place, they still vibrate because of their thermal energy. It may be difficult to think of ice as having heat energy, but even in ice the water molecules, though held firmly in the crystal lattice, vibrate around an equilibrium position. This vibrational motion is an expression of the thermal energy of ice.
As the temperature of a solid is increased, its molecules vibrate with greater and greater energies until they gain enough vibrational energy to overcome the intermolecular attractive forces. They then break loose from their fixed positions in the crystal lattice and move about more or less freely. The substance now assumes the shape of its container but maintains a constant volume. In other words, the substance has melted and is now a liquid.
Melting is a change of state, or a phase change. The temperature at which melting takes place varies from substance to substance (water and iron melt at different temperatures) but is the same for a given material at a given pressure (at atmospheric pressure water always melts at 32o F). One reason amorphous substances are not considered solids are that they do not have definite melting points. As the temperature is raised, they simply flow more easily.
Phase changes can work in reverse. If the temperature of a liquid is gradually decreased, a point is eventually reached at which the intermolecular forces are strong enough to bind the molecules despite the disruptive thermal motions. Then a crystal forms: the substance has frozen. The temperature at which this liquid-to-solid phase change takes place is the freezing point. The freezing point of a substance occurs at the same temperature as its melting point.
This theory of matter can also explain the liquid-to-gas change of state, a process called vaporization. As heat is applied to a liquid, some molecules gain sufficient thermal energy to overcome the intermolecular attraction--surface tension--exerted by molecules at the surface of the liquid. These high-energy molecules break free from the liquid and move away. Such molecules are now in the gaseous state. As more heat is applied, more molecules gain enough energy to move away until at a temperature called the boiling point of the liquid all the molecules can gain enough energy to escape from the liquid state.
The average distance between molecules in the gaseous state is extremely large compared to the size of the molecules, so the intermolecular forces in a gas are quite weak. This explains why a gas fills the entire volume of its container. Since intermolecular forces are so small, a gas molecule moves until it strikes either another gas molecule or the container wall. The net effect of the many molecules striking the container walls is observed as pressure.
Sometimes a substance will pass directly from the solid state to the gaseous state without passing through the liquid state. This process is called sublimation. Dry ice (solid carbon dioxide) sublimates at atmospheric pressure. Liquid carbon dioxide can form if the gas is subjected to over five times atmospheric pressure.
The Fourth State of Matter
At extremely high temperatures atoms may collide with such force that electrons are knocked free from the nuclei. The resulting mixture of free negative and positive particles is not a gas according to the usual definition. Such material is called plasma. Some scientists consider the plasma state to be a fourth state of matter. Actually, about 99 percent of the known matter in the universe is in the plasma state. In stars, matter is hot enough, and in interstellar space it is diffuse enough, for the electrons to be completely separated from the nuclei. From an astronomical standpoint, somewhat unusual conditions exist on Earth, where plasmas are difficult to produce.
The universe may contain matter in forms other than the four states. Dying stars are thought to collapse into conditions far too dense to be reproduced on Earth. White dwarf stars, for example, may contain the equivalent of the mass of the entire sun packed into the volume of the Earth. Pulsars are believed to be rapidly spinning neutron stars. In a neutron star a mass equal to that of the sun would be compressed within a sphere less than 20 miles (32 kilometres) in diameter. In the most extreme case, matter becomes so dense that its gravitational attraction pulls in all matter and radiation within a certain critical distance. Since no light can escape from such a collapsed star, it is called a black hole. In 1994, the Hubble Space Telescope found the first conclusive evidence for the existence of a black hole, at the centre of galaxy M87, 50 million light-years from Earth.
GRAVITATION AND INERTIA
A completely different way of approaching matter is based on the concepts of inertia and gravitation. Matter can be defined as anything that has inertia and that experiences an attractive force when in a gravitational field.
Inertia
Isaac Newton's first law of motion describes inertia. A body at rest tends to remain at rest; a body in motion tends to keep on moving at the same speed and in a straight line. In order to move a resting body or to stop a moving body, some effort, called a force, is required. The tendency of a body to remain at rest or, once moving, to remain in motion is inertia.
The inertia of a body is related to its mass. More massive bodies possess greater inertia than less massive bodies. A body's mass can be measured by exerting a force on the body and observing the acceleration that results. Newton's second law of motion states that the mass (M) is equal to the force (F) divided by the acceleration
In principle, this measurement can be made anywhere.