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Atomic Theory of Matter.

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Atomic Theory of Matter

According to modern atomic theory, all substances are made up of tiny units called atoms. Each atom consists of a massive, positively charged centre called the nucleus, around which fly one or more negatively charged electrons.  

   The nucleus itself can contain two kinds of particles: neutrons, which have no electrical charge, and positively charged protons. A neutral atom has the same number of electrons as protons, so the electrical charges cancel.  

   The identity of an atom and its atomic number is determined by the number of protons in its nucleus. For example, there is one proton in the nucleus of a hydrogen atom, so hydrogen has atomic number 1. Oxygen, with eight protons, has atomic number 8; mercury has atomic number 80; and uranium has atomic number 92.  

   Substances that are composed of only one kind of atom are called elements. Only 92 elements occur naturally on Earth. The lightest is hydrogen; the heaviest is uranium.  

   The nuclei of a given element all have the same number of protons but may have a differing number of neutrons. For example, about 99.8 percent of the oxygen nuclei in nature contain eight neutrons as well as eight protons. But a very few oxygen nuclei contain nine neutrons, and some even contain ten neutrons. Each kind of nucleus is a different isotope of oxygen. Each isotope has a different number of neutrons.  

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   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

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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.  


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.

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