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| Atoms and Molecules |
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The idea of the atom Models and mechanisms of how particles and other materials behave have been proposed for thousands of years. Especially in the last few centuries, however, these models have been constantly improved and specified. In the following chapters, a cross section of these developments will be presented, leading all the way to our present model of the atom, which will be explained along with all of the laws that govern its behaviour.The first model of matter which included elements and atoms was proposed in ancient times. The Greek philosopher Leukippos (around 500-400 B.C.) and his student Democritus (around 460-370 B.C.) were the first to describe the matter present in our world as a collection of atoms (Greek: indivisible). Their theory was based on the idea that if any body is divided into its smallest constituent parts, at some point the parts are so small that they can no longer be divided. They used the word indivisible to describe this remaining matter. According to this theory, atoms are small bodies which are not able to be divided.Atoms of different materials must differ in their composition and size. The characteristics of materials must therefore be determined by differences in their individual atoms: differences in their size, grouping and mutual arrangement. At the beginning of the 19th century, the Greek atomic model was expanded upon and specified by J. Dalton (1766-1844). According to his theory, elements are composed of small particles called atomsAtoms of individual materials differ in their mass and size. During chemical reactions, atoms themselves remain unchanged. Of course, the number and position of individual atoms in the reactant compounds can and does change. They are combined in certain proportions, only to change those combinations and proportions during a reaction. In more advanced atomic models, atoms are composed of a nucleus and electrons.The atom, of course, is composed of elementary particles. In an atom's nucleus are neutrons (uncharged) and positively charged protons. Atoms of the same element always contain the same amount of protons. Only the number of neutrons can differ slightly (in isotopes). Isotopes are actually different atoms of the same element differing only in the number of neutrons they contain and their atomic weight. Otherwise, isotopes of one element generally have the same chemical and physical characteristics as the element itself. The average atomic nucleus is relatively small compared to the atom itself, but it makes up the greatest part of an atom's mass. The mass of protons and neutrons has been designated with the relative number 1. The number of protons in an atom determines its atomic number. This number is also used to symbolize the atom, or element, in the periodic table of the elements. (hydrogen (H)=1, Helium (He)=2, etc.). Electrons (negatively charged particles) revolve around the nucleus of an atom in electronic orbitals, designated areas where they can be found. Their mass is relatively small - 1/1836 the mass of protons and neutrons. There is the same amount of electrons as the number of protons in the nucleus. For this reason, every atom, in its natural state, is neutral. Atoms can lose one or more of their electrons. When they do, they become positively charged. Or, atoms can gain electrons, which makes them negatively charged. When an atom gains or loses electrons, it is called an ion. The outer reaches of an atom, its shell, away from the inner nucleus and where electrons are found, makes up the greatest part of its size. This area is mostly empty space. Electrons move in certain designated areas around the atomic nucleus. Some electrons are closer to the nucleus than others (inner orbital, or shielded electrons). Others are further away from the nucleus (outer orbital electrons). The nucleus of an atom does not change during a chemical reaction. For this reason, it does not appear to be very important. Of course, an atom's electrons determine its chemical behaviour (mostly these are outer orbital electrons).The energy of a specific electron is defined with the help of both letters and numbers, according to the orbital where the electron is found. Of great importance is an electron's distance from the nucleus. The exact placement of an atom's electrons at any one time is impossible to determine, because location and direction of an individual electron are not able to be calculated (The Heisenberg Uncertainty Principle).The more accurately we try to determine the location of a specific electron, the less accurate is our ability to determine its direction. Why? Because it is impossible to tell which direction that electron will move in the moment we have determined its location. Unfortunately, only the probability of where an electron might be found can be calculated. On the other hand, if we know the direction an electron is moving, its exact location becomes impossible to locate. The spacial limitation, more simply the area where an electron of a certain energy can be found with greatest probability, is called the atomic orbital. ¹²5Ò³: ÉÏÒ»Ò³ 1 [2] [3] [4] [5] ÏÂÒ»Ò³
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