1g- molecules and molecular ions are represented by chemical formula A molecular formula is a concise description of two characteristics of the molecule: the atomic composition indicated by the symbols, and the ratios in which the atoms are combined indicated by a numeric subscript. 1-h Chemical reactions can be explained with the atomic model A chemical reaction is a process that transforms one set of chemical substances into another. Most of the chemistry is about the reorganization of electrons in chemical reactions while the atomic nuclei remain unchanged. As a consequence the substances resulting from the reaction (the products) are different than the substances at the beginning of the reaction (the reactants) but the number of each type of atom remain unchanged. The number of protons and neutrons can also change in nuclear reactions such as fusion, fission, bombardment by high energy particles or radiation of some type of decay, but this is beyond the scope of the course. If the nuclear reaction results in a change of the number of protons the reaction will produce a different type of atom. Electrons that are the furthest from the nucleus can be transferred to another atom or shared between atoms. By this mechanism atoms can be bonded into molecules or other complexes such as ionic compounds. 1-i Reactions are represented by balanced chemical equations A chemical equation is, like in mathematics, a compact notation replacing a complex statement. The idea is to identify the number of molecules of reactants and the number of molecules of products in such a way that for each element there is an identical number of atoms on the reactant side than on the product side. The reactants are indicated to the left of an arrow (or a double arrow if the reaction leads to an equilibrium) and the products to the right. Balancing a chemical equation is a trial and error process requiring a little bit of practice by the beginner chemist.
2- Management of amounts in chemistry The microscopic study of atoms and molecules explains the qualitative behavior of matter but we often need to understand the changes of amount, the quantitative behavior. This requires the ability to measure. 2a- Atomic mass Mass is a mechanical concept. It is the property of a physical object that quantifies the amount of matter it is equivalent to. 2a- The mass of an atom can be measured If we strip an atom from one or more electrons and send it through a magnetic field the path of the positively charged atom will change according to its mass. Such a measuring device is called mass spectrometer. The mass of a nuclide is close to its mass number but not identical. This is due to a phenomenon called mass defect, a loss of some of the baryonic mass into binding energy.
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