Atoms, elements and compounds

The basic building block of all matter is called an atom. Atoms are a collection of various subatomic particles containing negatively charged electrons, positively charged protons and neutral particles called neutrons. Each element has its own unique number of protons, neutrons and electrons. Both protons and neutrons have mass, whereas the mass of electrons is negligible. Protons and neutrons exist at the centre of the atom in the nucleus. 

Electrons move around the nucleus, and are arranged in shells at increasing distances from the nucleus. These shells represent different energy levels, the outermost shell being the highest energy level. 

Chemistry for Pharmacy Students Satyajit D Sarker and Lutfun Nahar 2007 John Wiley Sons, Ltd. 

Nucleus is tiny relative to the size of the electron cloud 

The number of protons that an atom has in its nucleus is called the atomic number. The total number of protons and neutrons in the nucleus of an atom is known as the mass number. For example, a carbon atom containing six protons and six neutrons has a mass number of 12. 

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All matter exists as either elements, compounds, or mixtures. An element consists of only one type of atom. A compound contains two or more elements in chemical combination it exhibits different properties from its component elements. The elements of a compound occur in fixed parts by mass because each unit of the compound has fixed numbers of each type of atom. Elements and compounds are referred to as substances because their compositions are fixed. A mixture consists of two or more substances mixed together, not chemically combined. The components retain their individual properties and can be present in any proportion. 

One of the most significant sources of change in isotope ratios is caused by the small mass differences between isotopes and their effects on the physical properties of elements and compounds. For example, ordinary water (mostly Ej O) has a lower density, lower boiling point, and higher vapor pressure than does heavy water (mostly H2 0). Other major changes can occur through exchange processes. Such physical and kinetic differences lead to natural local fractionation of isotopes. Artificial fractionation (enrichment or depletion) of uranium isotopes is the basis for construction of atomic bombs, nuclear power reactors, and depleted uranium weapons. 

Fuger, J. Oetting, F.L. "The Chemical Thermodynamics of Actinide Elements and Compounds. Part 2. The Actinide Aqueous Ions" International Atomic Energy Agency Vienna, 1976. 

While Lavoisier had established a rational system for naming elements and compounds, Frankland developed the system that we use today for writing chemical formulas and for depicting the bonds between the atoms in molecules. As Frankland synthesized more and more isomers, compounds with the same formulas but different molecular structures, he found traditional formulas confusing they showed the types and numbers of elements but provided no clue as to how the atoms were arranged inside the molecule. To remedy the problem, Frankland depicted the atoms in functional groups and drew lines between them to indicate the bonds between the elements. 

An internationally accepted chemical notation makes use of symbols to represent elements and compounds, and advises on naming chemical compounds. In this notation, the elements are represented by one or two letters, many of which are drawn from the elements Latin or Greek names. The number of atoms of an element in a molecule is represented by a subscript written after the symbol thus Au (the first two letters of aurum, the Latin name for gold) represents an atom of gold Cu (the first two letters of cuprum, the Latin name for copper), an atom of copper and C (the first letter of carbon), an atom of carbon O represents an atom of oxygen and 02, a molecule of oxygen. The symbols listed below provide examples of the presently accepted form of chemical notation ... 

A chemical equation describes a chemical reaction in many ways as an empirical formula describes a chemical compound. The equation describes not only which substances react, but the relative number of moles of each undergoing reaction and the relative number of moles of each product formed. Note especially that it is the mole ratios in which the substances react, not how much is present, that the equation describes. In order to show the quantitative relationships, the equation must be balanced. That is, it must have the same number of atoms of each element used up and produced (except for special equations that describe nuclear reactions). The law of conservation of mass is thus obeyed, and also the "law of conservation of atoms. Coefficients are used before the formulas for elements and compounds to tell how many formula units of that substance are involved in the reaction. A coefficient does not imply any chemical bonding between units of the substance it is placed before. The number of atoms involved in each formula unit is multiplied by the coefficient to get the total number of atoms of each element involved. Later, when equations with individual ions are written (Chap. 9), the net charge on each side of the equation, as well as the numbers of atoms of each element, must be the same to have a balanced equation. The absence of a coefficient in a balanced equation implies a coefficient of 1. 

Determine the atomic weight of the unknown element for each case, identify the element and compound.. 

Even better agreement is observed between calorimetric and elastic Debye temperatures. The Debye temperature is based on a continuum model for long wavelengths, and hence the discrete nature of the atoms is neglected. The wave velocity is constant and the Debye temperature can be expressed through the average speed of sound in longitudinal and transverse directions (parallel and normal to the wave vector). Calorimetric and elastic Debye temperatures are compared in Table 8.3 for some selected elements and compounds. 

Elements and compounds constitute the world of pure substances. An element is a substance that cannot be decomposed by any chemical reaction into simpler substances. Elements are composed of only one type of atom and all atoms of a given type have the same properties. Pure substances cannot be separated into other kinds of matter by any physical process. We are familiar with many pure substances water, iron, mercury, iodine, helium, rust, diamond, table salt, sugar, gypsum, and so forth. Among the pure substances listed above, iron, mercury, iodine, diamond (pure carbon), and helium are elements. We are also familiar with mixtures of pure substances. These include the air that we breathe, milk, molasses, beer, blood, coffee, concrete, egg whites, ice cream, dirt, steel, and so on. 

Jons Jakob Berzelius (1779-1848), a Swedish chemist, is also considered one of the founders of modern chemistry. He prepared, purified, and identified more than 2,000 chemical elements and compounds. He also determined the atomic weight (mass) of several elements and replaced pictures of elements with symbols and numbers, which is the basis of our chemical notations today. 

The free-electron gas model is a good starting point for the sp-valent metals where the loosely bound valence electrons are stripped off from their ion cores as the atoms are brought together to form the solid. However, bonding in the majority of elements and compounds takes place through saturated... 

We should be able to calculate the potential energy of atoms in elements and compounds, once we have determined the magnitude of the forces acting between the atoms as a function of the distance by which the atoms are separated from each other in the elements and... 

Morveau appendices containing the nomenclature of some compound substances, which combine sometimes like simple bodies a memoir by de Fourcroy, explaining the tables of nomenclature (thirty-seven octavo pages) a directory of the new nomenclature in ninety-four pages, and the symbols prepared by Ilassenfratz and Adet, a chemical shorthand by which the names of elements and compounds could be replaced by symbols. This system never came into general use, and symbols, in so far as they were used by chemists, were of the already developed systems, until Dalton s concept of the atomic weights and symbols had been simplified by Berzelius (in 1815) into the system still in use. 

It is difficult to draw sharp boundaries between compounds containing simple Au—M bonds, compounds containing a gold atom bound to two or more transition elements, and compounds containing two or more gold atoms and one or more transition metal atoms. Thus compounds with gold-transition metal bonds may be found in Sections 55.21.2, 55.21.3 and in the following sections. 

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