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Chemistry I Notes - Chemistry I Notes Chapter One Water A...

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Chemistry I Notes Chapter One- Water: A Natural Wonder The phases of matter include solids, liquids and gases. Density is the amount of matter that occupies a given volume. It’s units are g/mL and is considered a physical property. A solid has a definite volume and shape that does not depend on the size and shape of the container. Molecules in a solid occupy fixed positions in an orderly closely packed array. A liquid has a definite volume but takes the shape of its container. Molecules in a liquid can move about from place to place but stay close together. A gas takes the volume and shape of its container. The density of a pure gaseous sample varies with the pressure and temperature of the gas because the volume of the sample depends strongly on these variables. Molecules in a gas are far apart and move almost independently of one another. The densities of solids and liquids also vary with temperature, but the variation is much smaller than for gases. The variation of solid and liquid densities with pressure is usually negligible. The melting point and the boiling point are the temperatures at which, respectively, the solid to liquid and liquid to gas phases occur for a pure substance at a pressure of one atmosphere. We often refer to both liquids and solids as condensed phases because the molecules are more tightly packed together than in the gas phase. Every atom is made up of a positively charged nucleus surrounded by negatively charged electrons. The nucleus consists of particles called protons and neutrons. The periodic table arranges all known elements into rows (called periods) and columns (called groups). Core electrons are nearest the nucleus and are so strongly attracted to the nucleus that they never interact with other atoms. The nucleus plus the core electrons are called the atomic core. Valence electrons are farther from the nucleus and more weakly attracted than the core electrons because the attraction is mainly from the core charge, which is less than the full nuclear charge. Valence electrons are responsible for interactions of atoms with one another. The valence shell diameters of the elements increase in size from the top to the bottom (smaller to larger atomic numbers) of a group. However, across a period from left to right, the sizes of the atoms generally decrease despite the fact that the numbers of electrons increase. Remember that the nuclear charge is also
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increasing across a period; the increased charge attracts all the electrons more strongly and draws them all closer to the nucleus. Line formulas are written on a single line of test. They provide no explicit information about the way the atoms are connected to each other.
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