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Chemistry Review 9-16-09 Atoms Atoms are the basic unit of matter. They are made up of 3 subatomic particles: Protons Neutrons Electrons Elements are pure substances that consist entirely of one type of atom. Atomic number- the number of protons in an atom. Atomic mass- the number of protons and neutrons in an atom. Isotopes- atoms of the same element that differ in the number of neutrons they contain. Compounds Compounds are substances formed by the chemical combination of 2 or more elements in definite proportions. Chemical bonds hold the atoms of a compound together. Ionic bonds- formed when one or more electrons are transferred from one atom to another. Example: Na + Cl → Na+ + Cl− → NaCl Cation- atom that loses one or more electrons. Anion- atom that gains one or more electrons. Covalent bonds- forms when electrons are shared between atoms. The electrons actually travel in both orbitals. • Metal – non-metal Molecules- formed when atoms are joined by a covalent bond. Water Molecules Polarity is present when there is an uneven distribution of electrons between atoms. A water molecule has this distribution because the Oxygen has a stronger attraction for electrons than the 2 Hydrogens. This uneven distribution causes one end of a molecule to have a slightly positive charge and one end to have a slightly negative charge. These slightly opposite charges attract other water molecules and they are then joined by a Hydrogen bond. Cohesion- an attraction between molecules of the same substance. Water is extremely cohesive. Adhesion- an attraction between molecules of different substances. Mixture- a material composed of two or more elements or compounds that are physically mixed together but not chemically combined. Example: salt and pepper mixed together. Solutions- mixtures that contain ions evenly dispersed throughout the solution. Solute- the substance that is dissolved. Solvent- the substance in which the solute dissolves. Suspensions- mixtures of water and a non-dissolved material. Acids range from 0 to 7 and have a higher concentration of H+ ions. Examples: Bases range from 7 to 14 and have a higher concentration of OH- ions. Examples: Pure water has a pH of 7 and has equal concentrations of H+ ions and OH- ions. It is neutral. Buffers are weak acids or bases that can react with strong acids or bases to prevent sharp, sudden changes in pH.