Definitions

• Anion:    A negatively charged ion, formed by addition of electrons to atoms or molecules (Reduction).

  Group Number Number of Electrons Gained Charge of Anion Formed IV 4 -4 V 3 -3 VI 2 -2 VII 1 -1 VIII 0 no tendency to form anions

• Cation:    A positively charges ion, formed by removal of electrons from atoms or molecules (Oxidation).

  Group Number Number of Electrons Lost Charge of Cation Formed I 1 +1 II 2 +2 III 3 +3 IV 4 +4

• Compound:    A chemical combination of atoms of different elements to form a substance in which the ration of combining atoms remains fixed and is specific to that substance.  The constituent atoms cannot be separated by physical means; a chemical reaction is required for the compounds to be formed or to be changed.  The existence of a compound does not necessarily imply that it is stable.  Many compounds have lifetimes less than a second.
  

• Electron:    An elementary particle of negative charge (- 1.602 x 10-19 C), and rest mass (9.109 x 10-31 kg).  Electrons are present in all atoms in shells around the nucleus.
  
  
• Empirical Formula:    The formula of a compound showing the simplest ratio of the atoms present.  The empirical formula is the formula obtained by experimental analysis of a compound and it can be related to a molecular formula only if the molecular weight is known.  For example, P₂O₅ is the empirical formula of phosphorus (V) oxide although its molecular formula is P₄O₁₀.
  
  
• Ion:    An atom or molecule that has a negative or positive charge as a result of losing or gaining one or more electrons.
  
  
• Ionic Bond (Electrostatic or Electrovalent Bond):    A binding force between the ions in compounds in which the ions are formed by complete transfer of electrons from one element to another element.  For example, Na + Cl becomes Na⁺¹ and Cl^-1. 
  
  
• Ionic Crystal:    A crystal composed of ions of two or more elements.  The positive and negative ions are arranged in definite patterns and are held together by electrostatic attraction.  Common examples are sodium chloride and cesium chloride.
  

• Ionization:     The process of producing ions.   In certain chemical reactions ionization occurs by transfer of electrons; for example, sodium atoms and chlorine atoms react to from sodium chloride, which consists of sodium ions (Na⁺¹) and chloride ions (Cl^-1).
  

• Lattice:    A regular three-dimensional arrangement of points.  a lattice is used to describe the position of the particles (atoms, ions, or molecules) in a crystalline solid.  The lattice structure can be examined by x-ray diffraction techniques.
  

• Metals and Non-Metals:    The metals and nonmetals are characterized by distinctly different physical and chemical properties. A characteristic difference involves their behavior during oxidation-reduction reactions. In these reactions metals always lose electrons, while nonmetals always gain electrons. One can quickly distinguish between metals and nonmetals based on their placement on the Periodic Table of the Elements. The metals are those elements (with the exception of hydrogen) that are found to the left of a group of elements referred to as the metalloids . The metalloids are a group of elements which have properties similar to both the metals and nonmetals. These metalloids are: Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium, and Polonium. The nonmetals are those elements found to the right of these metalloids, including the element, hydrogen.

• Molecular Formula:    The formula of a compound showing the number and types of the atoms present in a molecule, but not the arrangement of the atoms.  For example, C₂H₆O represents the molecular formula both of ethanol (C₂H₅OH) and methoxymethane (CH₃OCH₃). The molecular formula can be determined only if the molecular weight is known.

• Oxidation:    An atom, an ion, or a molecule is said to undergo oxidation or to be oxidized  when it loses electrons. 
  

• Oxidation State:    The number of electrons lost or effectively lost by the neutral atom, i.e. oxidation number.
  

• Periodic Table:    A table of the elements arranged in order of increasing proton number to show similarities in chemical behavior between elements.  Horizontal rows of elements are called periods.  Across a period there is a general trend from metallic to non-metallic behavior.  Vertical columns of related elements are called groups.  Down a group there is an increase in atomic size and in metallic (electropositive) behavior. (Los Alamos National Lab: Periodic Table of the Elements: http://www.we.fh-osnabrueck.de/fbwe/periodic/index.html )

• Redox:    Relating to the process of oxidation and reduction, which are intimately connected in that during oxidation by chemical agents the oxidizing agent itself becomes reduced, and vice versa.  Thus an oxidation process is always accompanied by a reduction process. 
  

• Reduction:    The gain of electrons by such species as atoms, molecules, or ions.  It often involves the loss of oxygen from a compound, or addition of hydrogen. 

• Representative Elements:     The Groups or Families can be further categorized into the Representative Elements and the Transition Metals. The Representative Elements are those elements within the first two families (Groups I and II on the far left) and the last six families or groups (on the right) of the Periodic Table. The Transition Metals are the elements in those Groups within the middle of the Table.

• Shell:    A group of electrons that share the same principal quantum number.  Early work on x-ray emission studies used the terms K, L, M, and these are still sometimes used for the first three shells: n = 1 K-shell; n = 2 L-shell;
  n = 3 M-shell.
  

• Transition Elements:     A class of elements occurring in the periodic table in three series: from scandium to zinc; from yttrium to cadmium; and from lanthanum to mercury.  The transition elements are all metals.  They owe their properties to the presence of d electrons in the atoms.  Transition elements have certain characteristic properties resulting from the existence of d levels:
          1. They have variable valences - i.e. they can form compounds with the metal in different oxidation states
               (Fe⁺² and Fe⁺³, etc.)
          2. They form a vast number of inorganic complexes, in which coordinate bonds are formed to
              the metal atom or ion.  
          3. Compounds of transition elements are often colored. 
  

• Valence Electrons:    Valence electrons are defined as electrons that are found in the outermost energy levels of an atom. The number of valence electrons in any atom of a Representative Element corresponds to its Group Number. All elements within a Group or Family, have the same number of electrons in their outermost energy levels. This outermost energy level is referred to as an element's valence shell, as sketched below.

  The product of the valence and the number of atoms of each element in a compound must be equal.  For example, in Al2O3 for the two aluminum atoms (valence 3) the product is 6 and for the three oxygen atoms (valence 2) the product is also 6. The valence of an element is generally equal to either the number of valence electrons or eight minus the number of valence electrons.   Transition metal ions display variable valence.

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