 average atomic mass  the weighted average of the atomic masses of the naturally occurring isotopes of an element  Because isotopes of an element have different masses, the periodic table uses an average atomic mass of each element.

periodic table  group  columns on the table (going up and down)  identifies the number of valence electrons  period  rows on the table (going across)  identifies the energy level of the valence electrons

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valence electrons  the outermost electrons in an atom’s electron cloud  determines the elements chemical properties  an atom has 8 valence electrons is considered stable, or chemically unreactive  atoms that have only one, two, or three valence electrons tend to lose electrons easily  atoms that have from four to seven valence electrons are more likely to gain electrons

valence electrons & the periodic table  an element’s number of valence electrons can be identified by its location on the periodic table

combining atoms  elements rarely occur in their pure form  compound a substance made up of atoms of two or more different elements joined by chemical bonds  molecule a group of atoms that are held together by chemical forces; a molecule is the smallest unit of matter that can exist by itself and retain all of a compound’s chemical properties

examples of compounds H2OH2O  CO 2  NH 3  NaCl

chemical formula  a combination of letters and numbers that shows which elements make up a compound and the number of atoms of each element that are required to make a molecule of a compound  In a chemical formula, the subscript that appears after the symbol for an element shows the number of atoms of that element that are in a molecule.  For example: H 2 O = 2 H (hydrogen atoms) + 1 O (oxygen atom)

chemical equation  a formula which shows how compounds or elements combine during a chemical reaction  reactants on the left, products on the right  Example: CH 4 + 2O 2 CO 2 + 2H 2 O methane + oxygen yields carbon dioxide + water

balanced equation  An equation is balanced when…  the number of atoms of each element on the right side of the equation is equal to the number of atoms of the same element on the left side  To balance an equation, you must put numbers called coefficients in front of chemical formulas.  A coefficient multiplies the subscripts in an equation.