Presentation on theme: " average atomic mass the weighted average of the atomic masses of the naturally occurring isotopes of an element Because isotopes of an element have."— Presentation transcript:
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
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 H2OH2O 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.