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

page 84-85

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

moleculea 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

CO2

NH3

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:H2O = 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: CH4 + 2O2 CO2 +

2H2O

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.