Learning Chemical Formulas

Chemical Formulas The subscript to the right of the symbol of an element tells the number of atoms of that element in one molecule of the compound. H 2 O 2 atoms H 1 atom O

Ions When atoms lose or gain electrons, they become ions. Cations are positive and are formed by elements on the left side of the periodic chart. Anions are negative and are formed by elements on the right side of the periodic chart.

Ionic Bonds Ionic compounds (such as NaCl) are generally formed between metals and nonmetals.

Writing Formulas Because compounds are electrically neutral, one can determine the formula of a compound this way: The charge on the cation becomes the subscript on the anion. The charge on the anion becomes the subscript on the cation. If these subscripts are not in the lowest whole-number ratio, divide them by the greatest common factor.

Simple Ionic Compounds The overall charge of ionic compounds must equal zero. Examples : Na 1+ Cl 1- Ca 2+ Br 1- Al 3+ O 2-

Polyatomic Ion Formulas Concept: Polyatomic ions are groups of atoms that behave as one unit. Some ions have more that one atom but their overall charge can be determined. Examples: (SO 4 ) = S +6 and O -8 = -2 (NO 3 )= N +5 and O -6 = -1 (NH 4 ) = N -3 and H +4 = +1

Polyatomic Ion Formulas These group ions, polyatomic ions, are treated like single ions in formulas, but must have parentheses when more than one is used in a formula. Examples: Ca 2+ (NO 3 ) 1- 2 Ga 3+ (SO 4 ) 2- (NH 4 ) 1+ 2 O 2- The polyatomic ions have oxygen and another nonmetal are often named "____ate" with the root of the other nonmetal in the blank. Examples: (NO 3 ) 1- is nitrate (SO 4 ) 2- is sulfate (ClO 3 ) 1- is chlorate

Transition Metal Compounds Transition metals have electrons in d orbitals and can donate different numbers of electrons, thus giving them several different positive charges. These can be determined from the Roman numeral which is written next to the metal's name. Example: Cu 1+ is Copper I Pb 2+ is Lead II Fe 3+ is Iron III Sn 4+ s Tin IV