Writing Chemical Formulae Copyright Sautter 2003

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CHEMICAL FORMULAE* HOW TO WRITE FORMULAS FROM NAMES AND NAMES FROM FORMULAS* * SOME BOOKS USE FORMULAE ENDING IN AE WHICH IS THE LATIN PLURAL INSTEAD OF FORMULAS

TYPE OF COMPOUNDS (1) IONIC COMPOUNDS – THOSE CONSISTING PRIMARILY OF METALS COMBINED WITH NONMETALS (A METAL COMBINED WITH A NONMETAL IS CALLED A SALT. SODIUM CHLORIDE OR “TABLE SALT” IS NOT THE ONLY SALT, THERE ARE MANY DIFFERENT KINDS OF SALTS. IT HOWEVER IS ONE OF THE MOST COMMON SALTS.) (2) MOLECULAR COMPOUNDS – THOSE CONSISTING OF NONMETALS

WHAT ARE IONS? CHARGED ATOMS OR GROUPS OF ATOMS THERE ARE TWO KINDS OF IONS, POSITIVE AND NEGATIVE POSITIVE IONS ARE ATOMS OR GROUPS OF ATOMS WITH MISSING ELECTRONS THESE IONS USUALLY CONSIST OF METALS THEY ARE ALSO CALLED CATIONS NEGATIVE ION ARE ATOMS OR GROUPS OF ATOMS WITH EXTRA ELECTRONS THESE IONS USUALLY CONSIST OF NONMETALS THEY ARE ALSO CALLED ANIONS (NOT DOG IONS AS YOU MIGHT EXPECT !!)

SOME COMMON CATIONS (POSITIVE IONS) NAME SODIUM ZINC POTASSIUM CALCIUM MAGNESIUM COPPER II (CUPRIC) IRON II (FERROUS) IRON III (FERRIC) AMMONIUM* *ONE OF THE FEW POLYATOMIC CATIONS FORMULAS Na+1 Zn+2 K+1 Ca+2 Mg+2 Cu+2 Fe+2 Fe+3 NH4+1

SOME COMMON ANIONS (NEGATIVE IONS) NAMES CHLORIDE SULFIDE BROMIDE SULFATE SULFITE NITRATE NITRITE CARBONATE CHROMATE FORMULAS Cl-1 S-2 Br-1 SO4-2 SO3-2 NO3-1 NO2-1 CO3-2 CrO4-2

SOME RULES REGARDING CATIONS (POSITIVE IONS) IONS FROM COLUMN I (ALKALI METALS) ON THE PERIODIC TABLE ARE ALWAYS +1 IONS FROM COLUMN II (ALKALINE EARTH METALS) ON THE PERIODIC TABLE ARE ALWAYS +2 IONS FROM COLUMN II (THE ALUMINIUM GROUP) ON THE PERIODIC TABLE ARE +3 THE TRANSITIONAL METALS HAVE VARIABLE CHARGES RANGING FROM +1 TO +5 EXAMPLES: COLUMN I COLUMN II Na+1 Ca+2 K+1 Mg+2 etc. etc. COLUMN III Al+3 Ga+3 TRANSITIONAL METALS Cu+2 Fe+2 Cr+3

TRANSITIONAL METALS WITH VARIABLE CHARGE STATES SOME TRANSITIONAL METALS CAN EXSIST AS IONS IN TWO DIFFERENT CHARGE STATES. SOME OF THESE ELEMENTS ARE COPPER, TIN, IRON, COBALT, LEAD, MERCURY, CHROMIUM AND MANGANESE. * THESE VARIABLE CHARGES HAVE NO PATTERN AND MUST BE MEMORIZED EXAMPLES: COPPER +1 OR +2 TIN +2 OR +4 IRON +2 OR +3 COBALT +2 OR +3 LEAD +2 OR +4 MERCURY +1 OR +2 CHROMIUM +2 OR +3 MANGANESE +2 OR +3 * THE SAME ATOM WITH DIFFERENT CHARGE STATES ARE DESIGNATED USING ROMAN NUMERIALS SUCH AS COPPER I FOR Cu+1 OR LEAD II FOR Pb+2

RULES REGARDING FORMULAS AND NAMES FOR ANIONS (NEGATIVE IONS) ANIONS FROM COLUMN VII (HALOGENS) ARE CHARGED –1 ANIONS FROM COLUMN VI (OXYGEN GROUP) ARE CHARGED –2 ANIONS FROM COLUMN V (NITROGEN GROUP) ARE CHARGED –3 WHEN THE IONS ARE SINGLE ATOMS (MONATOMIC) EXAMPLES: COLUMN VII Cl-1 Br-1 COLUMN VI S-2 O-2 COLUMN V N-3 P-3

NAMING MONATOMIC (SINGLE ATOM) ANIONS ANIONS THAT CONSIST OF ONLY ONE ATOM END IN ALWAYS “IDE”. ANIONS THAT CONSIST OF MORE THAN ONE ATOM BUT CONTAIN NO OXYGEN ALSO END IN “IDE” THE HYROXIDE ION IS AN EXCEPTION IN THAT IT CONTAINS OXYGEN AND POLYATOMIC BUT ENDS IN “IDE” CHLORIDE Cl-1 SULFIDE S-2 NITRIDE N-3 CYANIDE CN-1 THIOCYANIDE SCN-1 HYDROXIDE OH-1

POLYATOMIC IONS ARE THOSE THAT CONSIST OF MORE THAN ONE ATOM. MOST ARE ANIONS. POLYATOMIC IONS THAT CONTAIN OXYGEN END IN “ATE” OR “ITE” ONCE POLYATOMIC IONS ENDING IN “ATE” ARE KNOWN, IONS OF SIMILAR COMPOSITION BUT DIFFERING BY ONE LESS OXYGEN ATOM END IN “ITE” EXAMPLES: SULFATE SO4-2 SULFITE HAS ONE LESS OXYGEN ATOM SO3-2 NITRATE NO3-1 NITRITE HAS ONE LESS OXYGEN ATOM NO2-1

POLYATOMIC IONS ARE THOSE THAT CONSIST OF MORE THAN ONE ATOM. MOST ARE ANIONS. Polyatomic anions which contain one more oxygen than the “ate” form are given the prefix “per” ClO3-1 is chlorate, ClO4-1 is perchlorate Polyatomic anions which contain one less oxygen than the “ite” form are given the prefix “hypo” ClO2-1 is chlorite, ClO-1 is hypochlorite

FORMULAS OF IONIC COMPOUNDS (CATIONS ARE ALWAYS NAMED FIRST FOLLOWED BY THE NAME OF THE ANION IN THE COMPOUND) IONIC SALTS ARE FORMED WHEN CATIONS COMBINE WITH ANIONS THE NET CHARGE ON ALL SALTS MUST BE ZERO WHICH MEANS ALL THE POSITIVE CHARGES ON THE CATIONS MUST BALANCE ALL THE NEGATIVE CHARGE ON THE ANIONS EXAMPLE: WHEN SODIUM CHLORIDE IS FORMED FROM A POSITIVE SODIUM ION AND A NEGATIVE CHLORIDE ION, JUST ONE SODIUM WITH A +1 CHARGE IS REQUIRED TO BALANCE JUST ONE CHLORIDE WITH A –1 CHARGE. SO THE FORMULA IS JUST NaCl.

WHAT ABOUT FORMULAS FOR COMPOUNDS WHEN THE POSITIVE AND NEGATIVE CHARGES DON’T EXACTLY BALANCE ZINC CHLORIDE CONSISTS OF ZINC WITH A +2 CHARGE AND CHLORIDE WITH A –1 CHARGE. IN ORDER TO BALANCE THE CHARGES, ONE ZINC MUST COMBINE WITH TWO CHLORIDES. THE FORMULA FOR ZINC CHLORIDE IS ZnCl2. (+2) + ( 2 x (-1)) = 0 IRON III SULFIDE CONSISTS OF TWO IRON ATOMS AND THREE SULFUR ATOMS. EACH IRON III ION HAS A +3 CHARGE AND EACH SULFIDE ION HAS A –2 CHARGE. THE FORMULA FOR IRON III SULFIDE IS Fe2S3. (2 x ( +3)) + (3 x (-2)) = 0

IONIC COMPOUNDS CONTAINING POLYATOMIC IONS (CATIONS AND ANIONS) LEAD II NITRATE CONSISTS OF ONE LEAD II ION WITH A +2 CHARGE AND TWO NITRATE IONS EACH WITH A –1 CHARGE. IN ORDER TO BALANCE THE CHARGES, ONE LEAD II MUST COMBINE WITH TWO NITRATES. THE FORMULA FOR LEAD II NITRATE IS Pb(NO3)2. ( +2) + (2 x (-1)) = 0 AMMONIUM PHOSPHATE CONSISTS OF THREE AMMONIUM IONS EACH WITH A +1 CHARGE AND ONE PHOSPHATE WITH A –3 CHARGE. IN ORDER TO BALANCE THE CHARGES, THREE AMMONIUM IONS MUST COMBINE WITH ONE PHOSPHATE. THE FORMULA FOR AMMONIUM PHOSPHATE IS (NH4)3PO4. (3 x (+1)) + (-3) = 0

The Crisscross Method of Writing Formulae Aluminum Nitrate Aluminum = Al Nitrate = NO +3 3 -1 Al NO +3 3 -1 Al ( NO ) 3 3 The positive ion charge without its sign becomes the subscript of the negative ion The negative ion charge without its sign becomes the subscript of the positive ion 1 (+3) + 3(-1) = 0

FORMULAS AND NAMES FOR MOLECULAR COMPOUNDS (COMPOUNDS CONSISTING OF ONLY NONMETALS, NOT IONS) NONMETALS ARE THE ELEMENTS ON THE RIGHT SIDE OF THE PERIODIC TABLE. WHEN NAMING COMPOUNDS, PREFIXES ARE USED TO TELL THE NUMBER OF EACH KIND OF ATOM. PREFIXES USED MONO = 1 DI = 2 TRI = 3 TETRA = 4 PENTA = 5 HEXA = 6 HEPTA = 7 OCTA = 8 SIMPLE!

NAMES AND FORMULAS OF MOLECULAR COMPOUNDS SULFUR DIOXIDE CONSISTS OF ONE SULFUR ATOM (NO PREFIX IS USED) AND TWO (DI) OXYGEN ATOMS. BOTH SULFUR AND OXYGEN ARE NONMETALS. THE FORMULA FOR SULFUR DIOXIDE IS SO2 DINITROGEN PENTOXIDE CONSISTS OF TWO NITROGEN ATOMS (DI) AND FIVE OXYGEN ATOMS (PENTA). BOTH NITROGEN AND OXYGEN ARE NONMETALS. THE FORMULA FOR DINITROGEN PENTOXIDE IS N2O5.

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