FORMULA WRITING AND NAMING OF COMPOUNDS

FORMULA WRITING AND NAMING OF COMPOUNDSMolecules

A molecule is an aggregate of at least two atoms in a definite arrangement held together by chemical forces (also called chemical bonds)

Examples of moleculesO2, H2, Cl2 diatomic moleculesH2O, C12H22O11 polyatomic moleculesIonsIs an atom or a group of atoms that has a net positive or negative charge

Two basic types of ions:Positive ions or cations one or more electrons less than neutralNa+, Ca2+, Al3+ - monoatomic cationNH4+ - polyatomic cation

Negative ions or anions

one or more electrons more than neutralF-, O2- , N3- - monoatomic anionSO42-, PO43- - polyatomic anions

Chemical formula shows the chemical composition of the substance. Types of formulas:Molecular Formula shows the exact number of atoms of each element in the smallest unit of a substanceExample:Chemical Formulas ExamplesMolecular FormulaNameH2hydrogenO2oxygenO3ozoneH2OwaterEmpirical FormulaMolecular formulaNameNH2N2H4hydrazineEmpirical formulas tells us which elements are present and the simplest ratio of their atoms, but not necessary the actual member of atoms in a given molecule.

Example:

Formula of Ionic Compounds

Formula of ionic compounds is determined by the charges of the ions.Charge on the cations must equal the charge on the anions.The compound must be neutral, that is the sum of the charges on the cation and anion in each formula unit must be equal to zero.If they are different, we apply the following formula to make the formula electrically neutral: The subscript of the cation is numerically equal to the charge on the anion, and the subscript of the anion is numerically equal to the charge on the cation. (We call this the criss-cross method). If the charges are numerically equal, then no subscripts are necessary. This rule follows because the formulas of most ionic compounds are empirical formulas, and as such, the subscripts are reduced to the smallest whole number ratios. Examples are as follows

Potassium Bromide is formed by K+ and Br-. The formula is KBr.

Zinc Iodide is formed by Zn2+ and I-. The formula is ZnI2.

Aluminum Oxide is formed by Al3+ and O2-. Al 3 + O 2 - = Al2O3The formula is Al2O3. The sum of the charges is therefore zero: 2 x (+3)= (+6) for Al and 3 x (-2) = (-6) for O therefore (+6) + (-6) = 0.

Practice Exercise 2.3: Which of the following compounds are likely to be ionic? Which are likely to be molecular? a. CH4 b. NaBr c. BaF2 d. CCl4 e. ICl f. CsCl Naming Compounds

Ionic CompoundsBinary Compounds compound that is composed of two elements, the cation derived from metal atoms and a nonmetallic anion.Cation with only one charge ( Alkali metal cations, Alkaline earth metal cations, Ag+, Al3+, Cd2+, Zn2+)Rule: name the (+) ion + name the stem of the (-) ion + -ide

The -ide Nomenclature of Some Common Monatomic Anion According to Their Position in the Periodic TableGroup 4AGroup 5AGroup 6AGroup 7AC4- CarbideN3- NitrideO2- OxideF1- FluorideSi4- SilicideP3- PhosphideS2- SulfideCl1- ChlorideSe2- SelenideBr1- BromideTe2- TellurideI1- IodideFormula NameNaClsodium chlorideAl2O3aluminum oxideKBrpotassium bromideZnI2zinc iodideExample:

Formula NameFeCl2iron (II) chlorideFeCl3iron (III) chlorideMnOmanganese (II) oxide Mn2O3manganese (III) oxideMnO2manganese (IV) oxideCation with more than one charge (Transition metals except Ag+, Al3+, Cd2+, Zn2+)Rule: name the metal + (Roman numeral of the charge of the metal cation) + name the stem of the (-) ion + -ideExample:Formula Name LiOHLithium hydroxide KCNPotassium cyanide BaSO4Barium sulfate Ca3PO4Calcium phosphate KMnO4Potassium permanganateTernary Compounds compounds consisting of three elements (contains polyatomic ion)

Rule: name the (+) ion + name the (-) ionExample:Practice Exercise : Name the following compounds: a. Cu(NO3)2 b. KH2PO4 c. NH4ClO3 d. PbO e. Li2SO3f. NaHCO3

Practice Exercise: Name the following compounds: a. Fe(NO2)2 b. AlPO4 c. (NH4)2CO3 d. SnS e. ZnSO3f. BaCr2O7Practice Exercise 2.5: Write chemical formulas for the following compounds

a. mercury (I) nitrateb. cesium sulfidec. barium phosphateMolecular Compoundsmolecular compounds contain discrete molecular units. They are usually composed of nonmetallic elements.

Binary Molecular CompoundsGreek prefixes are used to indicate the number of atoms in each element present in a molecule of a compound.

NumericPrefixNumericPrefix1mono6hexa2di7hepta3tri8octa4tetra9nona5penta10decaNote: - The prefix mono may be omitted for the first element - For oxides, the ending a in the prefix is sometimes omitted.Formula NameFormula NameB2H6Diborane HClHydrogen chlorideCH4Methane SiCSlicon carbideSiH4Silane COCarbon monoxideNH3Ammonia CO2Carbon dioxidePH3Phosphine NO2Nitrogen dioxideH2OWater N2O4Dinitrogen tetroxideH2SHydrogen sulfide SO2Sulfur dioxideExample:

- Exceptions to the use of Greek prefixes are molecular compounds containing hydrogen. AcidsFormulaName in dry form Name in aqueous solutionHClHydrogen chlorideHydrochloric acidHFHydrogen fluorideHydrofluoric acidH2SHydrogen sulfideHydrosulfuric acidHCNHydrogen cyanideHydrocyanic acidNaming Acids an acid can be described as a substance that yields hydrogen ions (H+) when dissolved in water. Formulas for acids contain one or more hydrogen atoms as well as an anionic group.

1. Binary acid composed of hydrogen and nonmetal Rule 1: For dry acidHydrogen + stem of the non-metal + -ide

Rule 2: For aqueous acidHydro + stem of the nonmetal + -ic + acidExample:Ternary acid contains hydrogen, oxygen and another element (the central element) are called oxoacids.

Rule 1: For dry acidHydrogen + name of polyatomic ion

Rule 2: For aqueous acidName stem of the middle element + -ous/-ic + acid

The use of ous and -ic

Addition of one O atom to the -ic acid: The acid is calle per...-ic acid. Thus adding an O atom to HCLO3 changes the chloric acid to perchloric acid, HClO4.

Removal of one O atom from the -ic acid: The acid is called -ous acid. Thus nitric acid, HNO3 becomes nitrous acid, HNO2.Removal of two O atoms from the -ic acid: The acid is called hypo...ous acid. Thus when HBrO3 is converted to HBrO, the acid is called hypobromous aid.

FormulaName in dry form Name in aqueous solutionH2CO3Hydrogen carbonateCarbonic acidHNO3Hydrogen nitrateNitric acidHClO4Hydrogen perchloratePerchloric acidHClO3Hydrogen chlorateChloric acidHClO2Hydrogen chloriteChlorous acidHClOHydrogen hypochloriteHypochlorous acidExample:

Naming bases a base can be described as a substance that yields hydroxide ions (OH-) when dissolved in water. Examples sodium hydroxide (NaOH), potassium hydroxide (KOH) barium hydroxide {Ba(OH)2 Ammonia (NH3) - a molecular compound in the gaseous or pure liquid state, is also classified as a common base. This is because when dissolved in water it reacts with water and yield NH4+ and OH- ions.