Chemical Formulas and chemical compounds

Objectives: Explain the significance of a chemical formula. Determine the formula of an ionic compound formed between two given ions. Name an ionic compound given its formula. Using prefixes, name a binary molecular compound from its formula. Write the formula of a binary molecular compound given its name.

Chemical names and Formulas There are millions of natural and synthetic chemical compounds Calcium carbonate – limestone Sodium chloride – table salt Dihydrogen monoxide – water These are their chemical and common names Chemical names help to describe the atomic makeup of the compounds

Significance of a Chemical Formula Indicates the relative number of atoms of each kind in a chemical compound. Molecular formula Indicates the relative number of atoms of each kind in a molecule. (Covalently bonded) C8H18 Subscript indicates there are 8 atoms of carbon in a molecule of octane Subscript indicates there are 18 atoms of hydrogen in a molecule of octane

Al2(SO4)3 Chemical formula for ionic compound Ionic compound consists of lattice of positive and negative ions held together by mutual attraction. Chemical formula represented by one formula unit Simplest ratio of the compounds positive and negative ions Aluminum sulfate below consists of aluminum cations and sulfate ions Al2(SO4)3 Subscript 3 refers to everything inside the parentheses giving 3 sulfate ions, with a total of 3 sulfur atoms and 12 oxygen atoms Subscript 2 refers to 2 Aluminum atoms Subscript 4 refers to 4 oxygen atoms in the sulfate ion Note: when you only have one of an atom, no subscript is used Note: parentheses are used to identify polyatomic ion as one unit

Monatomic ions Ions formed from a single atom Examples Na+1 lose one electron Mg2+ S2- gain two electrons N-3 Cl1- Not all main-group elements readily form ions Examples Carbon & Silicon form covalent bonds d-block elements form variable charges examples Copper, can be Cu+1 or Cu+2 Iron, can be Fe+2 or Fe+3 Lead, can bePb+2, Pb+3, or Pb+4

Naming Monatomic ions Positive ions Negative ions Name of element Ex: K+ Potassium Mg+2 Magnesium Al+3 __________ Sr+2 __________ Negative ions Base of element + -ide ending Ex: F-1 Fluoride N-3 Nitride O-2 _______ Br-1 _______

Binary Ionic Compounds Compounds composed of two different elements Total # of positive charges must be equal to total # of negative charges Writing formulas, Ex: Aluminum oxide Write the symbols for ions (Cation first) Al+3 O-2 Cross over the charges as subscripts Al2O3 Check to make sure total charges are equal, divide by largest number, to give smallest whole-number ratio +3 -2 2 x (+3) = +6 3 x (-2) = -6

Naming binary ionic compounds Nomenclature Naming system Name Al2O3 Name cation first : full name of cation Aluminum Name Anion last : base of anion + -ide oxide Al2O3 aluminum oxide

Practice Naming and Writing Formulas Name AgCl ZnO SrF2 silver chloride zinc oxide strontium fluoride Write the formulas for Zinc iodide Zinc sulfide Aluminum sulfide ZnI2 ZnS Al2S3

Stock System of Nomenclature Some cations may have two or more different charges Use stock system of naming (usually with d-block elements) Roman numeral represents charge in parentheses Fe+2 Fe+3 Iron(II) Iron(III) Some cations that commonly form only one cation Do not use roman numerals ( main group elements) No Anions form more than one charge +2 -1 CuCl2 copper(II) chloride

Practice stock system Write formula and give name for compound formed by ions Cr+3 and F-1 CrF3 chromium(III) fluoride Write formulas and give name for the following ionic compounds: Cu+2 and Br-1 CuBr2 copper(II) bromide Fe+2 and O-2 FeO iron(II) oxide Fe+3 and O-2 Fe2O3 iron(III) oxide

Compounds containing Polyatomic Ions All but NH4+, ammonium ion, are negatively charged Most are oxyanions Examples NO3-1 NO2-1 nitrate nitrite Most common anion has –ate ending Anion with one less oxygen has –ite ending Anion with two less oxygen has hypo prefix and –ite ending Anion with one extra oxygen has per prefix and –ate ending ClO3-1 ClO2-1 ClO-1 ClO4-1 hypochlorite perchlorate chlorate chlorite

Naming Compounds with Polyatomic Ions Same as naming for ionic compounds except Name polyatomic ion as one unit Example: AgNO3 Use parentheses if more than one polyatomic ion Example Al2(SO4)3 silver nitrate Show 2 Al+3 ions and 3 SO4-2 ions aluminum sulfate

Writing Formulas and Naming Compounds with Polyatomic Ions Write the formula for these: tin(IV) sulfate calcium chloride lithium nitrate calcium nitrite potassium perchlorate Sn(SO4)2 CaCl2 LiNO3 Ca(NO2)2 KClO4 Write the names for these: Ag2O Ca(OH)2 NH4OH FeCrO4 KClO silver oxide calcium hydroxide ammonium hydroxide iron(II) chromate potassium hypochlorite

Naming Binary Molecular Compounds May use stock system to name these New system – must understand oxidation numbers Prefix system Old system – must know numerical prefixes mono- di- tri- tetra- penta- hexa- hepta- octa- nona- deca-

Rules for prefix system of Nomenclature less-electronegative element is given first Second element is named by combining Prefix indicating number of atoms Root of name of second element -ide ending First element only gets a prefix if it has more than one Ex: P4O10 Ex: tetra phosphorus dec ox ide The o or a at the end of a prefix is usually dropped when the word following the prefix begins with another vowel

The 6 binary compounds of Nitrogen and Oxygen N2O NO NO2 N2O3 N2O4 N2O5 dinitrogen monoxide nitrogen monoxide nitrogen dioxide dinitrogen trioxide dinitrogen tetroxide dinitrogen pentoxide Name the following molecular compounds Write formulas for the following molecular compounds SO3 ICl3 PBr5 sulfur trioxide carbon tetraiodide phosphorus trichloride oxygen difluoride CI4 iodine trichloride PCl3 Phosphorus pentabromide OF2