Writing Chemical Formulas

Writing Chemical Formulas General Chemistry Mrs. Amy Nare http://chem.pdx.edu/~wamserc/C335W00/gifs/MW2.gif

Objectives • Distinguish between molecular and ionic compounds • Contrast molecular formulas and formula units • Use the periodic table to determine the charge on an ion • Define monatomic and polyatomic ion and name the charges of common polyatomic ions • Write the formulas and names for binary and ternary ionic compounds • Write formulas and names for molecular compounds

Inspiration Concept Map

Chemical Bonding • Atoms – same number of protons (+) and electrons (-); electrically neutral • Ions – atoms w/ a (+) or (-) charge; have lost or gained electrons (e-) * Cations: (+) charge; has lost e-; metals * Anions: (-) charge; has gained e-; nonmetals

Chemical Bonding • Types of Compounds A. Molecular – atoms bonded together by shared pairs of e- (covalent bonds); formed between nonmetals ex: C6H12O6 B. Ionic – ions attracted to each other by opposite charges; formed between a metal and a nonmetal ex: NaCl

Chemical Bonding

Chemical Bonding Chemical Formulas – show the type and number of atoms in smallest unit of substance • Molecular Formula – type and number of atoms joined to form a molecule ex: C2H5OH • Formula Unit – smallest ratio of ions in an ionic compound ex: CaF2

Representing Chemical Compounds • Law of Definite Proportions – in any sample of a compound, the elements are always combined in the same proportions ex: H2O and H2O2 H2O – water – H:O ratio always 2:1 H2O2 – hydrogen peroxide – H:O ratio always 1:1

Identifying Ionic Charges • Group A elements – use the periodic table to determine ionic charge * elements in same group have same ionic charge * Group 4A and Noble gases – almost never form ions • Group B elements – many have more than one ionic charge

Identifying Ionic Charges http://wps.prenhall.com/wps/media/objects/476/488316/ch04.html Charge on cations corresponds to group #. Charge on anions is found by subtracting 8 by group number the number 8 is used b/c it represents # of valence e- in Noble gases

Naming Cations and Anions Monatomic Ions • Ions formed by one element • Cations * for Group A elements – just write element’s name ex: calcium ion (Ca2+) = calcium * for Group B elements – write element’s name, then Roman numerals in parentheses to denote charge ex: Fe2+ = Iron (II) and Fe3+ = Iron (III) • Anions – drop the end of the element’s name & add “–ide” ending ex: chlorine ion (Cl1-) = chloride

Naming Cations and Anions Polyatomic Ions • Ions formed by more than one type of element • Atoms of different elements held together by covalent bonds • Atoms always stay together and collectively have a single charge • Do not always have “-ide” ending ex: NH41- = ammonium ion CO32- = carbonate ion Learn names, formulas, and charges of polyatomic ions!

Potassium ion Copper (II) ion Chloride ion Oxide ion Ba2+ S2- Au3+ Nitrite ion Hydroxide ion Phosphate ion SO42- CrO42- ClO32- Naming Cations and Anions Stop Write the name or symbol w/ charge:

Binary Ionic Compounds • Compounds composed of 2 different monatomic elements • To write binary formulas – write cation first, then anion *criss-cross charges to determine how many of each ion you need *use subscripts to denote number of ions ex: Ca2+ + Cl1- CaCl2 Na1+ + Cl1- NaCl • To name binary compounds – write name of cation first, then anion (-ide) ex: CaCl2 = calcium chloride Li2O = lithium oxide

Ternary Ionic Compounds • Compounds containing at least one polyatomic ion; at least 3 different elements • To write ternary formulas: write cation first, then anion *criss-cross charges to determine how many of each ion you need *use subscripts to denote number of ions *must use parentheses around polyatomic if more than one is needed!!! ex: Na1+ + SO32- Na2SO3 Mg2+ + OH1- Mg(OH)2 [not same as MgOH2] • To name ternary compounds: write name of cation, then name of anion (not all end in “-ide”) **be careful with transition metals (more than one charge)** ex: CaCO3 = calcium carbonate PbSO4 = lead (II) sulfate Ag2CrO4 = silver chromate

NaNO3 CaSO4 (NH4)2O CuSO3 Fe(OH)3 NaF Lithium sulfide Iron (III) phosphide Magnesium fluoride Barium nitrate Aluminum hydroxide Potassium phosphate Ionic Compounds Stop Write the name or the formula for the following compounds: Practice making ionic compounds!

Binary Molecular Compounds • Two nonmetals joined by covalent bonds • Use prefixes for naming 1 = mono- 2 = di- 3 = tri- 4 = tetra- 5 = penta- 6 = hexa- 7 = hepta- 8 = octa- 9 = nona- 10 = deca-

Binary Molecular Compounds • To name binary molecular compounds: *first element gets a prefix if there is more than one *second element ALWAYS gets prefix, and “-ide” ending ex: N2O3 = dinitrogen trioxide CO = carbon monoxide (not monocarbon) • If element begins with vowel and prefix ends in “a” or “o”, then drop last vowel on prefix to form the name ex: Cl2O7 = dichlorine heptoxide (not heptaoxide)

P2O5 N2O NO2 CBr4 CO2 tetraiodine nonoxide sulfur hexafluoride nitrogen trioxide carbon tetrahydride phosphorus trifluoride Molecular Compounds Stop Write the name or formula for the following compounds:

Assess what you learned. Log on to the Internet and take the quiz. http://school.discovery.com/quizzes31/amy_trauth/FormulasQuiz.html

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Writing Chemical Formulas