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10/18/13

Chemical Formulas The Elements n A gives the numbers n The chemical formulas of most of the and types of that are found in a elements are simply their elemental substance. symbol: n When the substance is a discrete Na () Fe (iron) , then the chemical formula is He (helium) U (uranium) also its molecular formula. n These chemical formulas are said to be Fe (iron) is a chemical formula monatomic—only an in chemical

Fe2O3 is a molecular formula formula

The Elements Binary Compounds n There are seven elements that occur n A binary compound is one composed of naturally as diatomic — only two different types of atoms. molecules that contain two atoms: Rules for binary compound formulas

H2 () N2 () 1. Element to left in Periodic Table comes first except for hydrogen: O2 () F2 () KCl Cl2 () Br2 (bromine) PCl3 I2 (iodine) Al2S3 n The last four elements in this list are in Fe O the same family of the Periodic Table 3 4

Binary Compounds Other Compounds

2. Hydrogen comes last unless other n For compounds with three or more element is from group 16 or 17: elements that are not ionic, if it contains LiH, NH , B H , CH carbon, this comes first followed by 3 2 4 4 hydrogen. Other elements are then 3. If both elements are from the same listed in alphabetical order: group, the lower element comes first: C2H6O SiC, BrF3 C4H9BrO

CH3Cl

C8H10N4O2

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Other Compounds Structural Formulas n However, the preceding rule is often n Very often, chemists use structural ignored when writing organic formulas formulas to show where the atoms in a (molecules containing carbon, molecule are positioned. The atoms are hydrogen, and maybe other elements) connected to each other with bonds. in order to give a better idea of how the atoms are connected: n Bonds represent the attractive forces that hold molecules together. C2H6O is the molecular formula for , but nobody ever writes it this n There are three types of structural way—instead the formula is written formulas: line structures, ball-and-stick C2H5OH to indicate one H atom is models, and spacing-filling models. connected to the O atom.

Line Structures Line Structures n Lines represent the bonds between n For organic molecules, the carbon atoms atoms are often not shown. n C-H bonds are also omitted in line structures.

Line structure of , C3H8

Line Structures Ball-and-Stick Models

n Atoms are represent by balls that are either color-coded or labeled with the appropriate element. (Color code is given on p. xv of textbook)

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Space-Filling Models Comparison of Structural Models n Atoms are represent by balls that are color-coded and are approximately the correct relative size of the atoms. n This gives a better perspective of the space available in a molecule.

Naming Chemical Compounds Nonmetallic Binary Compounds Nonmetallic Binary Compounds 1. Element that appears first retains its n Molecules composed of only two elemental name. different non-metals 2. Second element begins with root derived from name and ends with -ide 1 H The Non-Metals He 2 B C N O F Ne As: arsenide Br: bromide 3 Al Si P S Cl Kr 4 Ge As Se Br Ar C: carbide Cl: 5 Sb Te I Xe 6 Po At Rn O: oxide F: fluoride 7 H: I: iodide N: nitride S: sulfide

Nonmetallic Binary Compounds Nonmetallic Binary Compounds 3. When more than one atom appears in A Series of Nitrogen Oxides chemical formula, name is prefixed by NO: nitrogen oxide (nitrogen monoxide) number of atoms present NO : nitrogen dioxide CO: 2 NO3: nitrogen trioxide SiO2: silicon dioxide N O: dinitrogen oxide () NI3: nitrogen triiodide 2 CCl4: N2O3: dinitrogen trioxide PCl5: phosphorous pentachloride N2O4: dinitrogen tetroxide

SF6: sulfur hexafluoride N2O5: IF7: iodine heptafluoride

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Binary Compounds with Hydrogen Binary Compounds with Hydrogen n When hydrogen combines with elements n When hydrogen combines with from groups 2 or 16, the resulting molecule elements from groups 1 or 17, a contains 2 H atoms. results. n The molecule is named according to the previous rules for nonmetallic binary n The molecule is named according to compounds, but the di is omitted: the previous rules for nonmetallic H S: binary compounds: 2 CaH2: hydride NaH: n Exception is oxygen: HCl: H2O:

LiH: H2O2:

Ionic Compounds Ionic Compounds

n are atoms or molecules that have a n The positive charge of a cation is equal net electrical charge to the number of electrons lost by the n Species may either lose electrons to species to form an become positively charge (cations) or n The negative charge of an anion is gain electrons to become negatively equal to the number of electrons gained charged (anions) by the species to form an ion Na+ sodium ion Cl- chlroide ion + - NH4 ion NO3 nitrate ion 3- 2+ PO4 phosphate ion Mg magnesium

Ionic Compounds Ionic Compounds

n Metals usually form cations n Group 16 and 17 elements usually form n Transition metals may form cations with anions. various positive charges: S2- sulfide ion F- fluoride ion Fe2+ or Fe3+ O2- oxide ion Br- bromide ion n The positive charge on a metal atom is n Polyatomic ions are also very common frequently referred to as its oxidation —composed of molecular ions, not state atomic ions. Fe(II) iron has an oxidation state of 2 Fe(III) iron has an oxidation state of 3

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Usual Atomic Charges of Main Group Elements +1 +2 +3 +4 +5 +6 +7

Polyatomic Ions and Their Names -5 -4 -3 -2 -1 Formula Name Formula Name + - NH4 ammonium NO3 nitrate + - H3O NO2 nitrite + 3- Hg2 (I) PO4 phosphate - - OH hydroxide MnO4 permanganate - 2- CN cyanide CrO4 chromate 2- 2- CO3 carbonate Cr2O7 dichromate - - CH3CO2 acetate ClO4 perchlorate 2- - Examples C2O4 oxalate ClO3 chlorate SO3 sulfur trioxide 2- - SO4 ClO2 chlorite CO2 2- - Al2O3 aluminum trioxide SO3 sulfite ClO hypochlorite IF7 iodine heptafluoride

Fig. 2-6, p.63

Hydrates (Molecular Weight)

n Many ionic compounds have a set n Atomic mass of an element listed in the number of water molecules associated Periodic Table is the mass of one mole with them in the solid phase. These are of the naturally occurring element. called hydrates. atomic number n Hydrates are denoted with the number 6 of water molecules in the structure by elemental symbol including •nH2O in the formula (n = C number of water molecules) atomic mass 12.011 electronic CuSO •5H O copper(II) sulfate pentahydrate 4 2 2s2 2p2 configuration

Al(NO3)3•9H2O aluminum nitrate nonahydrate

Molar Mass (Molecular Weight) Molar Mass (Molecular Weight)

C2H6: 1 mol H = 1.0079 g n Molecular mass of a molecule is the sum of 1 mol C = 12.011 g the atomic masses of all atoms comprising that molecule.

H2: (2 mol C) (12.011 g /mol C) = 24.022 g 1 mol H = 1.0079 g (6 mol H) (1.0079 g/mol H) = 6.0474 g 2 mol H = 1 mol H2 ⎛ 2 mol H ⎞⎛1.0079 g⎞ 2.0158 g 1 mol C H (molar mass) = 30.069 g ⎜ ⎟⎜ ⎟ = 2 6 ⎝1 mol H2 ⎠⎝ 1 mol H ⎠ mol H2 note use of significant figures!

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Molar Mass (Molecular Weight) Mole-Mass Conversions

n Using molar mass, we can now Na2Ni(NH3)6: (2 mol Na)(22.990 g/mol Na) = 45.980 g calculate the number of moles or the mass of any compound if we know the (1 mol Ni)(58.693 g /mol Ni) = 58.693 g other quantity. (6 mol N)(14.007 g/mol N) = 84.042 g How many moles in 6.358 g H ? (18 mol H)(1.0079 g/mol H) = 18.1422 g 2

(1 mol H2) 1 mol Na Ni(NH ) = 206.827 g 6.358 g H2 = 3.154 mol H2 2 3 6 (2.0158 g)

note use of significant figures!

Mole-Mass Conversions Percent Composition n We are sometimes given the percent of What is the mass of 23.706 mol C2H6? each element present in a compound by (30.069 g) 23.706 mol C2H6 mass. This can be determined (1 mol C2H6) experimentally by elemental analysis using mass spectrometry or atomic = 712.82 g C2H6 spectrometry. n The percentages must add to 100% • mass ⇒ moles: divide by molar mass n From the percent composition, we can then determine the empirical formula of • moles ⇒ mass: multiply by molar mass a compound.

Empirical Formula Empirical Formula n If we know the percentage of each n The empirical formula may be different element in a , we from the molecular formula can then determine its empirical formula n Glucose has a percent composition of —the chemical formula of the 40.00% carbon compound with the fewest possible 6.71% hydrogen number of atoms. dinitrogen tetroxide 53.29% oxygen n Resulting empirical formula: CH2O empirical formula: NO2 n Molecular formula of glucose: C6H12O6 molecular formula: N2O4

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Empirical Formula Empirical Formula A compound was determined to contain 2. Divide moles of each element by smallest 61.52% C, 5.16% H, 10.25% N, and mole value to determine number of atoms in empirical formula ’ 23.07% O. What is the compound s N has only 0.7318 moles—N has the smallest empirical formula? mole value in the compound 1. Assume 100 g of compound; find moles of C: 5.122 mol / 0.7318 mol = 6.999 each element H: 5.12 mol / 0.7318 mol = 7.00 61.52 g C / 12.011 g mol-1 C = 5.122 mol C N: 0.7318 mol / 0.7318 mol = 1.000 5.16 g H / 1.0079 g mol-1 H = 5.12 mol H O: 1.442 mol / 0.7318 mol = 1.970 10.25 g N / 14.007 g mol-1 N = 0.7318 mol N 23.07 g O / 15.999 g mol-1 O = 1.442 mol O Empirical formula = C7H7NO2

Empirical Formula If you are given the masses of the elements in a compound instead of the percent composition, you can go through a similar process to determine the empirical formula: 1. Calculate moles of each element 2. Divide by smallest value to determine empirical formula

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