Chapter 3 Molecules and Compounds
Molecules and Compounds -Chemical Formulas
1:1 1:2 1:3 2:3 1:4 etcCO H2O NH3 Al2O3 CH4
C + 4 H = CH4
Molecular model
Some compounds are IONIC - electrons are TRANSFERRED from a metal to a nonmetal; the compound is held together ELECTROSTATICALLY
Coulomb’s Law: force = k (n+)(n-)/d2
Some compounds are COVALENT - electrons are SHARED between two atoms
+
e-
+
e-
E. g. carbon dioxide
This commonly occurs for two or more NONMETALS
Ions
• An atom or group of atoms with a net charge caused by the net loss or gain of electrons
• CATION = positively charged ion• ANION = negatively charged ion
Predicting Whether an Atom Will Form a Cation or Anion in Order to Make an Ionic Compound
Metals LOSE electrons to form CATIONS, nonmetals GAIN electrons to form ANIONS
Valence Electrons in Ionic Compounds
• The A-group (representative) elements follow the OCTET RULE; they obtain an inert gas valence (outer) shell that contains 8 electrons
• Metals - lose # electrons = group numbere.g. Ca Ca2+ + 2e- (Ar outer shell)
• Nonmetals - gain electrons = 8 - group #e.g. N + 3e- N3- (Ne outer shell)
Valence Electrons for Covalent Compounds
• Covalent compounds form between two or more nonmetals
• In this case the nonmetals can either LOSE all of their valence electrons,or -
• GAIN enough electrons to obtain an OCTET
Examples -
SO3 - oxygen (VIA) gains 8-6 = 2
O2- ion forms
- sulfur (VIA) loses all 6
S6+ ion formsNOTE: There is a rule that states that oxygen is ALWAYS -2. These rules are coming up!
Example 3.3 - Predicting Ion Charges When Forming Ionic Compounds
• Metals lose electrons, Nonmetals gain them
• Al = group IIIA metal, so LOSES 3 ELECTRONS
Al Al3+ + 3e-
• S = group VIA nonmetal, so GAINS 8 - 6 = 2 ELECTRONS
S + 2e- S2-
Polyatomic Ions• Contains 2 or more atoms COVALENTLY
bonded, and the complete unit contains a net
charge, e.g. nitrate, NO3-
Ionic Compound = Metal + Nonmetal or a Metal + Polyatomic Ion
• Compound held together electrostatically• Very strong forces hold the lattice together, so
ionic cmpd’s have very high melting points
NaCl crystal latticem.p. = 800 oC
Predicting Formulas of Ionic Compounds
Balance positive and negative charges to produce a neutral molecule
Ca2+ + Cl-
Ca2+ + CO32-
Ca2+ + PO43-
Al3+ + O2-
Oxidation Numbers
• A number assigned to each element in a compound in order to keep track of the electrons during a reaction
Mg2+ = +2Cl- = -1O2- = -2N3- = -3
Rules for Assigning Oxidation Numbers(Chap. 5, p. 207)
Rules higher up take precedence over lower rules
1 The O.N. for an atom in its pure, uncombined state = 0.
2 The sum of the O.N.’s for a neutral molecule = 0. For a polyatomic ion, the sum = charge.
3 Group IA = +1Group IIA = +2
4 H = +1 UNLESS combined with IA or IIA, then = -1
5 Oxygen = -2
6 For binary ionic compounds only -Group VA = -3Group VIA = -2Group VIIA = -1
Rules cont’d
Chemical Nomenclature Examples (More Detail in Lab)
• Ionic Compounds
NaCl sodium chlorideAl2S3 aluminum sulfideFeSO4iron(II) sulfateKClO3 potassium chlorate
• Covalent Compounds
SO2 sulfur dioxideP2O5 diphosphorus pentaoxideN2O dinitrogen oxide
The Mole - The mole is the chemist’s counting unit
Avogadro’s Number (NA) = 6.022 X 1023
pair = 2 Dozen = 12
Gross = 144
Ream = 500
• By definition, 12C = 12.000 amu
• How many particles does it take to have 12.000 grams of 12C ?
• NA = 6.022 X 1023 (as determined by experiment)
Where Does Avogadro’s Number Come From?
Significance of the Mole
Mass in amu’s
Mass in grams/mole
NA of carbon atoms weighs
NA of iron atoms weighs
Molar Mass - the mass in grams of one mole of any element
• Molar mass of sodium (Na) = mass of 1 mol of Na atoms= 22.99 g/mol= mass of 6.022 X 1023 Na atoms
• Molar mass of lead (Pb) = mass of 1 mol of Pb atoms= 207.2 g/mol= mass of 6.022 X 1023 Pb atoms
Mass Moles Conversion
Moles to Mass
moles • grams = grams 1 mole
Molar mass
Mass to Moles
grams • 1 mole = moles grams
1 / Molar mass
Example 3.6 - Mass to Moles
How many moles are represented by 125 g of silicon, an element used in semiconductors?
Mole Calculation Using DensityThe graduated cylinder in the photograph contains 25.0 cm3 of Hg. If the density of Hg = 13.534 g/cm3 at 25 oC, how many moles of Hg are in the cylinder? How many atoms of Hg are there?
Example 3.9 - Molar Mass & Moles
You have 16.5 g of the common compound oxalic acid, H2C2O4. Calculate -
1. The number of moles2. The number of molecules3. The number of C atoms4. The mass of one molecule
Other Fun Stuff1 molecule contains - 2 carbon atoms 1 oxygen atom 6 hydrogen atoms
1 mole contains - 2 moles of carbon atoms 1 mole of oxygen atoms 6 moles hydrogen atoms
46.07 g contains - 2(12.01) = 24.02 g of carbon 1(16.00) = 16.00 g of oxygen 6(1.008) = 6.05 g of hydrogen
C2H5OHMW = 46.07
Conversion factors for C2H5OH -
• 2(12.01) g C/ 46.07 g C2H5OH
OR
24.02 g C/ 46.07 g C2H5OH
• 6 moles H/ mole C2H5OH
• 1 mole oxygen/ 2 moles C
More Problems -How many grams of Na are there in 200. g of Na2CO3 ?
How many moles of oxygen are there in 25.0 mol of SO2 ?
More Problems -How many aluminum atoms are there in 150. g of Al2O3 ?
How many oxygen atoms are there in 500. mL of a 30.0 % solution of H2SO4 with a density of 1.250 g/cm3 ? (MW = 98.1)
Empirical & Molecular Formulas
Empirical = simplest ratio of atoms in the molecules
Molecular = actual ratio
Calculating Empirical FormulasFormulas of unknown compounds are determined from the percent composition of each element by mass.
Assume 100 g and divide by atomic weight
Divide by fewest number of moles
Calculating Molecular FormulasThe molecular weight must be known. It is obtained from a separate experiment
Benzene empirical formula = CHformula weight = 12.01 + 1.008 = 13.018
If the MW = 78.11, then what is the molecular formula?
Example 3.10Eugenol is the active component of oil of cloves. It has a MW of 164.2 g/mol and is 73.14 %C and 7.37 %H; the remainder is oxygen. What are the empirical and molecular formulas?