Ch. 5: Molecules and Ch. 5: Molecules and CompoundsCompounds

Dr. Namphol Sinkaset

Chem 152: Introduction to General Chemistry

I. Chapter OutlineI. Chapter Outline

I. IntroductionII. Chemical FormulasIII. Views of Elements/CompoundsIV. Naming “Type I” CompoundsV. Naming “Type II” CompoundsVI. Polyatomic IonsVII. Naming AcidsVIII. Naming “Type III” CompoundsIX. Molecular Masses

I. SugarI. Sugar

• Sugar is composed of carbon, oxygen, and hydrogen atoms.

• Properties of sugar completely different than elements from which it’s made.

I. Sodium and Chlorine…I. Sodium and Chlorine…

I. Versus Sodium ChlorideI. Versus Sodium Chloride

I. Elements in CompoundsI. Elements in Compounds

• When an element forms a compound, its properties change completely.

• Generally, properties of the compound have no correlation to the original elements.

• In this chapter, we see how elements become compounds and cover chemical nomenclature.

I. Molecules of a Compound I. Molecules of a Compound Are the SameAre the Same

• Law of Constant Composition: all samples of a given compound have the same proportions of their constituent elements.

• Generally, this is expressed as a mass ratio.

I. Water’s Mass RatioI. Water’s Mass Ratio

• If 18.0 g of water is decomposed into it’s elements O and H, there would be 16.0 g of O and 2.0 g of H.

• The O:H mass ratio is thus 8.0:1.0.

• Any sample of water would have this exact same ratio.

II. Representing CompoundsII. Representing Compounds

• Chemical formulas are used to refer to compounds.

• chemical formula: a way to show the elements present in a compound and the relative numbers of each elemental atom.

• The most common is the molecular formula.

II. Chemical FormulasII. Chemical Formulas• There are three types of

formulas. molecular: gives the actual

number of atoms of each element in a molecule of a compound (e.g. H2O2)

empirical: gives the relative number of atoms of each element in a compound (e.g. HO)

structural: uses lines to represent covalent bonds and shows interconnectivity

II. Writing Molecular FormulasII. Writing Molecular Formulas

• The more metallic element is generally listed first. Metallic character increases to the left and

down on the periodic table.

• Subscripts indicate the number of that type of atom in the compound.

• If groups of atoms behave as an independent entity, parentheses are used.

II. Molecular FormulasII. Molecular Formulas

II. Chemical ModelsII. Chemical Models

• Formulas lead to models which give an idea of the 3-D shape of a molecule.

II. From Macroscopic to II. From Macroscopic to SymbolicSymbolic

III. Pure SubstancesIII. Pure Substances

III. Atomic ElementsIII. Atomic Elements

• If element exists as individual atoms, it is named as “atomic.”

• e.g. atomic mercury

III. Molecular ElementsIII. Molecular Elements

• Some elements occur naturally as groups of two or more atoms.

• These are named “molecular” or “diatomic” (for two).

III. Molecular CompoundsIII. Molecular Compounds

• Compounds formed from two or more nonmetals.

III. Ionic CompoundsIII. Ionic Compounds

• Comprised of cations and anions.

• A formula unit is the smallest electrically-neutral collection of ions.

IV. Chemical NomenclatureIV. Chemical Nomenclature

• Like any specialized field, chemistry has its own language.

• The ability to name and recognize names of chemical entities is very important.

• The naming system is LOGICAL!!• The periodic table is indispensable

when you are first learning nomenclature.

IV. Type I CompoundsIV. Type I Compounds• Type I compounds are ionics that have a

metal from Groups 1 or 2 and a nonmetal from Groups 14-17.

• Examples: NaCl = sodium chloride MgBr2 = magnesium bromide

K2S = potassium sulfide

IV. Type I CompoundsIV. Type I Compounds

• To get a formula from a name, remember that a compound must be neutral.

• Ion charges can be found by locating the element on the periodic table.

• “The charge on one becomes the subscript of the other.”

IV. Type I CompoundsIV. Type I Compounds

IV. Sample ProblemIV. Sample Problem

• Give the correct name or formula for the compounds below.a) sodium nitride

b) CaCl2c) potassium sulfide

d) MgO

V. Transition MetalsV. Transition Metals

• Transition metals are found in the “Valley,” Groups 3-12, of the periodic table.

• Transition metal cations often can carry different charges, e.g. Fe2+ and Fe3+.

• Thus, a name like “iron chloride” is ambiguous.

V. Type II Compounds V. Type II Compounds

• Type II compounds are ionics that have a transition metal (Groups 3-12) and a nonmetal (Groups 14-17).

• Examples: FeCl2 = iron(II) chloride

FeCl3 = iron(III) chloride

V. Sample ProblemV. Sample Problem

• e.g. Give the correct name or formula for the compounds below.

a) MnO2

b) copper(II) chloride

c) AuCl3d) molybdenum(VI) fluoride

e) W2O3

V. Some Transition Metal CationsV. Some Transition Metal Cations

VI. Additional ComplicationsVI. Additional Complications

• To make naming ionic compounds harder, sometimes polyatomic ions are involved.

• polyatomic ion: two or more atoms that are bonded covalently and have a net positive or negative charge

VI. Common Polyatomic IonsVI. Common Polyatomic Ions

VI. Oxyanion FamiliesVI. Oxyanion Families

• oxyanion: anion containing oxygen

• There are families of oxyanions, and they have a systematic naming system.

• Have either two- or four-member families. e.g. NO2

- and NO3-

e.g. ClO-, ClO2-, ClO3

-, and ClO4-

VI. Two-Member FamiliesVI. Two-Member Families

• For a two-member family, oxoanion with fewer O atoms is given the “–ite” suffix while the one with more O atoms is given the “–ate” suffix. e.g. NO2

- = nitriteand NO3

- = nitrate

VI. Four-Member FamiliesVI. Four-Member Families

• For the four-member families, the prefixes “hypo-” and “per-” are used to indicate fewer or more oxygen atoms.

• e.g. the chlorine oxyanions ClO- = hypochlorite ClO2

- = chlorite

ClO3- = chlorate

ClO4- = perchlorate

VI. Oxoanion Naming VI. Oxoanion Naming SummarySummary

VI. Sample ProblemVI. Sample Problem

• e.g. Give names or formulas for the following compounds.

a) Na2CO3

b) magnesium hydroxide

c) potassium nitrate

d) CoPO4

e) nickel(II) sulfate

f) NaClO2

VII. AcidsVII. Acids

• Acids are special ionic compounds that have H+ as the cation.

• There are two categories of acids that have different naming rules.

1) Binary acids contain only hydrogen and a nonmetal.

2) Oxyacids contain hydrogen, a nonmetal, and oxygen.

VII. Naming Binary AcidsVII. Naming Binary Acids

• Examples: HCl = hydrochloric acid HBr = hydrobromic acid H2Se = hydroselenic acid

VII. Naming OxyacidsVII. Naming Oxyacids

• Set 1 HNO3 = nitric acid

H2SO4 = sulfuric acid

HClO3 = chloric acid

HClO4 = perchloric acid

H2CO3 = carbonic acid

H3PO4 = phosphoric acid

• Set 2 HNO2 = nitrous acid

HClO2 = chlorous acid

HClO = hypochlorous acid H2SO3 = sulfurous acid

Examples of oxyacids:

VII. Naming OxyacidsVII. Naming Oxyacids

-ate oxyanions become –ic acids.

-ite oxyanions become –ous acids.

VIII. Type III CompoundsVIII. Type III Compounds

• Type III compounds are covalent (nonmetal bonded to nonmetal).

• Naming rules:1) More metallic element is named 1st using the

normal element name EXCEPT when halogens are bonded to oxygen.

2) Second element is named using its root and the “-ide” suffix.

3) #’s of atoms indicated with Greek prefixes EXCEPT when there is only one atom of the first element.

VIII. Greek PrefixesVIII. Greek Prefixes

VIII. Type III CompoundsVIII. Type III Compounds

• Some examples: ClO2 = chlorine dioxide

N2O5 = dinitrogen pentoxide

S2Cl2 = disulfur dichloride

SeF6 = selenium hexafluoride

VIII. Naming PracticeVIII. Naming Practice• e.g. Indicate the “Type” and give the correct

formula or name of the compounds below.a) CoCl3b) dichlorine heptaoxidec) SrOd) magnesium hydroxidee) carbon tetrachloride

f) HF(aq)

g) sodium hydride

h) V2O5

i) Ru(ClO4)3

j) hydrosulfuric acid

k) H2SO4

l) titanium(IV) oxide

m) N2F2

IX. Masses of Compounds IX. Masses of Compounds

• Atomic masses are readily accessible via the periodic table, e.g. H = 1.008 amu.

• Formula masses (a.k.a. molecular masses or molecular weights) are calculated by adding up the masses of each atom in the compound.

IX. Molecular Mass of WaterIX. Molecular Mass of Water

• The formula for water is H2O, so it is comprised of 2 H atoms and 1 O atom.

IX. Formula MassIX. Formula Mass

• e.g. What is the formula mass of barium nitrate?