Ch. 5: Molecules and Compounds Dr. Namphol Sinkaset Chem 152: Introduction to General Chemistry

I. Chapter Outline I.Introduction II.Chemical Formulas III.Views of Elements/Compounds IV.Naming “Type I” Compounds V.Naming “Type II” Compounds VI.Polyatomic Ions VII.Naming Acids VIII.Naming “Type III” Compounds IX.Molecular Masses

I. 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. Versus Sodium Chloride

I. 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 Are 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 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 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 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. H 2 O 2 )  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 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 Formulas

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

II. From Macroscopic to Symbolic

III. Pure Substances

III. Atomic Elements If element exists as individual atoms, it is named as “atomic.” e.g. atomic mercury

III. Molecular Elements Some elements occur naturally as groups of two or more atoms. These are named “molecular” or “diatomic” (for two).

III. Molecular Compounds Compounds formed from two or more nonmetals.

III. Ionic Compounds Comprised of cations and anions. A formula unit is the smallest electrically-neutral collection of ions.

IV. 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 Compounds Type I compounds are ionics that have a metal from Groups 1 or 2 and a nonmetal from Groups Examples:  NaCl = sodium chloride  MgBr 2 = magnesium bromide  K 2 S = potassium sulfide

IV. 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 Compounds

IV. Sample Problem Give the correct name or formula for the compounds below. a)sodium nitride b)CaCl 2 c)potassium sulfide d)MgO

V. 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. Fe 2+ and Fe 3+. Thus, a name like “iron chloride” is ambiguous.

V. Type II Compounds Type II compounds are ionics that have a transition metal (Groups 3-12) and a nonmetal (Groups 14-17). Examples:  FeCl 2 = iron(II) chloride  FeCl 3 = iron(III) chloride

V. Sample Problem e.g. Give the correct name or formula for the compounds below. a)MnO 2 b)copper(II) chloride c)AuCl 3 d)molybdenum(VI) fluoride e)W 2 O 3

V. Some Transition Metal Cations

VI. 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 Ions

VI. 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. NO 2 - and NO 3 -  e.g. ClO -, ClO 2 -, ClO 3 -, and ClO 4 -

VI. 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. NO 2 - = nitrite and NO 3 - = nitrate

VI. 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  ClO 2 - = chlorite  ClO 3 - = chlorate  ClO 4 - = perchlorate

VI. Oxoanion Naming Summary

VI. Sample Problem e.g. Give names or formulas for the following compounds. a)Na 2 CO 3 b)magnesium hydroxide c)potassium nitrate d)CoPO 4 e)nickel(II) sulfate f)NaClO 2

VII. 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 Acids Examples:  HCl = hydrochloric acid  HBr = hydrobromic acid  H 2 Se = hydroselenic acid

VII. Naming Oxyacids Set 1  HNO 3 = nitric acid  H 2 SO 4 = sulfuric acid  HClO 3 = chloric acid  HClO 4 = perchloric acid  H 2 CO 3 = carbonic acid  H 3 PO 4 = phosphoric acid Set 2  HNO 2 = nitrous acid  HClO 2 = chlorous acid  HClO = hypochlorous acid  H 2 SO 3 = sulfurous acid Examples of oxyacids:

VII. Naming Oxyacids -ate oxyanions become –ic acids. -ite oxyanions become –ous acids.

VIII. Type III Compounds Type III compounds are covalent (nonmetal bonded to nonmetal). Naming rules: 1)More metallic element is named 1 st 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 Prefixes

VIII. Type III Compounds Some examples:  ClO 2 = chlorine dioxide  N 2 O 5 = dinitrogen pentoxide  S 2 Cl 2 = disulfur dichloride  SeF 6 = selenium hexafluoride

VIII. Naming Practice e.g. Indicate the “Type” and give the correct formula or name of the compounds below. a)CoCl 3 b)dichlorine heptaoxide c)SrO d)magnesium hydroxide e)carbon tetrachloride f)HF (aq) g)sodium hydride h)V 2 O 5 i)Ru(ClO 4 ) 3 j)hydrosulfuric acid k)H 2 SO 4 l)titanium(IV) oxide m)N 2 F 2

IX. Masses of Compounds Atomic masses are readily accessible via the periodic table, e.g. H = 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 Water The formula for water is H 2 O, so it is comprised of 2 H atoms and 1 O atom.

IX. Formula Mass e.g. What is the formula mass of barium nitrate?