Chapter 5: Elements and Compounds – Introduction to Chemical Compounds, Formulas, and Nomenclature

Chapter 5: Elements and Compounds

Compounds

Compounds are pure substances formed when two or more elements are chemically combined in a fixed proportion by mass. The composition of a compound is always constant, regardless of its source or method of preparation.

• Definition: A compound is a substance composed of two or more elements chemically combined in a definite ratio.

• Examples: Water ($H_2O$), Carbon dioxide ($CO_2$), Hydrogen peroxide ($H_2O_2$)

• Constant Composition: The ratio of elements in a compound is always the same (law of definite proportions).

• Types of Compounds:

  • Ionic Compounds

  • Covalent (Molecular) Compounds

Chemical Formulas

Chemical formulas represent the composition of compounds using element symbols and numerical subscripts and superscripts.

• Superscripts indicate the charge of an ion (e.g., $NH_4^+$ for ammonium ion). If there is no superscript, the species is neutral.

• Subscripts indicate the number of atoms of each element in a compound (e.g., $H_2O$ has two hydrogen atoms and one oxygen atom).

• Parentheses are used around polyatomic ions if more than one is present (e.g., $(NH_4)_2SO_4$).

Chemical Formulas: Oxidation States

Oxidation states (or oxidation numbers) are used to indicate the charge an atom would have if electrons were transferred completely. They are essential for determining the formulas of ionic compounds.

• Group 1 (IA): +1

• Group 2 (IIA): +2

• Group 13 (IIIA): +3

• Group 15 (VA): -3

• Group 16 (VIA): -2

• Group 17 (VIIA): -1

• Group 18 (VIIIA): 0 (noble gases)

Polyatomic Ions

A polyatomic ion is an ion composed of two or more atoms covalently bonded, carrying a net charge. These ions act as a single unit in chemical reactions and formulas.

• Examples:

  • $NH_4^+$ – Ammonium ion

  • $NO_3^-$ – Nitrate ion

  • $PO_4^{3-}$ – Phosphate ion

Ion	Name
$NH_4^+$	Ammonium
$NO_3^-$	Nitrate
$SO_4^{2-}$	Sulfate
$OH^-$	Hydroxide
$CO_3^{2-}$	Carbonate
$PO_4^{3-}$	Phosphate
$HCO_3^-$	Bicarbonate
$ClO_3^-$	Chlorate
$C_2H_3O_2^-$	Acetate
$CN^-$	Cyanide
$MnO_4^-$	Permanganate
$Cr_2O_7^{2-}$	Dichromate
$CrO_4^{2-}$	Chromate

Ionic Bonds and Ionic Compounds

An ionic bond is a chemical bond resulting from the electrostatic attraction between oppositely charged ions (cations and anions). Ionic compounds are typically formed between metals and nonmetals, but can also include polyatomic ions.

• Key Features:

  • Formed by transfer of electrons from metal to nonmetal

  • Resulting ions have opposite charges that attract each other

  • Formula unit is electrically neutral (charges must cancel out)

• Examples:

  • Magnesium nitrate: $Mg(NO_3)_2$

  • Magnesium phosphate: $Mg_3(PO_4)_2$

  • Iron(II) chloride: $FeCl_2$ (ferrous chloride)

  • Iron(III) chloride: $FeCl_3$ (ferric chloride)

Covalent (Molecular) Bonds and Compounds

Covalent bonds are formed when two or more nonmetal atoms share pairs of electrons. Compounds formed by covalent bonds are called molecular or covalent compounds.

• Key Features:

  • Atoms share electrons to achieve stable electron configurations

  • Usually formed between nonmetals

  • Discrete molecules are formed

• Examples:

  • Carbon dioxide: $CO_2$

  • Ethanol: $C_2H_6O$

  • Diatomic molecules: $H_2$, $O_2$, $N_2$, $F_2$, $Cl_2$, $Br_2$, $I_2$

Diatomic Gases

Certain elements exist naturally as diatomic molecules, meaning they are composed of two atoms bonded together.

• Diatomic Elements: $H_2$, $O_2$, $N_2$, $F_2$, $Cl_2$, $Br_2$, $I_2$

• Monatomic Gases: Noble gases such as He, Ne, Ar, etc.

Types of Chemical Formulas

• Molecular Formula: Shows the actual number of atoms of each element in a molecule (e.g., $C_6H_{12}O_6$ for glucose).

• Structural Formula: Shows how atoms are arranged and bonded in a molecule.

Nomenclature (Naming Compounds)

The International Union of Pure and Applied Chemistry (IUPAC) provides systematic rules for naming chemical compounds.

Ionic Compounds (Metals with Nonmetals)

• Cation (metal) retains its name.

• Anion (nonmetal) ends with -ide.

• For polyatomic anions, use the name as is (e.g., nitrate, sulfate).

• If the cation has variable charge, indicate the charge in Roman numerals in parentheses (e.g., iron(II) chloride).

• Examples:

  • $NaCl$ – sodium chloride

  • $NaNO_3$ – sodium nitrate

  • $FeCl_2$ – iron(II) chloride

Covalent (Molecular) Compounds (Nonmetal with Nonmetal)

• First element retains its name.

• Second element ends with -ide.

• Use prefixes to denote the number of each atom:

  • 1: mono- (not used for the first element)

  • 2: di-

  • 3: tri-

  • 4: tetra-

  • 5: penta-

  • 6: hexa-

  • 7: hepta-

  • 8: octa-

• Example: $CO_2$ – carbon dioxide

Acids

Acids are compounds that release hydrogen ions ($H^+$) in solution. Their names depend on the type of anion present.

• Binary Acids: Contain hydrogen and one other element. Name: hydro- + root of nonmetal + -ic acid (e.g., $HCl$ – hydrochloric acid).

• Oxyacids: Contain hydrogen, oxygen, and another element (usually a polyatomic ion). Naming depends on the polyatomic ion:

  • If the anion ends in -ate, the acid ends in -ic acid (e.g., $H_2SO_4$ – sulfuric acid from sulfate).

  • If the anion ends in -ite, the acid ends in -ous acid (e.g., $H_2SO_3$ – sulfurous acid from sulfite).

  • Prefixes such as per- (more oxygen) and hypo- (less oxygen) are used as needed.

Anion	Acid Name
$Cl^-$ (chloride)	hydrochloric acid ($HCl$)
$NO_3^-$ (nitrate)	nitric acid ($HNO_3$)
$NO_2^-$ (nitrite)	nitrous acid ($HNO_2$)
$SO_4^{2-}$ (sulfate)	sulfuric acid ($H_2SO_4$)
$SO_3^{2-}$ (sulfite)	sulfurous acid ($H_2SO_3$)

Summary Table: Common Polyatomic Ions to Remember

Name	Formula
Ammonium	$NH_4^+$
Phosphate	$PO_4^{3-}$
Nitrate	$NO_3^-$
Sulfate	$SO_4^{2-}$
Hydroxide	$OH^-$
Carbonate	$CO_3^{2-}$

Additional info: This guide covers the foundational concepts of chemical compounds, their formulas, and nomenclature, as typically presented in an introductory college chemistry course. For more detailed rules and additional polyatomic ions, refer to your textbook or instructor's materials.