Ionic and Molecular Compounds: Structure, Naming, and Formulas

Study Guide - Smart Notes

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Ionic & Molecular Compounds

Types of Chemical Bonds

Chemical bonds are the forces that hold atoms together in compounds. The two main types are ionic bonds and covalent bonds. L

Ionilc bonds: Formed when valence electrons are transferred from a metal to a nonmetal, resulting in the formation of ions.
Covalent bonds: Formed when atoms of nonmetals share valence electrons to create molecular compounds.
Octet Rule: Atoms tend to lose, gain, or share electrons to achieve eight electrons in their outer shell, resulting in greater stability.

Ions: Formation and Properties

An ion is an atom or molecule with a net electrical charge due to the loss or gain of electrons. Ions are classified as either cations (positive) or anions (negative).

Cations: Formed when atoms lose electrons (typically metals in Groups 1, 2, and 13).
Anions: Formed when atoms gain electrons (typically nonmetals in Groups 15, 16, and 17). The name of the anion changes to end with "-ide" (e.g., chlorine becomes chloride).

Example: Sodium (Na) loses one electron to become Na+; chlorine (Cl) gains one electron to become Cl-.

Sodium atom to sodium ion electron loss Magnesium atom to magnesium ion electron loss Chlorine atom to chloride ion electron gain

Ionic Charges from Group Numbers

The charge of ions formed by representative elements can be predicted using their group numbers:

Group 1A: loses 1 electron (charge +1)
Group 2A: loses 2 electrons (charge +2)
Group 3A: loses 3 electrons (charge +3)
Group 5A: gains 3 electrons (charge -3)
Group 6A: gains 2 electrons (charge -2)
Group 7A: gains 1 electron (charge -1)

Ionic Compounds

Formation and Chemical Formulas

Ionic compounds are formed by the combination of cations and anions in ratios that result in a net charge of zero. The chemical formula represents the lowest whole number ratio of ions.

Subscripts indicate the number of each ion needed to balance charges.
Always write the cation first, followed by the anion.
Use the lowest ratio of ions (formula unit).

Example: Magnesium (Mg2+) combines with two chloride ions (Cl-) to form MgCl2.

Formation of magnesium chloride from magnesium and chloride ions

Example: Sodium (Na+) combines with chloride (Cl-) to form NaCl.

Formation of sodium chloride from sodium and chloride ions

Example: Two sodium ions (Na+) combine with one sulfide ion (S2-) to form Na2S.

Formation of sodium sulfide from sodium and sulfide ions

Naming Ionic Compounds

The name of an ionic compound consists of the name of the metal (cation) and the nonmetal (anion), with the anion's name ending in "-ide." Subscripts are not used in the written name.

Example: NaCl is named sodium chloride.
Example: MgCl2 is named magnesium chloride.

Metals with Variable Charges

Transition metals (except Zn2+, Cd2+, and Ag+) can form ions with different charges. The charge is indicated by a Roman numeral in parentheses after the metal name.

Example: FeO is named iron(II) oxide.
Example: MnF2 is named manganese(II) fluoride.

Determining Variable Charge

To determine the charge and name of a compound with a variable charge metal:

Determine the charge of the cation from the anion.
Name the cation with its elemental name and Roman numeral for the charge.
Name the anion with the "-ide" suffix.
Write the cation name first, then the anion.

Example: MnF2 is manganese(II) fluoride.

Common Elements & Charges

Some transition elements have only a single charge:

Zinc: Zn2+
Cadmium: Cd2+
Silver: Ag+

Writing Formulas for Ionic Compounds from the Name

To write the formula for an ionic compound:

Identify the cation and its charge.
Identify the anion and its charge.
Balance the charges.
Write the formula with cation first, using subscripts as needed.

Example: Tin(II) chloride is SnCl2.

Table: Examples of Ionic Compounds

This table compares the composition and naming of several ionic compounds.

Compound	Metal Ion	Nonmetal Ion	Name
KI	K+	I-	Potassium iodide
MgBr2	Mg2+	Br-	Magnesium bromide
Al2O3	Al3+	O2-	Aluminum oxide

Table of ionic compounds, ions, and names

Polyatomic Ions

Naming with Polyatomic Ions

Polyatomic ions are groups of atoms with a net charge. Most end in "-ate"; if they have one less oxygen, the suffix "-ite" is used. "Per-" and "hypo-" indicate more or fewer oxygens, respectively.

Example: ClO4- is perchlorate, ClO3- is chlorate, ClO2- is chlorite, ClO- is hypochlorite.

Writing Formulas for Compounds with Polyatomic Ions

Polyatomic ions combine with ions of opposite charge. The total positive and negative charges must equal zero. Parentheses are used around the polyatomic ion if more than one is needed.

Subscripts for polyatomic ions are placed outside the parentheses.

Example: Magnesium nitrate is Mg(NO3)2.

Formation and formula of magnesium nitrate with polyatomic ions

Naming Compounds with Polyatomic Ions

Follow the same rules as for simple ionic compounds:

Write the cation name first.
Write the name of the polyatomic ion second.
Recognition of polyatomic ions is key for correct naming.

Example: Ca(NO3)2 is calcium nitrate; SnSO4 is tin(II) sulfate.

Additional info: Flashcards are recommended for memorizing common polyatomic ions and their charges.