Ionic and Molecular Compounds: Structure, Bonding, and Formulas

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Ch. 6: Ionic and Molecular Compounds

Introduction to Ions and Ionic Compounds

Ions are atoms or groups of atoms that have gained or lost electrons, resulting in a net electrical charge. Ionic compounds are formed from the electrostatic attraction between positively charged cations (usually metals) and negatively charged anions (usually nonmetals). Understanding the formation, naming, and formulas of these compounds is essential in introductory chemistry.

Ions: Formation and Charges

Cations are positively charged ions formed when an atom loses electrons (typically metals).
Anions are negatively charged ions formed when an atom gains electrons (typically nonmetals).
The charge of an ion is determined by its group position in the periodic table.
Transition metals can have variable charges, indicated by Roman numerals in their names (e.g., iron(II) is Fe2+, iron(III) is Fe3+).
Example: Sulfur (S) is in Group 6A, has 6 valence electrons, and gains 2 electrons to become S2−.

Periodic table showing common ion charges

Atoms, Molecules, and Compounds

Atoms are the basic units of elements, molecules are groups of atoms bonded together, and compounds are substances composed of different elements in fixed ratios.

Atoms: Single units of elements (e.g., H, Mg, N).
Molecules: Two or more atoms chemically combined. Can be elements (e.g., H2) or compounds (e.g., H2O).
Compounds: Contain different types of atoms in fixed proportions (e.g., H2O, CO2, NaCl).

Diagram showing atoms, molecules, and compounds

Chemical Bonds

Chemical bonds hold atoms together in molecules and compounds. The type of bond depends on the atoms involved:

Ionic bonds: Formed by the transfer of electrons from a metal to a nonmetal, resulting in the formation of ions.
Covalent bonds: Formed by the sharing of electrons between nonmetal atoms.
The octet rule states that atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight valence electrons.

Chemical Formulas

A chemical formula represents the elements in a compound and the ratio of their atoms. Subscripts indicate the number of each atom present.

Example: H2O contains 2 hydrogen atoms and 1 oxygen atom.
Example: Na2SO4 contains 2 sodium, 1 sulfur, and 4 oxygen atoms.
Example: Ba(OH)2 contains 1 barium, 2 oxygen, and 2 hydrogen atoms.

Counting Atoms in a Formula

To find the total number of atoms in a formula, add the number of each type of atom.
Example: Li2SO4 has 2 Li, 1 S, and 4 O, totaling 7 atoms.

Lewis Symbols and Valence Electrons

Lewis symbols are a visual shorthand to represent the valence electrons of an atom, which are the electrons in the outermost shell and are involved in bonding. The number of valence electrons corresponds to the group number for main group elements.

Example: Nitrogen (N) is in Group 5A and has 5 valence electrons.

Lewis periodic table showing valence electrons

Types of Chemical Bonds

Ionic Bonds: Form between a cation (metal) and an anion (nonmetal) by electron transfer.
Covalent Bonds: Form between nonmetals by sharing electrons.

Molecular structures of water, ammonia, carbon dioxide, and methane

Writing Formulas for Ionic Compounds

To write the formula for an ionic compound:

Determine the charge of each ion.
Balance the charges so the total positive and negative charges are equal (overall charge is zero).
Write the formula with the metal (cation) first, followed by the nonmetal (anion).

Example: Magnesium (Mg2+) and chlorine (Cl−) form MgCl2.
Example: Aluminum (Al3+) and oxygen (O2−) form Al2O3.

Practice: Predicting Ionic Compound Formulas

Lithium (Li+) and oxygen (O2−): Li2O
Magnesium (Mg2+) and chlorine (Cl−): MgCl2
Sodium (Na+) and sulfur (S2−): Na2S
Calcium (Ca2+) and nitrogen (N3−): Ca3N2
Aluminum (Al3+) and oxygen (O2−): Al2O3

Summary Table: Common Ions and Their Charges

Group	Common Ion Charge	Examples
1A	+1	H+, Li+, Na+, K+
2A	+2	Mg2+, Ca2+, Ba2+
3A	+3	Al3+
5A	−3	N3−, P3−
6A	−2	O2−, S2−
7A	−1	F−, Cl−, Br−, I−

Visualizing Atoms, Molecules, and Compounds

Diagram showing atoms, molecules, and compounds

Lewis Structures for Molecules

Lewis structures use dots to represent valence electrons around atomic symbols. They help visualize bonding in molecules and predict molecular shapes.

Example: Water (H2O) has two single bonds between oxygen and hydrogen.
Example: Carbon dioxide (CO2) has two double bonds between carbon and oxygen.

Course Structure and Study Tips

Follow the lecture schedule and complete quizzes and assignments on time.
Review the study guide and practice problems before exams.
Use the periodic table and equation sheet provided during exams.

Lecture schedule for chemistry course

Sample Lewis Dot Structures

Lewis periodic table showing valence electrons

Sample Molecular Structures

Molecular structures of water, ammonia, carbon dioxide, and methane

Equations and Notation

General ionic compound formation:
Example:
Lewis structure for water:

Additional info: The provided images and tables reinforce the concepts of ion charges, Lewis structures, and molecular geometry, which are foundational for understanding ionic and molecular compounds in introductory chemistry.