Atoms, Molecules, and Compounds

Definitions and Classifications

Understanding the basic units of matter is essential in chemistry. Atoms, molecules, and compounds are foundational concepts that describe how elements combine and interact.

• Atom: The smallest unit of an element, consisting of protons, neutrons, and electrons. Examples: H, Mg, N.

• Molecule: A group of two or more atoms chemically bonded together. Molecules can consist of the same or different types of atoms. Examples: H2 (element), H2O (compound), CO2 (compound).

• Compound: A substance containing two or more different types of atoms in fixed proportions. Examples: H2O, CO2, NaCl.

Chemical Bonds

Types of Chemical Bonds

Chemical bonds hold atoms together in molecules and compounds. The type of bond formed depends on the atoms involved and their tendency to gain, lose, or share electrons.

• Ionic Bonds: Formed by the transfer of electrons from a metal (cation) to a nonmetal (anion). Example: NaCl (sodium chloride).

• Covalent Bonds: Formed by the sharing of electrons between nonmetal atoms. Example: H2O (water), CO2 (carbon dioxide).

The octet rule states that atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight valence electrons.

Ions and Ionic Compounds

Formation of Ions

Atoms form ions by gaining or losing electrons to achieve a stable electron configuration. The charge of an ion depends on the number of electrons lost or gained.

• Cation: A positively charged ion formed when an atom loses electrons (usually metals).

• Anion: A negatively charged ion formed when an atom gains electrons (usually nonmetals).

For example, sulfur (S) in Group 6A has 6 valence electrons and gains 2 electrons to become S2−. Potassium (K) in Group 1A loses 1 electron to become K+.

Variable Charges of Transition Metals

Transition metals can form ions with different positive charges. The specific charge is indicated using Roman numerals in the ion's name (e.g., iron(II) is Fe2+, iron(III) is Fe3+).

• Transition metals always form positive ions (cations).

• Examples: Cu+ (copper(I)), Cu2+ (copper(II)), Fe2+ (iron(II)), Fe3+ (iron(III)).

Lewis Symbols and Valence Electrons

Lewis Symbols

Lewis symbols visually represent the valence electrons of an atom. The number of valence electrons corresponds to the group number for main group elements.

• Valence electrons are the outermost electrons and are involved in chemical bonding.

• Example: Nitrogen (N) is in Group 5A and has 5 valence electrons, represented as five dots around the symbol N.

Chemical Formulas

Writing Chemical Formulas

A chemical formula uses element symbols and subscripts to show the types and numbers of atoms in a compound.

• Examples: H2O (2 hydrogen, 1 oxygen), MgCl2 (1 magnesium, 2 chlorine), Na2SO4 (2 sodium, 1 sulfur, 4 oxygen).

• To determine the total number of atoms in a formula, add the subscripts for each element.

Example: Li2SO4 contains 2 lithium, 1 sulfur, and 4 oxygen atoms, for a total of 7 atoms.

Steps for Writing Ionic Compound Formulas

Procedure

To write the formula for an ionic compound:

1. Determine the charge of each ion.

2. Balance the charges so the total positive and negative charges are equal.

3. Ensure the overall charge of the compound is zero.

4. Write the formula with the metal (cation) first, followed by the nonmetal (anion).

Examples:

• Mg and Cl: Mg2+ and Cl− combine to form MgCl2.

• Na and S: Na+ and S2− combine to form Na2S.

• Ca and N: Ca2+ and N3− combine to form Ca3N2.

• Al and O: Al3+ and O2− combine to form Al2O3.

Visualizing Atoms, Molecules, and Compounds

Particle Diagrams

Diagrams can help visualize the differences between atoms, molecules, and compounds. Atoms are single spheres, molecules are groups of spheres, and compounds are repeating units of different atoms bonded together.

Examples of Molecular Structures

Common molecules such as water, ammonia, carbon dioxide, and methane have distinct shapes and bonding patterns.

Lewis Structures for Molecules

Lewis Structures

Lewis structures show how valence electrons are arranged among atoms in a molecule. They help predict the arrangement of atoms and the types of bonds formed.

• Each dot represents a valence electron.

• Shared pairs of electrons (bonds) are shown as lines between atoms.

Summary Table: Common Ions and Their Charges

The following table summarizes the common ions formed by main group elements and their charges:

Additional info:

• The lecture schedule confirms that these topics are covered in Ch. 6: Ionic and Molecular Compounds, which is part of the GOB college chemistry curriculum.