Atoms, Molecules, and Ions: Structure and Nomenclature

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Atoms, Molecules, and Ions

Introduction

This chapter explores the fundamental building blocks of matter: atoms, molecules, and ions. It addresses what atoms are, how they differ, how they combine to form compounds, and the rules for naming these compounds. Understanding these concepts is essential for mastering general chemistry.

Atomic Structure

What Makes Up an Atom?

Atom: The smallest unit of an element that retains the chemical properties of that element.
Composed of a nucleus (containing protons and neutrons) and electrons that orbit the nucleus.
The nucleus is extremely small compared to the overall size of the atom (if the atom were the size of a football stadium, the nucleus would be the size of a pea).

What Makes One Atom Different from Another?

Atomic Number (Z): The number of protons in the nucleus; defines the element.
Isotopes: Atoms of the same element with different numbers of neutrons.
Ions: Atoms or molecules that have gained or lost electrons, resulting in a net charge.

Elements and the Periodic Table

Classification of Elements

Alkali metals: Li, Na, K, Rb, Cs, Fr
Alkaline earth metals: Be, Mg, Ca, Sr, Ba, Ra
Chalcogens: O, S, Se, Te, Po
Noble gases: He, Ne, Ar, Kr, Xe, Rn

Molecules and Compounds

How Do Atoms Combine to Form Compounds?

Molecule: Two or more atoms bonded together.
Compound: A substance composed of two or more different elements chemically combined in fixed proportions.
Atoms combine via chemical bonds (ionic or covalent) to form compounds.

Ways to Represent Molecules

Chemical formulas: Indicate the types and numbers of atoms (e.g., H2O, CO2).
Structural formulas: Show how atoms are connected.
Space-filling and ball-and-stick models: Visualize the 3D arrangement of atoms.

Ions and Their Charges

Common Charges of Ions

Group 1A: +1 (e.g., Na+)
Group 2A: +2 (e.g., Ca2+)
Group 7A (halogens): -1 (e.g., Cl-)
Transition metals: Variable charges (e.g., Fe2+, Fe3+)

Common Cations and Anions

Formula	Name
H+	Hydrogen ion
Na+	Sodium ion
Ca2+	Calcium ion
NH4+	Ammonium ion
Cl-	Chloride ion
O2-	Oxide ion
SO42-	Sulfate ion
NO3-	Nitrate ion
CO32-	Carbonate ion
PO43-	Phosphate ion

Additional info: Table includes the most common ions students are expected to know.

Naming Inorganic Compounds

Naming Ionic Compounds

Name consists of the cation (positive ion) followed by the anion (negative ion).
For metals with variable charges, indicate the charge with a Roman numeral (e.g., FeCl2: iron(II) chloride).
Monatomic anions are named by replacing the ending of the element with -ide (e.g., O2-: oxide ion).
Polyatomic ions have specific names (e.g., NO3-: nitrate ion).

Naming Oxyanions

Oxyanions are polyatomic ions containing oxygen.
The ion with more oxygen atoms gets the -ate suffix; the one with fewer gets -ite (e.g., SO42-: sulfate, SO32-: sulfite).
Prefixes per- (one more O) and hypo- (one less O) are used for series (e.g., ClO4-: perchlorate, ClO-: hypochlorite).
Hydrogen or dihydrogen can be added to indicate the presence of H (e.g., HCO3-: hydrogen carbonate).

Naming Acids

If the anion ends in -ide, the acid name begins with hydro- and ends with -ic acid (e.g., HCl: hydrochloric acid).
If the anion ends in -ate, the acid name ends with -ic acid (e.g., HNO3: nitric acid).
If the anion ends in -ite, the acid name ends with -ous acid (e.g., HNO2: nitrous acid).
Prefixes per- and hypo- are retained in the acid name (e.g., HClO4: perchloric acid).

Naming Binary Molecular Compounds

Composed of two nonmetals or a nonmetal and a metalloid.
The element farther left (or lower if in the same group) in the periodic table is named first.
The second element is named with an -ide ending.
Prefixes indicate the number of each atom present:

Prefix	Number
mono-	1
di-	2
tri-	3
tetra-	4
penta-	5
hexa-	6
hepta-	7
octa-	8
nona-	9
deca-	10

Example: CO2 is carbon dioxide, N2O4 is dinitrogen tetroxide.

Examples and Applications

Table Salt (NaCl): Composed of sodium ions (Na+) and chloride ions (Cl-); forms a crystalline solid that dissolves in water.
Diamonds: Made of carbon atoms bonded in a network; hard and transparent due to strong covalent bonds.

Summary Table: Common Ions

Cation	Name	Anion	Name
Na+	Sodium ion	Cl-	Chloride ion
Ca2+	Calcium ion	O2-	Oxide ion
NH4+	Ammonium ion	NO3-	Nitrate ion
Fe2+	Iron(II) ion	SO42-	Sulfate ion

Key Equations

General formula for ionic compounds:

Example:

Additional info: This guide covers the essential concepts and nomenclature rules for atoms, molecules, and ions, as typically required in a first-semester General Chemistry course.