Atoms, Molecules, and Ions: Structure, Formulas, and Nomenclature

Atoms, Molecules, and Ions

Overview

This section covers the foundational concepts of atomic structure, molecular and empirical formulas, ions, and the systematic naming of inorganic and simple organic compounds. Mastery of these topics is essential for understanding chemical reactions and properties of substances.

The Atomic Theory and Structure

Atomic Number, Mass Number, and Isotopes

• Atomic Number (Z): The number of protons in the nucleus of an atom. It defines the element.

• Mass Number (A): The total number of protons and neutrons in an atom's nucleus.

• Isotopes: Atoms of the same element (same Z) with different numbers of neutrons (different A).

Equation for mass number:

$A = Z + N$

where $N$ is the number of neutrons.

The Periodic Table

Organization and Reading

• The periodic table arranges elements in order of increasing atomic number (Z).

• Each box lists the atomic number above the symbol and the atomic weight (average atomic mass) below the symbol.

• Elements are classified as metals, nonmetals, and metalloids based on their properties and position.

Chemical Formulas and Molecules

Types of Chemical Formulas

• Molecular Formula: Shows the exact number of atoms of each element in a molecule (e.g., $H_2O$, $C_2H_4$).

• Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound (e.g., $CH_2$ for $C_2H_4$).

• Structural Formula: Indicates the connectivity of atoms but not the 3D shape.

• Perspective, Ball-and-Stick, and Space-Filling Models: Visualize the three-dimensional arrangement of atoms.

Diatomic Molecules: Molecules consisting of two identical atoms (e.g., $H_2$, $N_2$, $O_2$, $F_2$, $Cl_2$, $Br_2$, $I_2$).

Empirical vs. Molecular Formulas

• Molecular formulas give the actual number of atoms of each element in a molecule.

• Empirical formulas give the lowest whole-number ratio of atoms.

• If the molecular formula is known, the empirical formula can be determined, but not vice versa without additional information.

Examples:

• Molecular: $C_2H_4$; Empirical: $CH_2$

• Molecular: $H_2O_2$; Empirical: $HO$

Ions and Ionic Compounds

Formation of Ions

• Cations: Positively charged ions formed when atoms lose electrons (e.g., $Na^+$, $Ca^{2+}$).

• Anions: Negatively charged ions formed when atoms gain electrons (e.g., $Cl^-$, $O^{2-}$).

• Polyatomic Ions: Ions composed of multiple atoms (e.g., $SO_4^{2-}$, $NH_4^+$, $PO_4^{3-}$).

Key Principle: Metal atoms tend to lose electrons (form cations), nonmetal atoms tend to gain electrons (form anions).

Predicting Ionic Charges

• Group 1A: $+1$ ions

• Group 2A: $+2$ ions

• Group 6A: $-2$ ions

• Group 7A: $-1$ ions

• Other groups: Variable charges, especially transition metals

Writing Formulas for Ionic Compounds

• Ionic compounds are electrically neutral; total positive charge equals total negative charge.

• Subscripts are used to balance charges (e.g., $Mg^{2+}$ and $N^{3-}$ combine to form $Mg_3N_2$).

• Reduce subscripts to the lowest whole-number ratio.

Examples:

• $Al^{3+}$ and $Cl^-$ → $AlCl_3$

• $Mg^{2+}$ and $NO_3^-$ → $Mg(NO_3)_2$

Naming Ions and Compounds

Positive Ions (Cations)

• Cations from nonmetals end in -ium (e.g., ammonium ion $NH_4^+$).

• Cations from metals use the element name (e.g., sodium ion $Na^+$).

• Transition metals with variable charge use Roman numerals (e.g., iron(II) $Fe^{2+}$, iron(III) $Fe^{3+}$) or -ous/-ic endings (ferrous/ferric).

Common Cations

Charge	Formula	Name	Formula	Name
1+	H+	hydrogen ion	NH4+	ammonium ion
1+	Li+	lithium ion	Cu+	copper(I) ion
2+	Mg2+	magnesium ion	Fe2+	iron(II) ion
3+	Al3+	aluminum ion	Fe3+	iron(III) ion

Negative Ions (Anions)

• Monatomic anions use the ending -ide (e.g., chloride $Cl^-$, sulfide $S^{2-}$).

• Polyatomic anions containing oxygen are called oxyanions:

  • Fewer oxygens: -ite (e.g., nitrite $NO_2^-$)

  • More oxygens: -ate (e.g., nitrate $NO_3^-$)

  • Series with more than two: per-...-ate (most O), hypo-...-ite (least O)

• Oxyanions with hydrogen: add hydrogen or bi- (one H), dihydrogen (two H)

Common Anions

Charge	Formula	Name	Formula	Name
1-	F-	fluoride ion	NO3-	nitrate ion
2-	O2-	oxide ion	SO42-	sulfate ion
3-	N3-	nitride ion	PO43-	phosphate ion

Acid Nomenclature

• Acids yield hydrogen ions ($H^+$) when dissolved in water.

• If the anion ends in -ide: hydro-...-ic acid (e.g., $Cl^-$ → hydrochloric acid, $HCl$)

• If the anion ends in -ate: ...-ic acid (e.g., $NO_3^-$ → nitric acid, $HNO_3$)

• If the anion ends in -ite: ...-ous acid (e.g., $NO_2^-$ → nitrous acid, $HNO_2$)

Common Acids:

• Sulfuric Acid ($H_2SO_4$)

• Phosphoric Acid ($H_3PO_4$)

• Perchloric Acid ($HClO_4$)

• Nitric Acid ($HNO_3$)

Nomenclature of Binary Molecular Compounds

• Binary molecular compounds contain two nonmetal elements.

• The element farther left or lower in the group is named first (exceptions apply).

• Prefixes indicate the number of atoms (mono-, di-, tri-, tetra-, penta-, etc.). Mono- is not used for the first element.

• The second element ends in -ide (e.g., $CO_2$: carbon dioxide).

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

Nomenclature of Organic Compounds: Alkanes and Alcohols

Alkanes

• Alkanes are hydrocarbons containing only single bonds between carbon atoms.

• The prefix indicates the number of carbons (meth- = 1, eth- = 2, prop- = 3, etc.), followed by -ane.

• Examples: methane ($CH_4$), ethane ($C_2H_6$), propane ($C_3H_8$).

Alcohols

• Alcohols are derived from alkanes by replacing a hydrogen with an -OH group.

• The name is based on the parent alkane, ending in -ol (e.g., methanol, ethanol, propanol).

Summary Table: Key Ions to Memorize

Name	Formula
Carbonate	CO32-
Nitrate	NO3-
Phosphate	PO43-
Sulfate	SO42-
Perchlorate	ClO4-

Additional info: This guide covers the essential aspects of atomic structure, chemical formulas, ions, and nomenclature, providing a foundation for further study in general chemistry.