Chapter 3: Compounds—How Elements Combine (Study Notes for GOB Chemistry)

Chapter 3: Compounds—How Elements Combine

3.1 Electron Arrangements and the Octet Rule

Understanding how electrons are arranged in atoms is essential for predicting how elements combine to form compounds. The arrangement of electrons in energy levels (shells) determines the chemical properties and reactivity of elements.

• Energy Levels: Electrons occupy specific energy levels around the nucleus, filling the lowest energy levels first.

• Maximum Electrons per Level: The maximum number of electrons in an energy level is given by the formula $2n^2$, where n is the energy level number.

• Valence Electrons: The electrons in the outermost energy level (valence shell) are called valence electrons. These are responsible for chemical bonding.

• Groups and Periods: In the periodic table, groups (columns) indicate the number of valence electrons for main-group elements, while periods (rows) indicate the highest occupied energy level.

• Octet Rule: Atoms tend to gain, lose, or share electrons to achieve eight electrons in their valence shell, a stable configuration similar to noble gases. Hydrogen and helium are exceptions, aiming for two electrons (duet rule).

Table: Electron Arrangements for Selected Elements

Element	Group Number	Total Electrons	n=1	n=2	n=3	n=4
H	1A	1	1
He	8A	2	2
Li	1A	3	2	1
Be	2A	4	2	2
B	3A	5	2	3
C	4A	6	2	4
N	5A	7	2	5
O	6A	8	2	6
F	7A	9	2	7
Ne	8A	10	2	8
Na	1A	11	2	8	1
Mg	2A	12	2	8	2
Al	3A	13	2	8	3
Si	4A	14	2	8	4
P	5A	15	2	8	5
S	6A	16	2	8	6
Cl	7A	17	2	8	7
Ar	8A	18	2	8	8
K	1A	19	2	8	8	1
Ca	2A	20	2	8	8	2

3.2 In Search of an Octet, Part 1: Ion Formation

Atoms can achieve a stable octet by gaining or losing electrons, resulting in the formation of ions. Ions are charged particles that play a crucial role in the formation of compounds.

• Anions: Atoms that gain electrons become negatively charged anions.

• Cations: Atoms that lose electrons become positively charged cations.

• Isoelectronic: Ions with the same number of electrons as a noble gas are called isoelectronic with that noble gas.

• Trends: Main-group metals (Groups 1A, 2A, 3A) form cations with a charge equal to their group number. Nonmetals (Groups 5A, 6A, 7A) form anions with a charge equal to 8 minus their group number.

• Polyatomic Ions: Groups of atoms that carry a charge, such as sulfate (SO42−) or ammonium (NH4+).

Table: Common Polyatomic Ions and Their Uses

Ion	Name	Formula	Common Uses
Hydroxide	OH−	Antacids
Hydronium	H3O+	Acidity (pH)
Ammonium	NH4+	Indicator of organ function
Nitrate	NO3−	Preservative
Sulfate	SO42−	Epsom salts
Phosphate	PO43−	Energy transfer in cells

Table: Common Cations and Anions in the Body

Ion	Function	Sources
Na+	Regulates fluids outside cells	Table salt, seafood
K+	Maintains ion concentration in cells	Dairy, bananas
Ca2+	Muscle contraction, bones	Dairy, leafy vegetables
Cl−	Fluid balance	Table salt
HCO3−	Acid–base balance	Produced in body

Naming Ions

• Metals: Add "ion" to the metal name (e.g., sodium ion).

• Transition Metals: Use Roman numerals to indicate charge (e.g., iron(II) ion).

• Nonmetals: Change the ending to "-ide" (e.g., chloride).

• Polyatomic Ions: Most end in "-ate" or "-ite" (e.g., sulfate, nitrite). Exceptions include hydroxide, hydronium, and cyanide.

3.3 Ionic Compounds—Electron Give and Take

Ionic compounds are formed when metals transfer electrons to nonmetals, resulting in the attraction between oppositely charged ions (ionic bonds).

• Ionic Bond: The electrostatic attraction between cations and anions.

• Formula: The ratio of ions in an ionic compound is such that the total positive and negative charges balance to zero.

• Writing Formulas: Use subscripts to indicate the number of each ion needed to balance charges. Charges are not shown in the final formula.

• Naming: Combine the names of the cation and anion (e.g., sodium chloride). For transition metals, include the Roman numeral for charge. For polyatomic ions, use the ion's name unchanged.

3.4 In Search of an Octet, Part 2: Covalent Bond Formation

Nonmetals achieve octets by sharing valence electrons, forming covalent bonds. The resulting compounds are called covalent (or molecular) compounds, and their smallest unit is a molecule.

• Covalent Bond: A shared pair of electrons between two nonmetal atoms.

• Lewis Structures: Electron-dot symbols (Lewis structures) show how atoms share electrons to complete their octets.

• Bonding Capacity: The number of covalent bonds an atom forms equals the number of electrons needed to complete its octet (e.g., N forms 3, O forms 2, F forms 1, C forms 4).

• Multiple Bonds: Atoms like carbon can form single, double, or triple bonds to achieve an octet.

• Molecular Formula: Indicates the actual number of each atom in a molecule (e.g., glucose is C6H12O6).

Naming Covalent Compounds

• Name the first element in the formula.

• Name the second element, changing the ending to "-ide".

• Use Greek prefixes to indicate the number of each atom (mono-, di-, tri-, tetra-, etc.).

• Some compounds have traditional names (e.g., water for H2O, ammonia for NH3).

Table: Greek Prefixes for Covalent Compounds

Prefix	Number
mono-	1
di-	2
tri-	3
tetra-	4
penta-	5
hexa-	6
hepta-	7

Summary Table: Distinguishing Ionic and Covalent Compounds

Type	Constituents	Bonding	Naming
Ionic	Metal + Nonmetal	Electron transfer (ionic bond)	Cation + Anion (no prefixes)
Covalent	Nonmetal + Nonmetal	Electron sharing (covalent bond)	Prefixes + -ide ending

Additional info: These notes cover the foundational concepts of electron arrangements, the octet rule, ion formation, ionic and covalent bonding, and the basics of naming compounds, as required for GOB Chemistry students.