Chapter 5: Chemical Bonding

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Chapter Overview & SLOs

What is a chemical bond? A chemical bond is a force of attraction that holds atoms together in a compound. Atoms form chemical bonds to achieve stability by attaining a noble gas electronic configuration.

What are the octet and duplet rules?

• Octet rule: Atoms tend to gain, lose, or share electrons to achieve a stable configuration of 8 electrons in their valence shell (like noble gases except helium).
• Duplet rule: Hydrogen and helium achieve stability with 2 electrons in their valence shell (like helium).
• Examples: Hydrogen (H) achieves duplet by sharing or gaining 1 electron; Nitrogen (N) achieves octet by sharing 3 electrons in $N_2$.

What are the primary types of chemical bonds?

• Ionic Bond (Electrovalent Bond): Formed by complete transfer of electrons from a metal to a nonmetal, resulting in the formation of cations (positive ions) and anions (negative ions).
• Covalent Bond: Formed by mutual sharing of electrons between nonmetal atoms.

How do atoms form ions? Metals lose electrons to form positive ions (cations), while nonmetals gain electrons to form negative ions (anions).

• Cation formation: $Al \rightarrow Al^{3+} + 3e^-$ (Aluminum loses 3 electrons), $Sr \rightarrow Sr^{2+} + 2e^-$ (Strontium loses 2 electrons).
• Anion formation: $S + 2e^- \rightarrow S^{2-}$ (Sulfur gains 2 electrons to complete its octet).

What are the types of covalent bonds based on electron sharing?

• Single Covalent Bond: One pair of electrons is shared between two atoms. Example: $H_2$ (H-H), $F_2$ (F-F).
• Double Covalent Bond: Two pairs of electrons are shared between two atoms. Example: $O_2$ (O=O).
• Triple Covalent Bond: Three pairs of electrons are shared between two atoms. Example: $N_2$ (N≡N).

What are Lewis dot structures (cross-dot diagrams)? Lewis dot structures represent valence electrons as dots (or crosses) around the symbol of an element. They help visualize bonding patterns and electron sharing or transfer.

• Example: $H_2$ can be represented as H• + •H → H:H (shared pair)
• Example: $O_2$ as O::O (double bond) with each oxygen having two lone pairs

How do we determine the number of valence electrons? For main group elements, the group number (IA to VIIIA) equals the number of valence electrons.

• Boron (B) is in Group IIIA → 3 valence electrons
• Arsenic (As) is in Group VA → 5 valence electrons

What are isotopes? Isotopes are atoms of the same element with the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers).

• Sulfur isotopes: S-32 (16 protons, 16 neutrons), S-33 (16 protons, 17 neutrons), S-34 (16 protons, 18 neutrons).

How do we calculate relative atomic mass? Relative atomic mass is the weighted average mass of all naturally occurring isotopes of an element, relative to 1/12th the mass of carbon-12.

• Formula: $$\text{Relative atomic mass} = \frac{\sum (\text{isotopic mass} \times \text{abundance})}{100}$$
• Example calculation for sulfur given abundances of S-32, S-33, S-34

Comparison of Ionic and Covalent Bonds:

| Property | Ionic Bond | Covalent Bond | |----------|------------|---------------| | Formation | Electron transfer | Electron sharing | | Participants | Metal + Nonmetal | Nonmetal + Nonmetal | | Solubility | Soluble in water | Insoluble in water (generally) | | Melting point | High | Low | | Conductivity | Conducts when molten/dissolved | Does not conduct |

These notes are strictly aligned with the Student Learning Outcomes (SLOs) for the FBISE 2026 annual examination.

• How do we apply stability rules to bonding? Define and provide examples of the octet rule (atoms achieve 8 valence electrons, e.g., Nitrogen in $N_2$) and duplet rule (hydrogen and helium achieve 2 electrons), explaining how atoms like hydrogen and nitrogen achieve noble gas configurations through sharing (covalent) or transfer (ionic).
• How do we represent ion formation? Use electron dot structures and electronic configurations to show how metal atoms lose electrons to form stable cations ($Al \rightarrow Al^{3+} + 3e^-$, $Sr \rightarrow Sr^{2+} + 2e^-$) and how nonmetals gain electrons to form anions ($S + 2e^- \rightarrow S^{2-}$).
• How do we distinguish between covalent bond varieties? Draw Lewis cross-dot diagrams to illustrate the formation of single covalent bonds (one shared pair, e.g., $H_2$, $F_2$), double covalent bonds (two shared pairs, e.g., $O_2$), and triple covalent bonds (three shared pairs, e.g., $N_2$) in diatomic molecules.
• How do we analyze isotopes and atomic mass? Define isotopes as atoms with same number of protons but different neutrons (e.g., sulfur isotopes S-32, S-33, S-34), identify their subatomic particle counts (protons, neutrons, electrons), and calculate the relative atomic mass of an element based on isotopic abundance using the weighted average formula.

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