10.3 Chemical Bonding: Ionic, Covalent & Metallic Bonds

Chemical Bonding: Ionic, Covalent & Metallic Bonds

Quick Answer: Chemical bonding occurs when atoms gain, lose, or share valence electrons to achieve the stability of a filled valence shell, typically 8 electrons (the octet rule). Ionic bonds form between metals and nonmetals via complete electron transfer, producing cations and anions held in a rigid crystal lattice with high melting points and electrolytic conductivity when dissolved. Covalent bonds form between nonmetals via shared electron pairs, classified as nonpolar (equal sharing) or polar (unequal sharing, creating dipoles). Metallic bonds feature positive metal ions surrounded by a delocalized "sea of electrons," conferring electrical conductivity, malleability, and ductility.

HiSET questions test your ability to classify bond types, compare properties, evaluate polarity, and interpret chemical formulas.

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The Driving Force of Bonding: The Octet Rule

Bonding lowers potential energy to achieve stability.

The octet rule states that atoms gain, lose, or share electrons until their outer shell contains 8 valence electrons ($ns^2 np^6$), matching noble gas stability.

• Duet Rule: Light elements like hydrogen ($H$) and helium ($He$) achieve stability with 2 valence electrons ($1s^2$), filling shell $n=1$.

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Ionic Bonding: Electron Transfer and Crystal Lattices

Ionic bonding occurs between a metal (loses electrons) and a nonmetal (gains electrons).

Mechanism of Ionic Bonding

1. Electron Transfer: Sodium ($Na: 1\text{ valence } e^-$) transfers its electron to chlorine ($Cl: 7\text{ valence } e^-$).
2. Ion Formation: Sodium forms cation $Na^+$; chlorine forms chloride anion $Cl^-$.
3. Coulombic Attraction: Oppositely charged ions attract via strong electrostatic forces ($F \propto \frac{q_1 q_2}{r^2}$).

Crystal Lattice & Physical Properties

Ionic compounds do not form isolated molecules. Alternating ions pack into a 3D crystal lattice ($NaCl$).

• Melting Points: High energy breaks the continuous lattice ($NaCl$ melts at 801°C).
• Brittleness: Impact shifts ion layers, aligning like charges ($+/+$ or $-/-$). Repulsion shatters the crystal.
• Conductivity: Solid ionic salts insulate. Dissolved or melted ions move freely, conducting electricity as electrolytes.

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Covalent Bonding: Electron Sharing in Molecular Compounds

Covalent bonding occurs between nonmetals, which share electron pairs to form discrete molecules.

Bond Multiplicity

• Single Bond (1 shared pair): Longest, lowest energy ($H-H$, $Cl-Cl$).
• Double Bond (2 shared pairs): Shorter and stronger ($O=O$ in $O_2$, $CO_2$).
• Triple Bond (3 shared pairs): Shortest and strongest bond ($N \equiv N$ in $N_2$).

Electronegativity Difference ($\Delta\text{EN}$) and Polarity

• Nonpolar Covalent ($\Delta\text{EN} < 0.4$): Symmetrical sharing; no dipole ($H_2, O_2, CH_4$).
• Polar Covalent ($0.4 \le \Delta\text{EN} \le 1.7$): Unequal sharing. The more electronegative atom acquires a partial negative charge ($\delta^-$), leaving the other partially positive ($\delta^+$).
  • Water ($H_2O$): Oxygen ($\text{EN} = 3.44$) draws electrons from hydrogen ($\text{EN} = 2.20$). Bent shape prevents dipole cancellation, creating a polar molecule.
• Ionic Bond ($\Delta\text{EN} > 1.7$): Complete electron transfer creates formal ions ($NaCl$).

Covalent molecular compounds have low melting points and do not conduct electricity.

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Metallic Bonding: The "Sea of Electrons"

Metallic bonding occurs among atoms of metals. Metal atoms release valence electrons into a delocalized cloud surrounding fixed positive cations.

• Conductivity: Delocalized electrons move freely, conducting electric current and heat.
• Malleability and Ductility: Non-directional bonds permit cation layers to slide past one another without fracturing.
• Luster: Mobile electrons absorb and re-emit visible light.

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Interpreting Chemical Formulas and Subscripts

Chemical formulas communicate elemental composition and stoichiometric ratios:

• Subscripts: Indicate atom counts. An omitted subscript denotes 1 ($H_2O$ has 2 H and 1 O).
• Polyatomic Parentheses: Subscripts outside parentheses multiply every atom inside.
  • Calcium Nitrate, $Ca(NO_3)_2$: 1 Ca, $1 \times 2 = \mathbf{2\text{ N}}$, $3 \times 2 = \mathbf{6\text{ O}}$, totaling $\mathbf{9\text{ atoms}}$.
• Coefficients: Numbers before a formula multiply the whole unit ($2H_2O$ has 4 H and 2 O atoms).
• Lewis Structures: Dot diagrams show valence electrons and shared pairs, illustrating octet completion.

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Comparison of Bonding Types

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HiSET Scenario Walkthrough: Classifying Unknown Substances

Scenario: A technician tests three unknown solid substances:

• Substance 1: Melts at 801°C, shatters under a hammer, conducts electricity only when dissolved in water. $\rightarrow$ Ionic compound ($NaCl$).
• Substance 2: Melts at 186°C, dissolves in water without conducting electricity. $\rightarrow$ Polar covalent compound (sucrose, $C_{12}H_{22}O_{11}$).
• Substance 3: Melts at 1085°C, malleable, conducts electricity as a solid. $\rightarrow$ Metallic solid (copper, $Cu$).

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Common HiSET Pitfalls & Exam Traps

[!CAUTION] Trap 1: Calling $NaCl$ a "molecule." Ionic compounds form extended lattices of formula units, never discrete molecules.

[!WARNING] Trap 2: Forgetting to multiply subscripts inside parentheses. In $Ca(NO_3)_2$, multiply nitrogen ($1 \times 2 = 2$) and oxygen ($3 \times 2 = 6$).

[!NOTE] Trap 3: Confusing polar covalent bonds with ionic bonds. Polar covalent bonds share electrons unequally ($\delta^+/\delta^-$), whereas ionic bonds involve full electron transfer ($+/ -$).