Chapter 4: When Atoms Share Electrons

Foundations of Covalent Bonding

• Covalent bonds occur when atoms of nonmetals share electrons to fill valence shells, aiming for the configuration of the nearest noble gas.

• Gilbert N. Lewis: Developed dot structures based on the octet rule; first proposed the concept of the covalent bond in his 1916 paper, The Atom and the Molecule.

• Electronegativity: A measure of an atom's ability to draw electrons toward itself. Fluorine is the most electronegative element, while cesium is the least.

• Nonmetals achieve stability either by taking electrons from metals (ionic bond) or sharing with other nonmetals (covalent bond).

Bond Mechanics: Energy and Length

• Bond Energy: The energy required to break a specific chemical bond, measured in kilojoules per mole ($kJ/mol$).

• Bond Length: The equilibrium distance between nuclei where attractive forces (protons and shared electrons) balance repulsive forces (nucleus to nucleus).

• Bond Order Relationships:

  • Single Bonds: Share $2$ electrons; characterized by the lowest bond energy and longest bond length.

  • Double Bonds: Share $4$ electrons; intermediate energy and length.

  • Triple Bonds: Share $6$ electrons; characterized by the highest bond energy and shortest bond length.

• Stability: Shorter bond lengths (triple bonds) generally correspond to higher stability due to stronger bond energy.

Classifying Molecules

• Organic Molecules: Contain carbon-carbon ($C-C$) or carbon-hydrogen ($C-H$) bonds; known as the molecules of life.

• Inorganic Molecules: Lack the characteristic carbon-carbon or carbon-hydrogen pairings of organic molecules (e.g., $O_2$, $N_2$, $CO_2$, $H_2O$, and $NH_3$).

• Abundance: Only $17$ elements are nonmetals, and only $11$ of those combine to form the majority of substances in the world.

Comparison: Ionic vs. Molecular Compounds

• Physical State: Ionic compounds are typically solid crystal lattices; molecular compounds are often softer solids, liquids, or gases under standard conditions.

• Melting/Boiling Points: Molecular compounds have lower melting and boiling points due to weaker intermolecular forces compared to ionic lattice attractions.

• Solubility and Conductivity: Most covalent compounds do not dissociate in water and conduct electricity weakly or not at all.

• Complexity: Covalent bonding allows for the formation of complex macromolecules with hundreds of atoms.

IUPAC Nomenclature for Inorganic Molecular Compounds

• Naming Priority: The least electronegative element (farthest from Fluorine) is listed first.

• Suffix: Inorganic binary molecular compounds use the $-ide$ ending.

• Numerical Prefixes:

  • $1$: mono-

  • $2$: di-

  • $3$: tri-

  • $4$: tetra-

  • $5$: penta-

  • $6$: hexa-

  • $7$: hepta-

  • $8$: octa-

  • $9$: nona-

  • $10$: deca-

• Note: The "mono-" prefix is omitted for the first element but required for the second (e.g., $CO$ is carbon monoxide).

Molar Mass and Conversions

• Molecular Mass: The sum of the molar masses ($g/mol$) of all elements in a molecular formula.

• Examples from text:

  • $CO$: $12.01\,g/mol + 16.00\,g/mol = 28.01\,g/mol$

  • $H_2O$: $2 \times 1.01\,g/mol + 16.00\,g/mol = 18.02\,g/mol$

  • $P_2O_5$: $2 \times 30.97\,g/mol + 5 \times 16.00\,g/mol = 141.94\,g/mol$

• Gram-to-Mole Conversion: Divide the sample mass by the molecular mass.

  • Example: $92.2\,g$ of ethanol ($C_2H_5OH$, molecular mass $46.08\,g/mol$) equals $2.00\,mol$.