Chemical Bonding and Molecular Structure Principles

Atomic Energy Levels and Nuclear Charge

In the n = 1 shell, electrons experience nearly the full nuclear charge, resulting in a strong electrostatic interaction between the electrons and the nucleus. The energy of the n = 1 shell also decreases tremendously (the filled 1s orbital becomes more stable) as the nuclear charge increases. For similar reasons, the filled n = 2 shell in argon is located closer to the nucleus and has a lower energy than the n = 2 shell in neon.

Molecular Geometry and Polarity

CH₄ is a tetrahedral, non-polar molecule. Because it is non-polar, it is a gas at SATP (Standard Ambient Temperature and Pressure). CH₃OH is tetrahedral around the carbon atom but bent around the oxygen atom. CH₃OH is a polar molecule, and this substance (methanol) is a liquid at SATP.

Properties of Ionic Solids

Ionic solids are composed of oppositely charged ions, consisting of positively charged cations and negatively charged anions. When ionic solids are dissolved in water, the cations and the anions separate and become free to move about in the water, allowing the solution to conduct electrical current. Lattice energy is defined as the heat of formation for ions of opposite charge in the gas phase combining into an ionic solid. For example, the lattice energy of sodium chloride (NaCl) is the energy released when gaseous Na⁺ and Cl^- ions come together to form a lattice of alternating ions in the NaCl crystal.

Forces Governing Chemical Structures

• Intramolecular forces are the forces that hold atoms together within a molecule.
• Intermolecular forces are forces that exist between molecules.

Key forces include:

1. Strong ionic attraction (recall lattice energy and its relation to solid properties).
2. Intermediate dipole-dipole forces.
3. Weak London dispersion forces (or van der Waals' forces); these forces always operate in any substance.
4. Hydrogen bonds.
5. Covalent bonding.
6. Metallic bonding.

Electronegativity and Molecular Attraction

Since Y has a higher electronegativity than the surrounding atoms, the resulting molecule will exhibit polarity. Since this molecule has a trigonal pyramidal geometry and is polar, the polar molecules interact strongly with other polar molecules because the positive end of one molecule is attracted to the negative end of another molecule. Intermolecular forces will be higher as the distance to the more electronegative element (Y) decreases.

Intermolecular Forces and Evaporation Rate

Stronger intermolecular forces lead to less evaporation. Evaporation occurs when the particles of a liquid escape the liquid and become gas particles.