Modern Atomic Theory and Molecular Geometry

Chapter 11: Modern Atomic Theory

Evolution of Atomic Models

Rutherford's Model: This model contains a dense nucleus with electrons outside and consists of mostly empty space. However, it could not explain electron arrangement or why electrons do not collapse into the nucleus.

Bohr Model: Bohr incorrectly assumed that electrons travel around the nucleus in fixed orbits.

Wave Mechanical Model: This is the modern model where electrons have both particle and wave properties instead of fixed orbits; it utilizes orbitals.

Electromagnetic Radiation and Energy

Electromagnetic Radiation is energy transmitted through space and light. Wavelength (λ) is the distance between wave peaks, while Frequency (ν) is the number of waves passing a point per unit of time.

• Short Wavelength / High Frequency: High energy.
• Long Wavelength / Low Frequency: Low energy.
• Spectrum: X-ray > UV > Visible > IR > Microwave > Radio.
• Visible Light: Blue light has higher energy; red light has lower energy.

Excitation, Emission, and Quantized Energy

Atoms gain energy by absorbing a photon or lose energy by emitting a photon. The Ground State is the lowest energy or unexcited state of an atom. Quantized energy means electrons only exist in specific energy levels. Moving between levels absorbs or releases fixed energy as photons; consequently, each element has a unique emission spectrum.

Orbitals and Electron Configuration

• Level 1 (Bottom): Contains only the 1s-sphere; it can hold 2 electrons and has the lowest energy (closest to the nucleus).
• Level 2: Contains the 2s sphere and three 2p double lobes; each can hold 2 electrons.
• Level 3: Contains 3s, three 3p, and five 3d-clover orbitals.
• Level 4: Contains 4s, three 4p, five 4d, and seven 4f-"freaks."

The Pauli Exclusion Principle states that two electrons in the same orbital must have opposite spins. Valence electrons are those in the outermost principal energy level. Note that 4s is lower in energy than 3d. Potassium (K) and Transition Metals are special cases. Shorthand configurations use Noble Gases.

Periodic Trends

Metals tend to lose electrons (found at the lower left of the table), while non-metals (NM) gain electrons (found at the upper right). Ionization Energy is the energy required to remove an electron; it decreases down a group and increases across a period.

Chapter 12: Chemical Bonding

Types of Chemical Bonds

• Ionic Bond: Involves the transfer of electrons, resulting in high melting and boiling points (Metal + Non-metal).
• Covalent Bond: Involves shared electrons (Non-metal + Non-metal).
• Polar Covalent: A middle ground where one atom "hogs" the shared electrons.

Electronegativity and Polarity

Electronegativity (EN) is the ability of an atom in a molecule to attract electrons. It increases across a period and decreases down a group; Fluorine (F) is the most electronegative element.

Electronegativity Difference (ΔEN)

• 0–0.4: Nonpolar covalent.
• 0.4–2: Polar covalent.
• >2: Ionic.

In a Dipole, the arrow points toward the negative end. Asymmetric molecules are polar, while symmetric molecules are nonpolar.

Ionic Charges and Lewis Structures

Common charges by group: Group 1 = +1, Group 2 = +2, Group 13 = +3, Group 15 = -3, Group 16 = -2, Group 17 = -1. Metals and non-metals exchange electrons until they reach a noble gas configuration or even electron count.

Lewis Dot Structures: Dots represent valence electrons. One pair of electrons forms a bond between each pair of bound atoms (often represented by a line). Arrange remaining electrons to satisfy the Duet Rule (Hydrogen only needs 2 electrons) and the Octet Rule (2nd-row non-metals want 8 electrons). Some electron pairs may need to be shared, denoted with double lines, to reach full valence. A shared pair is a bonding pair, while an unshared pair is a lone pair. Resonance structures occur when multiple valid Lewis structures exist for one molecule.

VSEPR Model and Molecular Geometry

The VSEPR Model states that electron pairs want to be as far from each other as possible. When determining VSEPR geometry, lone pairs matter.

Common Shapes and Angles

• Bent: 105°
• Linear: 180°
• Trigonal Planar: 120°
• Tetrahedral: Four items sticking out in four symmetric directions with an angle of 109.5°.