Question

Please explain and define molecular geometry.

How does molecular geometry relate to global warming?

Answer 1

The molecular geometry describes the shape of a molecule and the relative position of the atomic nuclei of a molecule

Temperature is a measure of the average energy of molecular motion in a sample of matter: to and fro translation, intramolecular vibration (and lattice vibration in solids), and rotation (both entire molecules and intramolecular portions). The sum of these motions’ energies can be described as the “thermal energy” of the sample. Thermal energy and, hence, temperature can change as various forms of energy, including electromagnetic (light/photons), interact with the sample and change the average energy of motion.

The electromagnetic spectrum covers a wavelength range of about 14 orders of magnitude, from the shortest, most energetic, high frequency gamma rays to the longest, very low energy, low frequency radio waves). The range of interest for climate science and atmospheric warming is approximately in the middle of this spectrum from about 0.1 to 100 μm (10^–1 to 10² μm). This includes the visible region of the spectrum (0.4 to 0.7 μm) and the adjacent higher energy (shorter wavelength) near ultraviolet (UV) and lower energy (longer wavelength) infrared (IR), including much of the “thermal infrared”. Incoming solar radiation is in the shorter wavelength (higher energy) part of this range from UV through near IR, between about 0.1 to 4 μm. Radiation from the warmed earth is mainly in the thermal IR region between 4 and 30 μm.

Molecular vibrations and some energetic rotations have energy level spacing that correspond to energies in the IR region of the electromagnetic spectrum (most rotations are in the microwave range which runs between thermal IR and radio wavelengths). Thus IR radiation absorbed by molecules causes increased vibration. Collisions between these energized molecules and others in the sample transfer energy among all the molecules, which increases the average thermal energy and, hence, raises the temperature. Conversely, molecules that emit IR radiation lose their vibrational energy and their collisions with other molecules decrease the average thermal energy and lower the temperature.

The wavelength unit used here and in most discussions of greenhouse gases is the micrometer, μm, which is usually called a “micron”. Frequencies of radiation in the IR are often given in units of reciprocal centimeters, cm^–1, called “wave numbers” (number of waves per centimeter). In order for molecular vibrations to absorb IR energy, the vibrational motions must change the dipole moment of the molecule. All molecules with three or more atoms meet this criterion and are IR absorbers. While the Earth’s (dry) atmosphere is predominantly composed of non-IR absorbers, N₂ (78%), O₂(21%), and Ar (~0.9%), the 0.1% of remaining trace gases contains many species that absorb IR.