Glossary
Bond Energy
The energy required to break a specific bond in one mole of gaseous molecules.
Example:
A higher bond energy indicates a stronger bond, meaning more energy is needed to break it, like the strong triple bond in N2.
Bond Length
The average distance between the nuclei of two bonded atoms in a molecule.
Example:
A carbon-carbon triple bond has a shorter bond length than a carbon-carbon single bond because of the increased electron density pulling the nuclei closer.
Bond Order
The number of chemical bonds between a pair of atoms, indicating the strength and length of the bond.
Example:
The carbon-carbon bond in ethane (C2H6) has a bond order of 1, while in ethene (C2H4) it's 2, and in ethyne (C2H2) it's 3.
Electron Domain Geometry
The arrangement of all electron domains (both bonding and lone pairs) around a central atom, which dictates the overall spatial orientation of electron groups.
Example:
For a molecule like NH3, even though its molecular shape is trigonal pyramidal, its electron domain geometry is tetrahedral because it has four total electron domains (three bonding, one lone pair).
Electron Domains
Regions around a central atom where electrons are located, including bonding pairs (single, double, or triple bonds) and lone pairs.
Example:
In a water molecule (H2O), the central oxygen atom has four electron domains: two bonding pairs with hydrogen and two lone pairs.
Hybridization
The mixing of atomic orbitals on a central atom to form new, degenerate hybrid orbitals that are suitable for bonding and explain observed molecular geometries.
Example:
The carbon atom in methane (CH4) undergoes sp3 hybridization to form four equivalent hybrid orbitals, allowing it to bond with four hydrogen atoms in a tetrahedral arrangement.
Lone Pairs
Non-bonding pairs of electrons located on a central atom. They exert a greater repulsive force than bonding pairs, influencing bond angles and molecular shape.
Example:
The two lone pairs on the oxygen atom in water cause the H-O-H bond angle to be less than 109.5°, resulting in a bent molecular shape.
Molecular Geometry
The specific 3D arrangement of only the atoms in a molecule, determined by the repulsion of electron domains but ignoring the lone pairs themselves.
Example:
Although ammonia (NH3) has a tetrahedral electron domain geometry, its molecular geometry is trigonal pyramidal due to the presence of one lone pair.
Pi (π) Bonds
Covalent bonds formed by the side-by-side overlap of unhybridized p orbitals, with electron density concentrated above and below the internuclear axis.
Example:
In an ethene molecule (C2H4), the double bond consists of one sigma (σ) bond and one pi (π) bond.
Shape
The actual 3D arrangement of atoms in a molecule, which is a direct consequence of the electron domain geometry but specifically describes the positions of the nuclei.
Example:
Carbon dioxide (CO2) has a linear shape because its two double bonds arrange themselves 180° apart to minimize repulsion.
Sigma (σ) Bonds
Covalent bonds formed by the direct, end-to-end overlap of atomic orbitals, with electron density concentrated along the internuclear axis.
Example:
Every single bond, such as the C-H bond in methane, is a sigma (σ) bond.
Valence Shell Electron Pair Repulsion (VSEPR) theory
A theory used to predict the 3D arrangement of atoms in a molecule by minimizing the repulsion between electron pairs around a central atom.
Example:
Using VSEPR theory, you can predict that methane (CH4) will have a tetrahedral shape because its four bonding electron pairs repel each other equally.