Glossary
Bond order
The number of chemical bonds between a pair of atoms, which can be a fraction in molecules exhibiting resonance, indicating an average bond strength.
Example:
In the nitrate ion (NO3-), the nitrogen-oxygen bond order is 4/3, meaning each N-O bond is stronger than a single bond but weaker than a double bond.
Electronegativity
A measure of an atom's ability to attract shared electrons in a chemical bond towards itself.
Example:
Oxygen has a higher electronegativity than carbon, which is why in a C-O bond, the electron density is pulled more towards the oxygen atom.
Expanded octet
The ability of central atoms in the third period and beyond to accommodate more than eight valence electrons in their outermost shell, often by utilizing d-orbitals.
Example:
Sulfur in sulfur hexafluoride (SF6) exhibits an expanded octet, bonding to six fluorine atoms and having 12 electrons around it.
Formal charge
A theoretical charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally, used to determine the most stable Lewis structure.
Example:
When evaluating different Lewis structures for the thiocyanate ion (SCN-), calculating the formal charge on each atom helps identify the most plausible arrangement.
Lewis structure
A diagram that shows the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule, representing the valence electron arrangement.
Example:
Drawing the Lewis structure for water (H2O) shows two single bonds between oxygen and hydrogen, and two lone pairs on the oxygen atom.
Lone pair electrons
A pair of valence electrons that are not shared with another atom in a covalent bond and are instead associated solely with one atom.
Example:
In ammonia (NH3), the nitrogen atom has one lone pair electrons, which contributes to its trigonal pyramidal molecular geometry.
Resonance
A concept used when a single Lewis structure cannot fully describe the bonding in a molecule or ion, where the true structure is an average or hybrid of multiple contributing structures.
Example:
Benzene (C6H6) exhibits resonance, where the electrons in its ring are delocalized, making all carbon-carbon bonds identical in length, rather than alternating single and double bonds.
Resonance structures
Different valid Lewis structures that can be drawn for a molecule or ion, which collectively describe the delocalization of electrons and contribute to the overall resonance hybrid.
Example:
For the carbonate ion (CO3^2-), you can draw three equivalent resonance structures by moving the double bond among the three oxygen atoms.
Valence electrons
The electrons in the outermost shell of an atom, which are involved in forming chemical bonds and determining an atom's reactivity.
Example:
Carbon has four valence electrons, allowing it to form four bonds in molecules like methane (CH4).