Science Poetry

Shaping Ideas

“To consider electrons’ repulsions
In geometries under construction,
VSEPR theory
Provides first steps; here, see 
To molecular shapes, introduction.”  

The 14 October 2019 limerick alludes to a theory used in introductory chemistry courses to rationalize molecular geometries.   

“To consider electrons’ repulsions /
In geometries under construction…”
Yet another big idea in General Chemistry is that of molecular geometry: the three-dimensional shape in which a molecule exists.  The shapes of molecules have major implications for how these molecules can react and interact with one another, and so being able to predict a molecule’s geometry is an important first step for understanding its properties and reactions.  Three theories are typically introduced to rationalize molecular geometries: valence bond theory, molecular orbital theory, and valence-shell electron-pair repulsion theory.  All involve to some extent the central idea that negatively charged electrons repel one another.  

“VSEPR theory / 
Provides first steps; here, see /
To molecular shapes, introduction.”     
In general chemistry textbooks, the chapter on molecular geometry is a chapter in which Bent’s characterization of “strange terms for strange things” often seems particularly apt.  The three theories mentioned above are referred to as their abbreviations: VB theory, MO theory, and VSEPR theory, respectively.  (Further confusing the issue, VSEPR is often pronounced “vesper”!  However, in this poem, the rhyme scheme relies on spelling out the acronym.)       

VSEPR theory is the simplest of the three and is generally extensively explored in introductory coursework, providing important “first steps.”  As alluded to above, VSEPR stands for “valence-shell electron-pair repulsion.”  Valence electrons are the outermost electrons of an element; when elements combine into molecular compounds, the valence electrons participate in covalent bonds (with each bond involving two electrons) or exist in “lone pairs.”  In all of these cases, the electron pairs from the valence shells repel.  This ultimately results in characteristic shapes for molecules, as electron pairs will distance as far away from one another as is geometrically possible.