April 2019 Limerick Project

Covalent Bonds

“Covalent bonds: electrons’ sharing, 
In two orbitals’ overlap, pairing. 
H2’s bond length can be 
Found via minimized E 
As a function of H atoms’ bearings.”

Chemical compounds form when their component elements can stabilize one another via energetic interactions.  The 20 April 2019 limerick summarizes several important pieces of information related to one such type of interaction: a covalent bond.  In particular, the poem describes a hydrogen molecule and how its geometry is related to the energy of its covalent bond.  

“Covalent bonds: electrons’ sharing,/
In two orbitals’ overlap, pairing.” 
Chemical compounds bond in two main ways: ionic bonds, in which oppositely charged ions attract, and covalent bonds, in which atoms share their valence electrons.  This poem focuses on the latter case.  

To share electrons to form a covalent bond, two atoms’ orbitals (regions of space in which electrons are likely to be found) must overlap.  This is the substance of valence bond theory: as these orbitals overlap, the electron of one atom pairs with the electron of another in a covalent bond, stabilizing the molecule overall.  

“H2’s bond length can be/ Found via minimized E/
As a function of H atoms’ bearings.”
The simplest molecule is the hydrogen molecule, written as H2 and containing two hydrogen atoms covalently bonded together (again, my Twitter notation fails to subscript the numeral two!).  

These last three lines of the poem narrate the potential energy surface of a hydrogen molecule, with energy as a function of internuclear distance (bond length).  As we look at the graph, we can find the bond length of H2 by finding the minimum energy (the minimum point on the y-axis) and looking at what bond length corresponds (on the x-axis).  In other words, we look for “minimized E as a function of H atoms’ bearings”: minimized energy as a function of the hydrogen atoms’ distance from one another.  

Such a graph can be found in any introductory chemistry textbook and gives us two important pieces of information: the bond dissociation energy of H2 (how much energy is required to break the bond?), which is 432 kJ/mol, and the bond length of H2, which is 74 picometers (pm).