One half of the distance between the nuclei of two atoms bonded together by a single bond is known as

Answer is: an atomic radius.
The atomic radius of a chemical element is a measure of the size of its atom.The atomic radius varies with increasing atomic number, but usually increases because of increasing of number of electrons.The atomic radius decreases across the periods because an increasing number of protons, because greater attraction between the protons and electrons.

True, the standard unit for volume is liters which can be represented by either a lower case "l" or a capital "l"