Covalent bonds usually occur between atoms that have and . a. high molecular weights; large atomic radii b. low ionization energies; low electron affinities c. low molecular weights; small atomic radii d. high ionization energies; high electron affinities

Answer is: D. High ionization energies; high electron affinities.

Covalent bond is usually between nonmetals.

The ionization energy (Ei) is the minimum amount of energy required to remove the valence electron, when element lose  electrons, oxidation number of element grows (oxidation process).

Nonmetals are far right in the main group and they have highest ionization energy, because they have many valence electrons.

For example, fluorine has the greatest affinity for electrons and tendency to be reduced, than oxygen and chlorine.

Fluorine form molecule of fluorine F₂ with single nonpolar covalent bond.

a 20-cm-long, cylindrical graphite (pure carbon) rod is inserted into an oxidizing atmosphere at 1145 k and 1.013 x 105 pa pressure. the oxidizing process is limited by the diffu- sion of oxygen counterflow to the carbon monoxide that is formed on the cylindrical surface.