Group 6: cohesion increases from chromium to molybdenum to tungsten. partially occupied d bands make a major contribution to metallic cohesion. Relative to localized 3d orbitals in Cr, more extended 4d/5d orbitals support stronger overlap and broader bonding bands. Strong 5d bonding and the compact dimensions associated with lanthanide/relativistic effects help make W especially cohesive.
Group 11: cohesion increases from silver to copper to gold. the filled d shell changes the balance from Group 6. Going from Cu to larger Ag weakens overlap and lowers cohesion. Gold reverses that simple size trend: relativistic 6s stabilization/contraction, changes in the 5d orbitals and stronger s-d participation enhance bonding. A size-only argument cannot explain Au exceeding both Ag and Cu.
Group 15, As > Sb > Bi: bonding in these elemental solids involves directional p-orbital interactions. Increasing size and longer separations weaken overlap down the group, reducing enthalpy of atomization. Changes in detailed solid structure and relativistic effects refine the numbers, but the underlying weakening of p-derived bonding explains the trend.
Enthalpy of atomization measures removal from the entire standard-state solid to gaseous ground-state atoms; it is not simply the dissociation energy of one selected diatomic bond.
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