cohesive energy
/ koh-HEE-siv EN-er-jee /
Imagine taking a solid block of metal and pulling it apart atom by atom until you have a cloud of separate, free-floating atoms drifting far from one another. That takes effort — you have to fight the attractions holding everything together. The cohesive energy is exactly the price of that demolition: the energy you must supply to disassemble a solid into its isolated atoms.
More precisely, it is the difference in energy between the atoms locked together in the solid and the same atoms sitting alone, with nothing pulling on them. We usually quote it per atom, in units like electron-volts, and the bigger the number, the more tightly the solid is bound. It is the bookkeeping that adds up all the bonds an atom feels from its neighbors, not just one pair, which is why it can differ from a single bond energy.
Cohesive energy matters because it is a clean, measurable summary of how strongly a material holds itself together, and it tracks well with melting point, hardness, and stiffness. A subtle point: cohesive energy is about pulling atoms fully apart into a gas of free atoms, which is not the same as melting (where atoms still touch) or boiling. Confusing it with melting energy is a common slip — melting only loosens the arrangement, while cohesive energy measures complete separation.
Diamond has one of the largest cohesive energies of any element — around 7 electron-volts per atom — because every carbon atom grips four neighbors with strong shared-electron bonds. That is why diamond is so hard and melts only at extreme temperatures, while a soft metal like sodium, bound by only about 1 electron-volt per atom, can be cut with a knife.
Diamond's huge cohesive energy makes it the hardest natural material.
Cohesive energy is defined relative to free, neutral atoms. For an ionic crystal one instead often quotes the lattice energy, which refers to free ions — a different reference state, so the two numbers should not be compared directly.