The transition metals generally are hard, brittle, and have high melting points. Hardness and melting point correlate well with the number of impaired d electrons, as can be seen from the melting point curve below. This correlation arises because bonding between atoms in these metals is partially covalent. Potassium and calcium at the beginning of the fourth row of the table lose all their outer electrons, to become +1 and +2 ions in the metal. Their electrons are free to wander through the solid. The first transition metal, scandium, also loses all three of its outer electrons, but as the number of electrons in the d orbitals increases in successive elements, complete loss of electrons becomes more difficult. Chromium does not become a ion completely in the metal, but retains a partial hold on its outer-orbital electrons, sharing them to an extent with neighboring ions. If we could watch the electrons in a block of chromium, we might see them moving through the metal, but spending a disproportionate amount of time in positions that correspond to covalent bonds between adjacent Cr atoms. This partial covalent behavior creates stronger attractions between atoms in chromium and makes it harder, tougher, and higher melting than potassium or calcium. In spite of this tendency toward covalency, the transition metals are good conductors of electricity because the outer-shell electrons are still mobile, and there are relatively many of them per atom.