U2.4 · Structure of Metals and AlloysPractice · 金属与合金的结构

Conduction needs mobile charged particles. In a metal the cations are fixed in the lattice and only vibrate in place, while the valence electrons are delocalized over the whole piece and drift in one direction when a voltage is applied. A molten salt conducts by moving ions; a metal conducts by moving electrons.

Na has 1 valence electron (3s¹) and Mg has 2 (3s²). Mg’s cations carry +2 and there are twice as many delocalized electrons; Mg atoms are also smaller (covalent radius Mg 141 pm, Na 166 pm), so the cations and electron sea are closer. The metallic bonding is stronger and takes more energy to overcome. Both have the same number of shells, and mass doesn’t set the strength of metallic bonding.

A C atom is only 0.50 times the radius of an Fe atom, so C fits into the spaces between Fe atoms: an interstitial alloy, with no Fe atoms replaced. All the Fe atoms are still there, plus the mass of the C atoms, in nearly the same volume, so the density increases. “C is lighter than Fe” would lower the density only if C replaced Fe.

Sn and Cu are similar in size (ratio 1.07), so Sn replaces some Cu atoms in the lattice: a substitutional alloy, not an interstitial one. They aren’t exactly the same size, though, so the Sn atoms make the layers uneven, and the layers don’t slide past each other as easily. Bronze is harder than copper. It still has metallic bonding with delocalized electrons.

An interstitial alloy needs atoms much smaller than the host’s, small enough to fit into the gaps. Radius ratios to Fe: Cr 1.0, B 0.61, Ni 0.96, Zn 0.96. B atoms are much smaller and fit between Fe atoms; Cr, Ni, and Zn are about the same size as Fe and would replace Fe atoms, forming substitutional alloys.