Discussion
The energy band diagram for magnesium is
(The hatched and unhatched regions in the figure correspond to filled and unfilled regions of the band, respectively.)
- A.
- B.
- C.
- D.
Answer: D
The band diagram for magnesium is (D), where the s-band and the p-band overlap and the combined band is only partly filled.
Magnesium has the configuration [Ne]3s2. If the 3s and 3p bands were separate, the two electrons per atom would exactly fill the s-band and leave the p-band empty, as in (A), and magnesium would be an insulator. Magnesium is a good metal because the 3s and 3p bands overlap: electrons spill into the bottom of the p-band, leaving both partly filled, with empty levels immediately above the highest filled ones.
(B) shows both bands full, which would need more electrons than magnesium has. (C) shows a half-filled, separate s-band, which describes sodium (3s1), not magnesium.
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