In an isolated atom, the energy of an electron is fixed by the orbit in which it revolves. In a silicon crystal, the energy levels of the valence electrons spread out into a band of closely spaced levels. This happens because, inside the crystal,
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → each electron sees a slightly different pattern of charges around it
In a solid the atoms are very close to each other. The outer orbits of neighbouring atoms come very close or even overlap. So an electron in a crystal does not move as it would in an isolated atom.
Option (1) each electron sees a slightly different pattern of charges around it — Correct. Each electron has its own position in the crystal. The charges around it differ a little from those around any other electron, so its energy level also differs a little. These many close levels form a band.
Option (2) thermal energy gives each electron a slightly different energy — Incorrect. The valence band and the conduction band are present even at 0 K, when there is no thermal energy. Thermal energy only lifts a few electrons from one band to the other.
Option (3) the electrons lose different amounts of energy in collisions with the ion cores — Incorrect. Collisions scatter the electrons that move through the crystal. They do not decide the energy levels of the bound valence electrons.
Option (4) the outer electrons leave their atoms and move freely through the crystal — Incorrect. With no outside energy, all the valence electrons stay in the valence band. They are not free, yet the band is already there.
NCERT (2026-27):
On the basis of energy bands – “Inside the crystal each electron has a unique position and no two electrons see exactly the same pattern of surrounding charges. Because of this, each electron will have a different energy level. These different energy levels with continuous energy variation form what are called energy bands. …”