EC-5.2 Atoms, Quantisation, and Why Electrons Sit in Levels
You can explain why an electron in an atom can only hold certain energies, what a shell and an orbital are, and why the outermost electrons decide almost everything an element does.
Before:nothing requiredUnlocks:PE2-5. MEMSEC-6. Semiconductor Devices and Device Models
Everything about semiconductors follows from one fact: an electron bound to something cannot take any energy it likes. It has a list. That single restriction produces spectral lines, the periodic table, chemical bonding and eventually the band gap that makes a transistor possible. This topic exists so that the band diagram in the next one arrives as a consequence rather than as a picture to memorise, and it is deliberately physical rather than mathematical.
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Why a bound electron has a list of allowed energies rather than a range
Confining a wave to a region allows only the wavelengths that fit, exactly as a guitar string does, and an electron behaves as a wave when confined. The allowed wavelengths become allowed energies, and everything between them is forbidden.
NPTEL: Physics of Materials · CourseAbsorption and emission, and what a spectral line is evidence of
An electron moves between two allowed levels by taking in or giving out exactly the energy difference, which arrives as light of one particular colour. The sharp lines in a spectrum are direct evidence that the levels are discrete.
Shells, subshells and orbitals, and the rules that fill them
Electrons fill the lowest available levels first, no two in the same state, which produces the shell structure. That filling order is the whole reason the periodic table has the shape it does.
NPTEL: Physics of Materials · CourseValence electrons, and why only the outermost ones matter
Inner electrons are held tightly and take no part in anything chemical or electrical, so an element's behaviour is decided by the few in its outermost shell. Silicon having four of them is the reason it does what it does.
Bonding: what holds atoms together, and the three ways it happens
Atoms share electrons, hand them over, or pool them, and which one happens decides whether the result conducts. The shared-electron case is what a silicon crystal is, and the pooled case is what a metal is.
MIT OpenCourseWare 3.091SC: Introduction to Solid State Chemistry · CourseEnergy in electron volts, and why the unit is worth using
An electron volt is the energy an electron gains crossing one volt, and it makes the numbers in this subject small and comparable instead of tiny and awkward. A band gap of about one electron volt is a fact worth carrying forward.
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