Chemistry Flashcards
(39 cards)
Pauli Exclusive Principle
No two electrons in an atoms may be in the same quantum state.
In other words no two electrons can have the same four quantum numbers, n, l, mł, ms.
Aufbau Principle
Electrons in atoms (and molecules) generally exist in their lowest possible energy state. This is called the ground state.
Electron-electron repulsion
When we have more than one electron, the electrons within an orbital repel each other.
Orbital Shielding
When an orbital is occupied, it shields the interaction of the outer orbital with the nucleus. This alters the energy levels of the orbitals. (i.e. orbitals more shielded from nucleus has more energy and orbitals nearest nucleus have lowest energy)
Hund’s Rule
The lowest energy electron configuration in orbitals of equal energy is the one with the maximum number of unpaired electrons with parallel spins.
- Maximise number of parallel unpaired electron spins in degenerate orbitals
Which radiation is used for imaging and why?
Gamma radiation is used as it must be highly-penetrating so it can be detected and is not harmful to the patient.
Which element can be used for imaging and why?
Technetium-99m
- can be easily incorporated into many drugs
- is easily prepared from Mo-99
- it does not change its chemistry when it decays
- emits only highly-penetrating gamma rays, not harmful alpha and beta particles
What is the photoelectric effect?
Light caries energy. The photoelectric effect shows how the energy of light depends on its frequency and intensity.
- Light can behave as a wave and a particle
Light can eject electrons from a metal, but only if its frequency is above a threshold frequency (characteristic for each metal).
Converting wavelength nm to m
Multiply by 10^-9
What’s wrong with the Bohr model?
- Moving charged particles radiate energy so electrons should continually lose energy and spiral into the nucleus.
- Bohr’s model could only explain the emission spectra of single-electron atoms (it failed to predict the spectra of multi-electron atoms)
- Bohr could offer no reason why an electron should have discrete orbits or energies.
Louis de Broglie’s theory
Matter behaves like a wave (e.g. electrons also have wave properties)
Electron density
The probability of finding the electron at a particular point in space.
What will result when two 1s orbitals in the same phase mix?
The result is an orbital with lower energy.
- bonding orbital
What will result when two 1s orbitals in the opposite phase mix?
The result is an orbital with a node, i.e. higher energy.
- antibonding orbital
Paramagnetic molecules
Paramagnetic molecules have unpaired electrons and a net magnetic moment. Such substances will be drawn into magnetic fields, e.g. oxygen
Diamagnetic molecules
Diamagnetic molecules do not have unpaired electrons, and have no magnetic moment. They are weakly repelled by magnetic fields, e.g. nitrogen
What does HOMO and LUMO stand for and what is it?
HOMO = Highest Occupied Molecular Orbital
LUMO = Lowest Unoccupied Molecular Orbital
The HOMO-LUMO transition is the lowest energy electronic transition of a molecule.
What is the valence band?
The band of occupied orbitals.
What is the conduction band?
The band of unoccupied orbitals.
What is the band gap?
The band gap is the minimum energy a network solid must absorb to promote an electron from the valence band to the conduction band.
Relate the conduction in metals to its band gap
Metals do not have a band gap. Valence and conduction bands overlap, so metals can conduct electricity.
Relate insulator to band gap
Has large band gap
- electrons cannot be promoted to conduction band
Relate intrinsic semiconductor to band gap
Electrons can be promoted from valence to conduction upon heating
What is n-type doping?
In n-type doping, there are extra negative charge carriers (i.e. electrons). This is achieved by substituting with an element to the right on the periodic table, which has more electrons. e.g. substituting Si with P
The extra electrons reside in donor levels, just below the conduction band. As the material is heated, these electrons are promoted to the conduction band, and vastly outnumber any holes in the valence band.
