chapter 2 Flashcards
black body radiation
Any radiation leaking out of
the hole has been absorbed and re-emitted
inside so many times that it has come to thermal equilibrium
energy distribution in a black body at several temp
energy density increases in the region of shorter wavelengths as the
temperature is raised, and the peak shifts to shorter wavelengths.
remark about rayleigh frequency distribution formula
The energy radiated would increase continuously as the frequency ν
increases
In the photoelectric effect
electromagnetic radiation incident on a metal
surface causes the metal to emit electrons after acquiring enough energy to escape
Observation about photoelectric effect and photons
(1) no electrons are ejected, regardless of the intensity of the radiation, unless its frequency exceeds a threshold value characteristic of the metal.
(2) increasing the intensity of the light increases the number of electrons
emitted but does not affect the kinetic energy of the emitted electrons.
(3) increasing the frequency of the radiation increases the kinetic energy of
the emitted electrons.
The photoelectric effect can be understood
if an electron is ejected in a
collision with a particle-like projectile (later named photons), provided the
projectile carries enough energy (Ephoton = hν) to expel the electron from
the metal.
The emission of light at discrete frequencies can be understood if we suppose that:
–the energy of the atoms or molecules is confined to discrete values, as
then energy can be discarded or absorbed only in packets as the atom
or molecule jumps between its allowed states.
–the frequency of the radiation is related to the energy difference between
the initial and final states: ∆E = hν.
The Schrödinger Equation
The mathematical representation of the wave, that in quantum mechanics
replaces the classical concept of trajectory, is called a wave-function, ψ, and is a function of the coordinates of the particle and also a function of time.
Quantum Numbers
Principal quantum number: n = 1, 2, 3, . . .
Angular-momentum quantum number: l = 0, 1, 2, . . . , n − 1
Magnetic quantum number: m = −l, −l + 1, . . . , 0, . . . , l − 1
Pauli Exclusion Principle
to determine the manner in which these states are filled: electrons configuration
The Spin of an Electron
is an intrinsic angular momentum that every
electron possesses and that cannot be changed or eliminated
The Aufbau Principle
electron configurations is based on the the fact that the total energy of the atom should be minimum.
Group 0:
inert gases, filled electron shells and stable electron configuration.
Groups VIIA & VIA:
elements with one (halogens) or two deficient
electrons from stable structure.
Groups IA & IIA:
alkali and alkaline earth metals having, respectively,
one and two excess electrons vs. stable configuration.
Group IIIB to IIB:
transition metals with partially filled d-electron
states and in some case one or two electrons in the next higher energy shell.
Groups IIIA to VA:
elements with characteristics intermediate between
metals and nonmetals because of their valence electron structure.
metallicity can be defined
as the tendency of an atom
to donate electrons to metallic or ionic bonds.
Metallicity increases
from
top to bottom and from right to left on the periodic
Ionic bonding:
nondirectional bonding between metallic (easily give up their valence electrons) and nonmetallic elements (the horizontal extremities of the table of the elements), such as NaCl, MgO
(usually when the electronegativity difference > 2).
Covalent bonding:
stable electron configuration as a result of sharing of
electrons between adjacent atoms (directional bond), such as in nonmetallic elemental molecules (H2, Cl2), heterogeneous molecules (CH4, H2O) and elemental solids (Si, SiC,C). Usually when the electronegativity difference< 0.4)
Metallic bonding:
nondirectional, found in metals and their alloys; maximum 3 valence electrons not bound to any particular atom in the solid matrix and are free to drift throughout the entire metal generating an electron cloud and ion cores
Secondary Interatomic Bonds
–induced dipoles (due to the distortion of electric symmetry because of
vibrational motion of atoms that would induce the same with adjacent atoms),
i–nduced dipoles and polar molecules (permanent dipoles because of electric dissymmetry such as H2O), and
–polar molecules
