Electromagnetic radiation:
is a way by which energy travels through space.
Electromagnetic radiations are characterized by a frequency and wavelength related by the speed of light: c = 𝞴v
Energy can be transferred in only specific quantities of units called quanta of energy, E =hv
When the surface of a metal is struck with light, it emits electrons. This is called the photoelectric effect. Electrons will be emitted only if the photon have sufficient energy to eject the electron.
Photoelectron spectroscopy (PES): determines the energy needed to eject electrons from the material, not only the valence electrons but core electrons as well. Data from PES
provides a method to deduce the shell structure of an atom. The intensity of the photoelectron signal at a given energy is a measure of the number of electrons in that energy level.
Energy has mass calculated using Einstein’s equation:
E = mc²
All matter has a wave nature whose wavelength can be calculated using the de Broglie’s equation:λ = h/mv
Continuous spectrum: is the like the one that results when white light passes through a prism.
Line spectrum: only patches of light appear separated by patches of darkness.
Bohr’s quantum theory has many postulates, those that are still valid today include:
Classical physics does not apply to particles of atomic or subatomic sizes.
The e in an H-atom can have only specific amounts of energy, ie. its energy is quantized.
The allowed energy levels are calculated using the formula:
, where n is an integer and Z is the nuclear charge
A H-atom radiates energy only when its e jumps from one allowed stationary state (energy level) to a lower stationary state (energy level).
When an electron moves between allowed energy levels, the change in its energy can be calculated using:
The Heisenberg uncertainty principle: It is not possible to simultaneously know precisely the position and velocity of an electron.
Electron repulsions of core electrons decrease the actual nuclear charge felt by the
valence electrons.
Draw an orbital diagram by using squares to represent orbitals and arrows to represent
electrons.
The state of half-filled orbital is especially stable as it involves no electron-electron
repulsions within the same orbital.
When writing the electronic configuration of a cation remember to subtract the charge
from the atomic number to get the number of electrons, while for an anion remember to add the
charge to the atomic number to get the number of electrons.
When writing the electronic configuration of a transition metal cation of period 4,
electrons are removed from the 4s orbital before the 3d.
Transition metals have multiple oxidation states.
Ionization Energy: is the minimum amount of energy required to remove one mole of
loosely bounded electrons from one mole of gaseous atoms to form one mole of single positively
charged gaseous ions under standard conditions.
AP definition: is the change in energy needed to remove the least tightly held electron from
an atom or ion.
Screening or Shielding: Core or inner electrons “shield” or “screen” the valence
electrons from the full effect of the nuclear charge. As a result the valence electrons are not
attracted to the nucleus by the “actual” nuclear charge but less. The larger the number of core
electrons the greater the shielding and the less the attraction to the nucleus.
Trend in ionization energy across a period: Across a period, the number of core
electrons remains the same shielding remains the same. However, the nuclear charge
increases which increases the attraction of the valence electrons to the nucleus IE across a
period, in general, increases.
Between Groups II and III ionization energies decrease. The electron is removed
from ns2 orbital in Group II while from np1
in Group III. np electrons are more energetic than ns, so less energy will required to pull it out.
124. Between Groups V and VI ionization energies decrease. The e - is removed from np3
orbital in Group V while from np4 in Group VI. np4
electrons are less stable as they experience more repulsions due to pairing of electrons, so less energy will required to pull it out.
Trend in ionization energy down a group: Down a group, the number of core electrons
increases shielding increases. Although the nuclear charge increases, the increase in the
shielding decreases the attraction of the valence electrons to the nucleus. Furthermore, the
valence electrons are in higher shell numbers the electrons are more energetic the electrons
require less energy to be removed IE down a group decreases.
NB In answering these types of questions, it is essential to mention BOTH the shell number and orbital type.
Energy difference between removing e -s of the same n and l quantum numbers is small.
Energy difference between removing electrons of the same n but different l quantum
numbers is slightly larger
Energy difference between removing e,s of different n quantum numbers is very large.
Electron Affinity: energy change associated with the addition of an electron to a gaseous atom or ion. OR Energy change when 1 mole of electrons are added to 1 mole of gaseous atoms forming 1 mole of uni negatively charged gaseous ions.
Across a period, electron affinity becomes more negative.
Down a group, electron affinity becomes less negative.
Atomic radius: is half the distance between the nuclei in a molecule made of 2 identical atoms. OR is half the distance between the nuclei of 2 adjacent atoms in a crystal of an element.
Across a period, atomic radii decrease. Across a period, the number of core electrons remains the same shielding remains the same. However, the nuclear charge increases which increases the attraction of the valence electrons to the nucleus atomic radii decrease.
Down a group, atomic radii increase. Down a group, the number of electrons increases
electrons are placed in further and further shells atomic radii increase.
Group I metals react with cold water violently producing alkaline solutions and liberating
hydrogen gas.
Certain cations impart a color to a blue flame when burnt in it.
Study More , Explanation videos, track your work , get quizzes to test your understanding , Sign up free here.
Or Go Back to the Main Chapter Menu
