Modern Atomic Structure

The Research

Planck's hypothesis
- energy is given off in little packets called quanta (singular = quantum), rather than continuously
- a photon is a quantum of light energy
- one quantum of light energy is related to the frequency of light by the equation
  E = hv, where h is a constant known as Planck's constant
Bohr
- pointed out that the absorption of light at different wavelengths corresponds to definite changes in the energy of the electron.
- electrons can absorb or emit energy only in whole numbers of photons (quanta)
- the size of the smallest orbit the electron can occupy (one closest to the nucleus) is its ground state
Louis de Broglie
- proved correct 2 years later
Heisenburg Uncertainty Principle
- impossible to know both the exact position and momentum of an object at the same time
- observing one changes the other
Shrodinger
- treated electron like wave and developed mathematical equation to describe its behavior.
- Max Born applied this equation to give the probability of the location of the hydrogen electron
- the area of highest probability was equal to the distance calculated by Bohr
- if points of highest probability are connected, a 3-D shape is created
- the most probable location for the electron is somewhere on the surface of this 3-D shape
- since the volume occupied by an electron is somewhat fuzzy, it is better to refer to it as an electron cloud

Quantum Theory Quantum mechanics

describes the behavior of extremely small particles at velocities near the speed of light
important to study because the chemical behavior of an atom is determined by the number and arrangement os its electrons
allows us to predict the arrangement of these electrons
each electron in an atom can be described by a set of four quantum numbers, n, l, m, and s

principle quantum number, n
- is the number of the energy level and describes the relative electron cloud size
- the higher the value of n, the greater the energy and the larger the electron cloud
- the maximum number of electrons an energy level can hold is 2n²
- if n = 1, then maximum # of electrons is 2; if n = 2, maximum electrons = 8, etc.
Second Quantum Number, l
- each energy level is made of many energy states, or sublevels that are closely grouped together
- the l quantum number describes the shape of the sublevel in which the electron is located
- the number of sublevels possible for each energy level is equal to the value of the principle quantum number for that level
- these sublevels are designated s, p, d, and f
- if n = 1, then only s sublevels are possible, if n = 2, then s and p sublevels may be present, etc.
Third Quantum Number, m
- each s sublevel can hold 1 pair of electrons, p can hold 3 pairs, d -5 pairs, and f - 7 pairs
- the space occupied by one pair of electrons is called an orbital
- for example, an f sublevel has 7 orbitals
- each orbital of the same type has the same amount of energy, but a different spatial orientation
- the m quantum nuber distinguishes between these orbitals
- the sum of all electron clouds in any energy level is a spherical cloud
Fourth Quantum Number, s
- distinguishes between the two electrons located in a particular orbital
- describes the spin of the electron
- can be clockwise or counterclockwise

Electron Configurations

The Rules

Aufbau Principle
- electrons fill lowest energy levels first
- electrons fill energy levels in this order:
  1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p 8s
- Aufbau diagram shows this relationship
Pauli exclusion principle
- no two electrons will have the same set of 4 quantum numbers
- if they occupy the same orbital, one must be spinning clockwise and the other spinning counterclockwise.
Hund's rule
- when filling degenerate orbitals, electrons will take an empty orbital, if possible
- in other words, fill in clockwise spins first, then counterclockwise spins.

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