MCAT – Chemistry

Pauli Exclusion Principle



Pauli Exclusion Principle: No 2 electrons in a given atom can possess the same set of 4 quantum numbers

The Quantum Numbers

(name, symbol, what is specifies, what its value can be, and how many electrons allowed in each region)

  1. Principle Quantum Number (n) – size – n= positive integer – the greater n is, the higher the energy level and radius of the electron’s orbit is.
    – max number of electrons in each energy level is 2n2.
  2. Azimuthal (angular momentum) Quantum Number (l) – shape – l=0 to n-1 – refers to the subshells that occur within each principle energy level – max number of electrons in each subshell is 4l+2.
  3. Magnetic Quantum Number (ml) – orientation of the orbitals – ml= l to -l – specifies the particular orbital within a subshell where an electron is likely to be found – max number of electrons is 2.
  4. Spin Quantum Number (ms)ms= -1/2 or 1/2 – specifies in which direction an electron is spinning.

Hund’s Rule
Hund’s Rule: Every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin.

Planck’s Quantum Theory


 Bohr’s Model

Planck’s Quantum Theory – energy emitted as electromagnetic radiation from matter comes in discrete bundles call quanta…E=hf


Bohr’s Model – depicts that atom as a small, positively charged nucleus, surrounded by electrons orbiting the nucleus in distinct circular paths, much like a solar system, but with electrostatic forces providing the attraction rather than gravity.

Aufbau Principle
Aufbau Principle – Electrons fill the lowest energy levels first
Equation for Formal Charge
Formal Charge= Valence Electrons – 1/2(Bonding electrons) – Nonbonding electrons
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