ELECTRON
CONFIGURATION
I. Rules of Electron Configuration
  The ground-state electron configuration of an atom
represents its arrangement of electrons that have the
lowest possible energy.
  They are based on the 4 quantum numbers.
  There are three rules that dictate how we order
electrons for each element.
I. Rules of Electron Configuration
1.  The Aufbau Principle – an electron occupies the
lowest-energy orbital that can receive it.
  In general…
  The first energy level fills before the second.
  s-orbitals fill before p-orbitals
  Some unexpected results…
  The 4s orbital is actually lower in energy than the
3d orbital, so it fills first.
I. Rules of Electron Configuration
I. Rules of Electron Configuration
2.  The Pauli Exclusion Principle – No two electrons in
the same atom can have the same 4 quantum
numbers.
  In other words, electrons in the same orbital have
opposite spins.
  In an orbital diagram we represent these electrons
as an up and down arrow.
I. Rules of Electron Configuration
3.  Hund’s Rule – orbitals of equal energy
receive one electron each before doubling
up.
  This dictates how we draw orbital diagrams
which explains chemical bonding
principles.
I. Rules of Electron Configuration
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There are three ways to represent electron
configurations:
  Orbital diagrams: an arrow represents each
electron and lines or boxes represent each
orbital
  Electron configuration: shorthand, keeps track
of each electron and which energy level and
orbital shape they are in
  Noble gas notation: abbreviates portions of an
electron configuration by substituting a Noble
gas symbol (because they are stable).
I. Rules of Electron Configuration
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Let’s draw some configurations for these elements!
H He Li C O Ne Si Cr