Electron Configuration

How are electrons arranged?

• the first energy level (closest to the nucleus) holds a maximum of two electrons
• the second energy level holds a maximum of eight electrons
• electrons are paired in twos, this represents the filling of orbitals
  • each energy level has different orbitals, and each orbital can hold two electrons
• any unpaired electron is always in the outermost energy level
  • for main group elements, lower energy levels fill completely before electrons are placed into the next level
    • main group: elements in the s and p blocks of the periodic table

Shells (energy levels)

• quantized energy levels that electrons can occupy
  • electrons can not exist at energies between levels
• labeled $n=1,n=2,n=3$ starting from the nucleus
  • the higher the shell number, the greater the energy of electrons in that shell
• the shells represent electrons’ energy levels, NOT their position or path
  • electrons do not move in circular paths around the nucleus
• sometimes, the Bohr model may show rings that get closer together as the shell number increases
  • represents how the difference between energy levels decreases with greater $n$
  • exact spacing of the rings is not important
• maximum number of electrons per shell is determined by the formula: $\text{max \# of electrons}=2\cdot n^2$
• electrons in the outermost shell are called valence electrons

Subshells, and orbitals

• subshells — the different configurations/shapes of a shell
  • composed of orbitals
• orbitals — each unique orientation of the subshells
  • each orbital can hold up to 2 electrons
  • specifies the shape and position of the regions that electrons occupy

Electron configuration notation

• to explain how an atom’s electrons are arranged in orbitals
• written as the element symbol with subshell labels superscripted with the number of elections in each subshell
  • ex. $\mathrm{H}=1\mathrm{s}{}^1\mathrm{P}=1{\mathrm{s}}^{2}2{\mathrm{s}}^2{\mathrm{p}}^{6}3{\mathrm{s}}^2{\mathrm{p}}^3$
• can be condensed by building upon the most previous noble gas, instead of listing every shel, subshell, and electron count
  • ex. $\mathrm{He}=1\mathrm{s}{}^2\mathrm{Ne}=[\mathrm{He}]2{\mathrm{s}}^{2}2{\mathrm{p}}^6\mathrm{Ar}=[\mathrm{Ne}]3{\mathrm{s}}^{2}3{\mathrm{p}}^6$ etc.
• for ions, exactly the same but add/remove electrons to/from the highest energy orbital
• atoms/ions that have the same electron configuration are called isoelectronic

Aufbau principle

• German for “building-up” principle
• electrons first fill up subshells of the lowest available energy, to form the most stable electron configuration possible
• the elements in the periodic table can be broken up into “blocks” that group them by the orbitals their valence electrons fill
• only accurate for the first 20 elements, up to calcium, then does not accurately predict electron placement