http://www.ck12.org Chapter 5. Electrons in Atoms
FIGURE 5.19
The Aufbau principle is illustrated in the
diagram by following each red arrow in
order from top to bottom: 1s, 2s, 2p, 3s,
etc.principle,which states that no two electrons in an atom can have the same set of four quantum numbers. The energy
of the electron is specified by the principal, angular momentum, and magnetic quantum numbers. Because there are
two possible values for the spin quantum number, each orbital can hold up to two electrons. TheFigure5.20
describes how to depict electron configurations in an orbital filling diagram.
FIGURE 5.20
In an orbital filling diagram, a square represents an orbital, and arrows
represent electrons. An arrow pointing upward represents one spin
direction, while an arrow pointing downward represents the other spin
direction.View an animation of electron spin at http://www.dlt.ncssm.edu/core/Chapter8-Atomic_Str_Part2/chapter8-Animati
ons/ElectronSpin.html.
Hund’s Rule
The last of the three rules for determining ground state electron configurations gives information on how to arrange
electrons in a set of orbitals that are all within the same sublevel.Hund’s rulestates that orbitals of equal energy are
each occupied by one electron before any orbital is occupied by a second electron, and all of the unpaired electrons
must have the same spin. A slight preference for keeping electrons in separate orbitals helps minimize the natural
repulsive forces that exist between any two electrons. TheFigure5.21 shows how a set of threeporbitals would be
filled with one, two, three, and four electrons.