5.3. Electron Arrangement in Atoms http://www.ck12.org
FIGURE 5.21
The arrangement of electrons within the 2psublevel for the elements
boron (Z= 5), carbon (Z = 6), nitrogen (Z= 7), and oxygen (Z = 8).
According to Hund’s rule, as electrons are added to a set of orbitals of
equal energy, one electron enters each orbital before any orbital receives
a second electron.Orbital Filling Diagrams
An orbital filling diagram provides a more visual way to represent the arrangement of electrons in a particular atom.
In an orbital filling diagram, the individual orbitals are shown as circles (or squares), and orbitals within the same
sublevel are drawn next to each other horizontally. Sublevels can be shown where energy increases as you move up
the page (Figure5.18), or, to save space, all sublevels can simply be shown horizontally one after the other. Each
sublevel is labeled by its principal energy level and sublevel. Electrons are indicated by arrows inside the circles. An
arrow pointing upward indicates one spin direction, while a downward pointing arrow indicates the other direction.
The orbital filling diagrams for hydrogen, helium, and lithium are shown below.
According to the Aufbau principle, sublevels and orbitals are filled with electrons in order of increasing energy.
Since thessublevel consists of just one orbital, the second electron simply pairs up with the first electron, as in
helium. The next element, lithium, requires the use of the next available sublevel. The third electron must be placed
in a 2sorbital, because the 1sorbital is completely filled.
Electron Configuration Notation
Electron configuration notation eliminates the circles and arrows of orbital filling diagrams. Each occupied sublevel
is written down, along with a superscript indicating the number of electrons present in that sublevel. For example,
the configuration of a hydrogen atom is 1s^1 , and the configuration of helium is 1s^2. Multiple occupied sublevels are
written one after another. The electron configuration of lithium is 1s^22 s^1. The sum of the superscripts in an electron
configuration is equal to the number of electrons in that atom, which is in turn equal to its atomic number.
Sample Problem 5.5: Orbital Filling Diagrams and Electron Configurations
Draw the orbital filling diagram for carbon and write its electron configuration.