Rule and Principle of Electrons Help

By — McGraw-Hill Professional
Updated on Aug 28, 2011

Orbital Filling

While some scientists were studying the energy levels as element energy increased, Friedrich Hund worked on figuring out the lowest energy that electrons could be arranged in a subshell.

Hund's rule states that all orbitals of a given sublevel must be occupied by a single electron before pairing begins.


Example 1

A simple example is carbon, atomic number (Z) = 6, orbital configuration of 1s 2 2s 2 2p 2 :

Electron Configurations Orbital Filling

Example 2

A more complex example is vanadium, a metal additive to steel, with an atomic number (Z) = 23, orbital configuration of 1s 2 2s 2 p 6 3s 2 p 6 d 3 4s 2 :

Electron Configurations Orbital Filling

Quantum State Of Electrons

The Austrian physicist, Wolfgang Pauli, won the Nobel Prize for Physics for his exclusion principle; that no two electrons can have the same quantum state (position, momentum, mass, and spin) simultaneously.

The Pauli exclusion principle states that no two electrons in the same atom can be in the same configuration at the same time.

Since each orbital can only have two electrons and those must be opposite in charge (or spin), then there are only two possible values for m s . Look at the examples on the next page. Are all the orbital diagrams possible?


Electron Configurations Quantum State Of Electrons

Did you get (a) yes, (b) no, (c) yes, (d) no, and (e) no?

Spin Magnetism

In 1921, two scientists, Otto Stern and Walter Gerlach, experimenting with silver atoms and a specially designed magnet found that electrons act like tiny magnets themselves. Figure 6.5 shows how. By sending a beam of atoms through a magnet, the beam is split into two beams, one bending one way and the other bending the other way. They decided that the electrons must be attracted and repelled by opposite and like charges just like magnets attract and repel opposite and like charges. This was called spin magnetism .

Electron Configurations Spin Magnetism

Fig. 6.5. Electrons attracted and repelled like magnets.


This example shows an orbital diagram of electrons in the s and p orbitals.

Electron Configurations Spin Magnetism

The electrons in orbital diagrams are written as up and down arrows for (up) m s = +1/2 and (down) m s =  -1/2. Electrons spinning through their orbitals act like spinning marbles of electrical charge. This spinning electrical charge circulates in the orbitals creating mini magnetic fields. This spin magnetism value is written as m s .

Additionally, just as opposites charges attract and like charges repel, so it is with pairs of electrons in orbital subshells. Two electrons with the same spin cannot be placed in orbitals together. If one electron is spinning positively, then the other must be spinning negatively.

Subshells And The Periodic Table

The Periodic Table helps chemists write the atomic makeup of a molecule fairly simply. The arrangement of a main group element uses the following formula:

n s a n p b

where n = the outer shell number and period (row) of the element and a + b = total number of valence electrons that you can get from the group (column) number.

In general, groups IA and IIA fill the s subshell, groups IIIA-VIIIA fill the p subshell, and groups IIIB- VIIIB fill the ( n - 1) d subshell.

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