6.3. Trends in the Periodic Table http://www.ck12.org
of a cation, except a minus sign is used instead of a plus sign. A chlorine atom that gains one electron becomes Cl−,
and a sulfur atom that gains two electrons becomes S^2 −.
It is important to realize that atoms become ions only when the number of electrons increases or decreases. The
number of protons and neutrons is not changing. Therefore the reactivity of the element may change as the valence
shell configuration is changing, but the element itself remains the same.
Noble Gas Configurations
How can we predict the number of electrons that an element is likely to gain or lose in order to form a stable ion? To
answer that, we look to the noble gases. Certain physical and chemical properties were found to repeat themselves in
a regular pattern when the elements are arranged by their atomic number. For example, the elements on the far left of
the periodic table (groups 1 and 2) tend to be quite reactive in their pure form, whereas the elements in group 18 (the
noble gases) are almost completely unreactive. Because they are so unlikely to react with other chemical substances,
most of them were discovered quite a bit later than the elements just before and after them on the periodic table. This
lack of reactivity can largely be explained by electron configurations. The configurations of the noble gas elements
are shown inTable6.3.
TABLE6.3: Electron Configurations of the Noble Gases
Element (Symbol) Electron configuration
helium (He) 1s^2
neon (Ne) 1s^2 2s^2 2p^6
argon (Ar) 1s^2 2s^2 2p^6 3s^2 3p^6
krypton (Kr) 1s^2 2s^2 2p^6 3s^2 3p^6 3d^10 4s^2 4p^6
xenon (Xe) 1s^2 2s^2 2p^6 3s^2 3p^6 3d^10 4s^2 4p^6 4d^10 5s^2 5p^6
radon (Rn) 1s^2 2s^2 2p^6 3s^2 3p^6 3d^10 4s^2 4p^6 4d^10 5s^2 5p^6 4f^14 5d^10 6s^2 6p^6Except for helium, each of these elements has a configuration ofns^2 np^6 in its highest occupied principal energy
level. In other words, each noble gas has 8 valence electrons, sometimes referred to as a complete octet. Having
completely filled s and p orbitals in the outermost energy level represents an especially stable configuration, so noble
gases have very little driving force to react any further. Conversely, other elements will readily gain, lose, or share
electrons in order to achieve a stable octet of valence electrons. The number of electrons that needs to be lost or
gained in order for this to occur helps us to predict the charges of ions formed by the main group elements.
Helium may seem to be an exception, since it has only two valence electrons. This has to do with the fact that the n
= 1 energy level has one s orbital and no p orbitals. As a result, the first energy level can be completely filled by just
two electrons. It is the presence of a filled valence shell that gives noble gases their unusual stability, not anything
intrinsic about the number 8. In addition to helium, the first few elements such as lithium, beryllium, and boron,
have a particularly stable configuration with a pair of valence electrons rather that an octet.
Ionization Energy
To make an electron jump from a lower energy level to a higher energy level, there must be an input of energy.
Removing the electron from the atom entirely requires even more energy. This is called an ionization process.
Ionization energyis the energy required to remove an electron from an atom (X→X++ e−). An equation can be
written to illustrate this process for a sodium atom.
Na+energy→Na++e−