Ionization Energy
Atoms hold their valence electrons, then, with different amounts of energy. If
enough energy is supplied to one outer electron to remove it from its atom, this
amount of energy is called the first ionization energy. With the first electron
gone, the removal of succeeding electrons becomes more difficult because of the
loss of repulsive effects that were present with a greater number of electrons. It
should be noted that the lowest ionization energies are found with the least
electronegative elements.
TIP
Know this definition of the first ionization energy.Ionization energies can be plotted against atomic numbers, as shown in the
graph below. Follow this discussion on the graph to help you understand the
peaks and valleys. Not surprisingly, the highest peaks on the graph occur for the
ionization energy needed to remove the first electron from the outer energy level
of the noble gases, He, Ne, Ar, Kr, Xe, and Rn, because of the stability of the
filled p orbitals in the outer energy level. Notice that, even among these elements,
the energy needed gradually declines. This can be explained by considering the
distance of the involved energy level from the positively charged nucleus. With
each succeeding noble gas, a more distant p orbital is involved, therefore making
it easier to remove an electron from the positive attraction of the nucleus. Besides
this consideration, as more energy levels are added to the atomic structure as the
atomic number increases, the additional negative fields associated with the
additional electrons screen out some of the positive attraction of the nucleus.
Within a period such as that from Li to Ne, the ionization energy generally
increases. The lowest occurs when a lone electron occupies the outer s orbital, as
in Li. As the s orbital fills with two electrons at atomic number 4, Be, the added
stability of a filled 2s orbital explains the small peak at 4. At atomic number 5, B,
a lone electron occupies the 2p orbital. This electron can be removed with less
energy, and therefore a dip occurs in the graph. With the 2p orbitals filling
according to Hund’s Rule (refer to Table 2), with only one electron in each orbital
before pairing occurs, again a slightly more stable situation and, therefore,
another small peak occurs at atomic number 7. After this peak, a dip and continual
increases occur until the 2p orbitals are completely filled with paired electrons at
the noble gas Ne. As you continue to associate the atomic number with the line in
the chart, you find peaks occurring in the same general pattern. These peaks are
always related to the state of filling of the orbitals involved and the distance of
these orbitals from the nucleus.