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Periodic Trends 121
8-2 Ionization Energy
Don’t Forget!
Don’t Forget!
Neutral atoms may gain or lose electrons to form ions. Ions are atoms (or groups
of atoms) that have an overall charge. We will only consider ions consisting of one
atom in this chapter. Ions of only one atom are monatomic ions. If electrons are
lost, the resultant ion has more protons than electrons and therefore has a positive
charge. An ion having a positive charge is a cation. The cation has one positive
charge for each electron lost. How easily one or more electrons are lost
depends on the atomic radius and the effective nuclear charge.
Atoms or groups of atoms may lose electrons to form positive ions, known as
cations.
The ionization energy (IE) is the energy required to remove an electron from a
gaseous atom in its lowest possible energy state, its ground state. This process
begins with the electrons furthest from the nucleus. It requires energy to overcome
the attractive force of the nucleus. The closer the electron is to the
nucleus, the greater the attraction. The greater the attraction, the greater the
energy required to remove that electron. Therefore, ionization energies tend to
decrease going towards the bottom of a family. This is because the valence electrons
are farther from the nucleus. This is the reverse of the atomic radii trend.
Larger atoms have smaller ionization energies.
It is possible to remove more than one electron. This yields a second or a third
IE, and so on. Successive ionization energies require more energy than the preceding
one. This is true because the increase in positive charge creates a greater
attraction for the remaining electrons, pulling those electrons closer to the
nucleus. More energy is necessary to overcome this increased attraction.
Positives attract negatives. In order to pull the negative electron away from the
positive nucleus, energy will be required to break the attractive force.