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Wize High School Grade 12 Chemistry Textbook > Atomic Models and Properties of Atoms

Ionization Energy

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Ionization Energy (IE)

The ionization energy is the amount of energy required to remove the outermost electron from an atom or ion in the gas phase
  • If removing an electron do we get an anion or a cation?
    cation

E→E++e−ΔE=Ionization Energy\textbf{E}\mathbf{\rightarrow}\textbf{E}^\mathbf{+}\mathbf{+}\textbf{e}^\mathbf{-}\quad\quad\Delta\text{E}=\text{Ionization Energy}
  • ↑IE means that the atom/ion requires more energy for a valence electron to be removed (has a tight pull on electron)


The Periodic Trend
  • As we move to the right across a period, Zeff increases and the electrons are held more tightly. Therefore, it takes more energy to remove an electron and the IE increases as we move to the right.

  • As we move down a group and the valence electrons are further away from the nucleus, they are held more weakly (lower Zeff) and IE decreases (it becomes easier to remove the outermost electron)




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Comparing 1st, 2nd, and 3rd Ionization Energies


How does the first IE compare to the second IE?


First Ionization Energy (IE1):    A (g)  →  A+ (g)  +  e−First\ Ionization\ Energy\ \left(IE_1\right):\ \ \ \ A\ \left(g\right)\ \ \rightarrow\ \ A^+\ \left(g\right)\ \ +\ \ e^-

Second Ionization Energy (IE2):    A+ (g)  →  A2+ (g)  +  e−Second\ Ionization\ Energy\ \left(IE_2\right):\ \ \ \ A^+\ \left(g\right)\ \ \rightarrow\ \ A^{2+}\ \left(g\right)\ \ +\ \ e^-

Third Ionization Energy (IE3):    A2+ (g)  →  A3+ (g)  +  e−Third\ Ionization\ Energy\ \left(IE_3\right):\ \ \ \ A^{2+}\ \left(g\right)\ \ \rightarrow\ \ A^{3+}\ \left(g\right)\ \ +\ \ e^-

1st ionization is always smaller than the 2nd because removing a negatively charged electron from a cation is more difficult.
Example:
Ca (g)  →  Ca+ (g)  +  e−    IE1 = 590 kJmolCa\ \left(g\right)\ \ \rightarrow\ \ Ca^+\ \left(g\right)\ \ +\ \ e^-\ \ \ \ IE_1\ =\ 590\ \frac{kJ}{mol}

Ca+ (g)  →  Ca2+ (g)  +  e−    IE2 = 1145 kJmolCa^+\ \left(g\right)\ \ \rightarrow\ \ Ca^{2+}\ \left(g\right)\ \ +\ \ e^-\ \ \ \ IE_2\ =\ 1145\ \frac{kJ}{mol}


Wize Tip
IE1 < IE2 < IE3 < IE4 etc


How does the IE2 and IE3 for magnesium compare?




A core electron is much more difficult to remove than a valence electron, for example: the 3rd ionization energy of magnesium is much, much, higher than the 2nd.

Wize Tip
Although the IE increases each time another electron is removed, the increase is not linear and is usually related to the electron configuration.

Ionizing Mg2+ would be very difficult!
Mg→  Mg+ +  e−    IE1 = 738 kJmolMg\rightarrow\ \ Mg^+\ +\ \ e^-\ \ \ \ IE_1\ =\ 738\ \frac{kJ}{mol}

Mg+→  Mg2+ +  e−    IE2 = 1451 kJmolMg^+\rightarrow\ \ Mg^{2+}\ +\ \ e^-\ \ \ \ IE_2\ =\ 1451\ \frac{kJ}{mol}

Mg2+→  Mg3+ +  e−    IE3 = 7733 kJmolMg^{2+}\rightarrow\ \ Mg^{3+}\ +\ \ e^-\ \ \ \ IE_3\ =\ 7733\ \frac{kJ}{mol}


Needing so much energy to remove the 3rd electron indicates that when 2 were removed it was stable/unstable:
stable
!


Practice: Highest 1st Ionization Energy

Which of the following species has the highest 1st ionization energy?


Practice: Trends in Ionization Energy

Rank the following atoms in order of increasing first ionization energy (1 = smallest; 6 = largest): Fe, O, Ba, Si, V, Zr


Fe
O
Ba
Si
V
Zr

Practice: Lowest Ionization Energy

Select the element with the lowest 3rd ionization energy.