Ionization energy is the energy needed to remove an electron from a gaseous atom or ion. It matters because it reveals how strongly an atom holds its electrons and helps explain reactivity, bonding, and periodic trends. Elements with low ionization energy tend to lose electrons easily, while elements with high ionization energy resist electron loss.
Chemists use ionization energy data to identify valence electrons and predict common ion charges.
Successive ionization energies remove electrons one at a time from the same atom or ion. The values usually increase because each electron is removed from a more positive ion, which attracts the remaining electrons more strongly. A dramatic jump appears when all valence electrons have been removed and the next electron would come from a lower, core energy level.
Reading this jump helps determine how many valence electrons an atom had before ionization began.
Key Facts
- First ionization energy: X(g) -> X+(g) + e-
- Second ionization energy: X+(g) -> X2+(g) + e-
- Successive ionization energies increase because the ion becomes more positively charged after each electron is removed.
- A large jump in successive ionization energy usually marks the transition from removing valence electrons to removing core electrons.
- Ionization energy generally increases from left to right across a period and decreases from top to bottom down a group.
- Higher effective nuclear charge makes electrons harder to remove, while greater shielding and larger atomic radius make electrons easier to remove.
Vocabulary
- Ionization energy
- The energy required to remove an electron from a gaseous atom or ion.
- Successive ionization energy
- The sequence of energies needed to remove electrons one at a time from an atom or ion.
- Valence electron
- An electron in the outermost occupied energy level of an atom that is most involved in bonding.
- Core electron
- An inner electron that is closer to the nucleus and usually much harder to remove than a valence electron.
- Effective nuclear charge
- The net positive attraction felt by an electron after accounting for shielding by other electrons.
Common Mistakes to Avoid
- Assuming ionization energy always decreases during successive removals, which is wrong because each removal leaves a more positive ion that attracts the remaining electrons more strongly.
- Ignoring the large jump in a successive ionization energy chart, which is wrong because that jump often identifies when core electrons begin to be removed.
- Confusing atomic radius trends with ionization energy trends, which is wrong because larger atoms usually hold outer electrons less tightly and often have lower ionization energy.
- Using ionization energy values for liquids or solids, which is wrong because standard ionization energy is defined for gaseous atoms or ions.
Practice Questions
- 1 An element has successive ionization energies of 590, 1145, 4910, 6490, and 8120 kJ/mol. Between which two removals is the largest jump, and how many valence electrons does the atom most likely have?
- 2 The first ionization energies of three elements are 496 kJ/mol, 738 kJ/mol, and 2081 kJ/mol. Rank them from easiest to hardest to ionize, and explain your ranking using the numerical values.
- 3 An atom shows a huge increase after its third ionization energy. Explain what this reveals about its electron arrangement and the charge of the ion it is most likely to form.