Ionization Energy (College Board AP® Chemistry)
Study Guide
Written by: Martín
Reviewed by: Stewart Hird
Ionization Energy
Ionization energy is the energy required to remove an electron from an atom or ion in gaseous state
E.g. The equation for the ionization energy of sodium is shown below
It is always positive because removing an electron always takes energy
Coulomb’s Law helps to estimate the energy based on the distance from the nucleus and the effective charge of the nucleus
Factors affecting the Ionization Energy
Nuclear Charge: The greater the positive charge in the nucleus, the stronger the Coulombic attraction to electrons, leading to higher ionization energy
Distance from Nucleus: Electrons closer to the nucleus experience stronger Coulombic attraction, resulting in higher ionization energy
Shielding (Effective Nuclear Charge): Core electrons shield outer electrons from the full Coulombic attraction from the nucleus, reducing the ionization energy for valence electrons
Examiner Tips and Tricks
When analyzing the ionization energy between atoms, the shielding effect (effective nuclear charge) and the distance from the nucleus have more impact than the nuclear charge
Worked Example
Estimate which first ionization energy is greater, F or Cl.
Answer:
Step 1: Analyze nuclear charge
The nuclear charge is determined by the number of protons in the nucleus
Chlorine (Cl): Z = 17 (17 protons)
Fluorine (F): Z = 9 (9 protons)
Chlorine has more protons (Z) than fluorine, so its nuclear charge (Z) is greater and the Coulombic attraction should be greater
Step 2: Analyze distance from the nucleus
Chlorine has its outermost electron in the 3p subshell
Fluorine has its outermost electron in the 2p subshell
The distance from the nucleus is greater in Cl, therefore the electron in Cl experiences less Coulombic attraction from the nucleus
Step 3: Analyzing the shielding effect (effective nuclear charge):
The valence electrons in Cl experience more shielding from the core electrons compared to fluorine, because there is an extra shell filled with electrons in Cl. Therefore, the effective nuclear charge is weaker in Cl
Step 4: Estimate ionization energy
Based on the factors we've analyzed, we can make an estimation
Even if the nuclear charge is greater in Cl,
The distance from the nucleus is smaller in F
The effective nuclear charge is greater in F
Therefore, the coulombic force of attraction experienced by the valence electrons is stronger in F. This means that it requires more energy to remove a valence electron from a F atom compared to a Cl atom
Examiner Tips and Tricks
It is compulsory to make reference to the Coulomb’s Law in every estimation of ionization energies between atoms
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