Ionization Energy: The amount of energy required to remove an electron from an atom or ion. We have previously called this “binding energy” when discussing the photoelectric effect.
First Ionization Energy: The amount of energy required to remove the first valence electron from a neutral atom. When discussing the trend of ionization energy, we will be specifically looking at first ionization energy.
X + Energy → X⁺ + e⁻ (chemical reaction for the first ionization energy of element “X”)
Keep in mind that as you increase the force of attraction between the nucleus and valence electrons, more energy is required to overcome that attraction and remove those electrons. Therefore, ionization energy increases as effective nuclear charge increases.
Moving down a group decreases ionization energy. Moving down a group adds more energy levels. The valence electrons are further from the nucleus, and therefore are less attracted to it. Additionally, there are more core electrons, further decreasing the attraction between the nucleus and valence electrons. This weakened attraction makes it easier to remove a valence electron (it requires less energy), so ionization energy decreases.
Moving left to right across a period increases ionization energy. Moving left to right increases the effective nuclear charge because there are more protons. The valence electrons are more attracted to the nucleus, so it requires more energy to remove them. This means that ionization energy increases.
Example Problem #1
What has a higher ionization energy?
Cl vs. I
Answer
Chlorine has a higher ionization energy. Chlorine has less principal energy levels, so its valence electrons are closer to the nucleus. Consequently, there is a stronger attraction between the nucleus and valence electrons. It requires more energy to overcome this attraction, so chlorine has a higher first ionization energy.
Example Problem #2
What has a higher ionization energy?
Cs vs. Ba
Answer
Barium has a greater first ionization energy. They have the same amount of principal energy levels, but barium has more protons than cesium. It has a greater effective nuclear charge, meaning more energy is required to remove an electron from barium.
Example Problem #3
What has a higher ionization energy?
Sr vs. S
Answer
Sulfur has a greater first ionization energy. Sulfur has less principle energy levels than strontium, so the valence electrons are closer to the nucleus. It also has a greater effective nuclear charge. Both of these factors contribute to sulfur having a stronger pull on its valence electrons. Therefore, it requires more energy to remove an electron from sulfur.
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