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Ionization energy: period trend | Atomic structure and properties | AP Chemistry | Khan Academy
Khan Academy
Overview
This video explains the trend of ionization energy across a period in the periodic table. It details how ionization energy generally increases from left to right due to increasing effective nuclear charge. The video also highlights exceptions to this trend, such as the slight decreases observed when moving from Beryllium to Boron and from Nitrogen to Oxygen, explaining these anomalies by examining electron configurations and orbital repulsions.
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Chapters
- Ionization energy generally increases as you move from left to right across a period on the periodic table.
- This increase is primarily due to a rise in effective nuclear charge (Zeff).
- Effective nuclear charge is the net positive charge experienced by an electron, calculated as the nuclear charge (Z) minus the shielding effect of inner electrons (S).
- As the number of protons increases across a period, the nucleus exerts a stronger pull on the valence electrons.
Understanding this trend helps predict how much energy is needed to remove an electron from an atom, which is crucial for understanding chemical reactivity and bonding.
Moving from Lithium (520 kJ/mol) to Neon, ionization energies generally increase, with Neon having a significantly higher ionization energy.
- Effective nuclear charge (Zeff) is calculated by subtracting the shielding constant (S) from the atomic number (Z).
- Inner shell electrons shield the outer valence electrons from the full positive charge of the nucleus.
- A higher Zeff means valence electrons are held more tightly to the nucleus.
- For Lithium (1s²2s¹), Zeff ≈ +1 (3 protons - 2 core electrons).
- For Beryllium (1s²2s²), Zeff ≈ +2 (4 protons - 2 core electrons).
This concept is the fundamental reason behind many periodic trends, including ionization energy, as it quantifies the attractive force experienced by valence electrons.
Lithium's outer electron experiences an effective nuclear charge of approximately +1, while Beryllium's outer electron experiences approximately +2, leading to a stronger pull on Beryllium's electron.
- Despite an increase in protons, ionization energy slightly decreases from Beryllium (900 kJ/mol) to Boron (800 kJ/mol).
- Boron's electron configuration is 1s²2s²2p¹, meaning the valence electron is in a higher-energy 2p orbital.
- The 2p electron is, on average, further from the nucleus than the 2s electrons.
- The 2s electrons provide additional shielding and repulsion to the 2p electron, making it easier to remove.
This exception demonstrates that electron configuration and orbital energies play a significant role, not just the overall effective nuclear charge.
Boron's single 2p electron is less tightly held than Beryllium's 2s electrons due to its higher energy level and repulsion from the 2s electrons.
- Ionization energy slightly decreases from Nitrogen (1400 kJ/mol) to Oxygen (1300 kJ/mol).
- Nitrogen's configuration is 1s²2s²2p³ (one electron in each 2p orbital).
- Oxygen's configuration is 1s²2s²2p⁴, meaning one 2p orbital contains a pair of electrons.
- The electron repulsion within the doubly occupied 2p orbital in Oxygen makes its removal easier compared to removing an electron from a singly occupied 2p orbital in Nitrogen.
This exception highlights how electron-electron repulsion within the same subshell can lower ionization energy, overriding the general trend of increasing Zeff.
The pairing of electrons in one of Oxygen's 2p orbitals creates repulsion, making it easier to remove one of those electrons than an electron from Nitrogen's half-filled 2p subshell.
Key takeaways
- Ionization energy generally increases across a period because the effective nuclear charge increases, pulling valence electrons more strongly.
- Effective nuclear charge is a key factor determining how tightly electrons are held.
- Electron configurations and the specific orbitals electrons occupy influence ionization energy.
- Electron-electron repulsion within a subshell can lead to exceptions where ionization energy decreases.
- The trend of increasing ionization energy across a period is a general rule with predictable exceptions based on electron arrangement.
- Understanding these trends is fundamental to predicting atomic behavior and chemical properties.
Key terms
Ionization energyPeriodic trendEffective nuclear charge (Zeff)Shielding effectElectron configurationValence electronsCore electronsOrbitalElectron repulsion
Test your understanding
- What is the general trend for ionization energy as you move from left to right across a period, and why does this trend occur?
- How does the concept of effective nuclear charge explain the increase in ionization energy across a period?
- Why is there a slight decrease in ionization energy when moving from Beryllium to Boron?
- What causes the dip in ionization energy observed when moving from Nitrogen to Oxygen?
- How do electron-electron repulsions within an orbital affect the energy required to remove an electron?