The general increase in ionization energy across a period in the periodic table can be attributed to several key factors:
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Nuclear Charge: As you move from left to right across a period, the number of protons in the nucleus increases. This increase in positive charge (nuclear charge) exerts a stronger attractive force on the electrons in the same energy level.
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Shielding Effect: While the nuclear charge increases, the electrons added across a period are primarily placed in the same principal energy level. Since the shielding effect (the repulsion between electrons) remains relatively constant across a period, the increased nuclear charge is more effectively felt by the outermost electrons. As a result, these outer electrons are held more tightly by the nucleus, making it more difficult to remove them.
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Atomic Radius: As you go from left to right across a period, the atomic radius tends to decrease due to the increased nuclear charge pulling the electrons closer to the nucleus. This decrease in distance between the nucleus and the outermost electrons leads to a stronger attraction, further contributing to the increase in ionization energy.
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Effective Nuclear Charge (Z_eff): The increasing nuclear charge experienced by the outer electrons can be quantified as the effective nuclear charge. This effective charge becomes greater as more protons are introduced without a significant addition of inner-shell electrons, making it more difficult to remove an electron.
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Electron Configuration Stability: Elements on the right side of a period typically have nearly filled or filled subshells, leading to greater stability. Removing an electron from these stable configurations requires more energy, contributing to the overall increase in ionization energy.
In summary, the increase in ionization energy across a period results from the combined effect of increasing nuclear charge, minimal shielding, a smaller atomic radius, and increased effective nuclear charge, all of which contribute to a stronger attraction between the nucleus and the valence electrons.
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