Ionization energy (IE) is the amount of energy required to remove an electron from an isolated gaseous atom or ion. This process results in the formation of a cation (positively charged ion). The ionization energy can be influenced by various factors including the atomic number, the electron configuration of the atom, and the effective nuclear charge experienced by the electrons.
Trends in Ionization Energy:
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Across a Period (Left to Right):
- Ionization energy generally increases as you move from left to right across a period in the periodic table. This is because as you move across a period, protons are added to the nucleus, increasing the positive charge and thus the effective nuclear charge that attracts the electrons more strongly. As a result, more energy is required to remove an electron.
- Exceptions: There are a few exceptions to this trend, particularly between groups 2 and 13 (e.g., Be to B) and groups 15 and 16 (e.g., N to O). These exceptions occur due to the stability of half-filled and fully filled subshells.
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Down a Group (Top to Bottom):
- Ionization energy generally decreases as you move down a group in the periodic table. This trend occurs because the outer electrons are further from the nucleus, experience greater electron shielding from inner-shell electrons, and thus experience a weaker effective nuclear charge. Consequently, it takes less energy to remove an outer electron.
Key Points:
- Unit: Ionization energy is measured in electronvolts (eV) or kilojoules per mole (kJ/mol).
- Multiple Ionizations: Atoms can have more than one ionization energy corresponding to the removal of multiple electrons (e.g., first ionization energy, second ionization energy, etc.). Each successive ionization energy is usually higher than the previous one due to the increasing positive charge of the ion and reduced electron-electron repulsion.
Overall, understanding these trends in ionization energy helps predict the reactivity of elements and their chemical behavior.