What is effective nuclear charge?

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Effective Nuclear Charge (Z_eff)

The <a href="https://www.wikiwhat.page/kavramlar/effective%20nuclear%20charge">effective nuclear charge</a> (Z_eff) is the net positive charge experienced by an electron in a polyelectronic atom. It's the actual nuclear charge (Z) minus the shielding or screening effect of inner-shell electrons.

• Concept: Electrons in an atom are attracted to the positively charged nucleus, but they are also repelled by other electrons. The inner-shell electrons shield the outer-shell electrons from the full force of the nuclear charge.

• Calculation: A simplified way to estimate Z_eff is:

  Z_eff = Z - S

  Where:

  • Z is the atomic number (number of protons in the nucleus)
  • S is the shielding constant (estimate of the shielding effect of inner electrons)

• Shielding: <a href="https://www.wikiwhat.page/kavramlar/shielding%20effect">Shielding effect</a> increases with the number of inner electrons and the closer the electrons are to the nucleus. Core electrons are very effective at shielding the valence electrons.

• Slater's Rules: More accurate methods for calculating S exist, such as <a href="https://www.wikiwhat.page/kavramlar/slater's%20rules">Slater's rules</a>, which provide a set of guidelines for determining the shielding constant for each electron.

• Trends:

  • Across a Period: Z_eff generally increases across a period in the periodic table. This is because the number of protons in the nucleus increases, while the number of core electrons remains the same, leading to a weaker shielding effect.
  • Down a Group: Z_eff remains approximately constant or increases slightly down a group. Although the number of protons increases, the number of core electrons also increases, roughly canceling out the effect.

• Significance: Z_eff affects various atomic properties:

  • Atomic Size: A larger Z_eff leads to a stronger attraction between the nucleus and electrons, resulting in a smaller atomic radius.
  • Ionization Energy: A larger Z_eff makes it more difficult to remove an electron, leading to a higher ionization energy.
  • Electronegativity: A larger Z_eff increases the attraction of an atom for electrons in a chemical bond, thus increasing its electronegativity.

In summary, <a href="https://www.wikiwhat.page/kavramlar/effective%20nuclear%20charge">effective nuclear charge</a> is a fundamental concept in understanding the behavior and properties of atoms and molecules.