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The secret of effective nuclear charge: Why do outer electrons never experience the full nuclear charge?
In atomic physics, effective nuclear charge is a concept that describes the strength of the nuclear charge felt by an electron in a multi-electron atom or ion. The importance of this concept lies in its ability to help us understand many physical and chemical properties of elements. As we delve deeper into the topic of effective nuclear charge, we can't help but ask: How exactly are the outer electrons shielded from fully feeling the nuclear charge?
The effective nuclear charge is the nuclear charge of the atom minus the shielding effect caused by other inner-shell electrons.
In a multi-electron atom, the effective nuclear charge felt by the outermost electrons (usually represented by the symbol Zeff) is reduced relative to the actual number of protons in the nucleus. This is because there is repulsion between electrons, which affects the electrostatic interaction between the outer electrons and the nucleus. For example, a 1s electron in steel (iron has atomic number 26) can feel the attraction of almost all 26 protons, but a 4s electron in the outermost shell can only feel an effective nuclear charge of about 5.43.
In multi-electron atoms, the difference between the nuclear charge and the effective nuclear charge is due to the screening effect of the inner-shell electrons.
The most basic calculation formula for effective nuclear charge can be expressed as: Zeff = Z - S, where Z is the number of protons in the atom and S is the screening constant. This formula reveals the shielding effect of inner electrons on outer electrons. Using this framework, we can apply Slater's rules to simplify the calculation of the screening constant. Slater's rule provides a simple way to estimate the screening effect of each electron and thus to calculate the effective nuclear charge more accurately.
In addition to Slater's rule, another more theoretical method is the Hartree-Fock method, which requires more complicated mathematical operations but is better than Slater in terms of accuracy. rule. This method combines the calculation of the screening constant with the wave function, making the calculation results more reliable.
In an atom, the outer electrons are not only attracted by the nucleus, but also repelled by the inner electrons, which forms a shielding effect.
The concept of effective nuclear charge is key to understanding the chemical behavior of elements. Not only that, the effective nuclear charge helps us predict and explain changes in properties such as ionization energy and chemical reactivity. In the periodic table, the effective nuclear charge decreases from top to bottom, but increases from left to right. This is because the shielding effect of the inner electrons on the outer electrons changes with the change of atomic radius. .
This way of thinking based on atomic structure not only helps scientists design new materials, but also guides us to understand how atoms interact with each other in chemical reactions. So, is it possible to advance science through a deeper understanding of effective nuclear charge?
