Shielded
Definition and meaning of Shielded in chemistry.
Shielded refers to an electron or nucleus protected from the full effect of an external force by intervening electrons. In atoms, inner electrons shield outer electrons from the nucleus's positive charge, reducing the effective nuclear charge they experience.
In more detail
In electron shielding, electrons in inner orbitals partially block the positive charge of the nucleus from reaching outer electrons. For example, in an oxygen atom, the two 1s electrons act as the core shielding the six valence electrons, reducing the effective nuclear charge they experience from +8 to roughly +6 (Z_eff = Z − S = 8 − 2). This concept explains key periodic trends: across a period, shielding remains roughly constant while nuclear charge increases, producing smaller atoms and higher ionization energies. In NMR spectroscopy, electron density also shields nuclei from external magnetic fields, causing chemical shifts that vary by chemical environment.
Key facts
| Field | General Chemistry |
|---|---|
| Formula | Zeff = Z − S (where S is the shielding constant) |
| Explains | Periodic trends in atomic radius and ionization energy |
| Also applies to | NMR spectroscopy and chemical shifts |
In oxygen (O), the two 1s core electrons shield the six valence electrons, reducing the effective nuclear charge they experience from +8 to approximately +6.
Frequently asked questions
Why does shielding make atoms larger?
Inner electrons reduce the effective positive charge, weakening the nucleus's attraction on outer electrons, so they orbit farther away.
What is the difference between actual and effective nuclear charge?
Actual nuclear charge (Z) is the total protons in the nucleus; effective nuclear charge (Zeff) is what outer electrons actually experience after accounting for shielding by inner electrons.