Effective nuclear charge
Effective nuclear charge (Zeff) is the pull an outer electron actually feels from the nucleus. Inner electrons shield some of the nucleus's positive charge, so Zeff is smaller than the full charge. Roughly, Zeff = Z − shielding.
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Analogy
Like standing behind a crowd at a concert: you still see the stage, but the people in front block part of the view.
Example
Na: Z = 11, inner electrons = 10Zeff ≈ 11 − 10 = 1Cl: Z = 17, inner electrons = 10Zeff ≈ 17 − 10 = 7
In practice
Zeff rises across a period, which is why atoms get smaller and hold their electrons tighter going left to right.
Practice
Flashcards
- What is effective nuclear charge?The net pull an outer electron feels after inner electrons shield part of the nucleus.
Interview questions
- Explain why atomic radius decreases across a period but increases down a group.
- Across a period: protons increase while electrons go into the same shell
- Zeff rises, so the pull on outer electrons grows
- Down a group: extra shells are added
- Outer electrons are further away and more shielded
Questions people ask
Why do electrons in the same shell shield poorly?
Electrons in the same shell are at about the same distance from the nucleus. Only electrons closer in can block the nucleus's pull well. So adding a proton and an electron in the same shell raises Zeff, because the new electron adds little shielding.
How does Zeff explain trends across a period?
Moving across a period, each step adds one proton but the extra electron goes into the same shell. Zeff grows, so the nucleus pulls the outer electrons harder. This shrinks the atom, raises ionization energy and makes the element hold on to its electrons more strongly.