# 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.

Chemistry (university) · Part 01: Atomic structure and periodic trends · Card 8 of 10 · Last updated 2026-10-19

## 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

**Sodium vs chlorine**

Na: Z = 11, inner electrons = 10
Zeff ≈ 11 − 10 = 1
Cl: Z = 17, inner electrons = 10
Zeff ≈ 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.

## Related concepts

- [Atomic and ionic radius](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-1/chemistry-university-part-1-concept-9)
- [Ionization energy](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-1/chemistry-university-part-1-concept-10)

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