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Effective Nuclear Charge Calculator

Chemistry

Calculate effective nuclear charge (Z*) using Slater's rules. Enter atomic number and electron configuration to find shielding constant σ and Z* = Z − σ.

154

Effective Nuclear Charge Z*

1.4
Shielding Constant (σ)
15.6
Electron Configuration
1s2 2s2 2p6 3s2 3p5
Interpretation
Low Z* (1.40) — effective nuclear charge close to 1; loosely held valence electrons

This calculator computes your Effective Nuclear Charge Z*, Shielding Constant (σ), Electron Configuration, Interpretation from the values you enter.

Inputs
Atomic Number (Z)Target Electron Shell
Outputs
Effective Nuclear Charge Z*Shielding Constant (σ)Electron ConfigurationInterpretation

What is a Z*?

The Effective Nuclear Charge Calculator computes Z* = Z − σ using Slater's rules. Select the atomic number (Z = 1–54) and the target electron shell. The calculator fills the electron configuration, applies the Slater shielding contributions from each electron group, and returns the effective nuclear charge Z*, shielding constant σ, and periodic interpretation.

Effective nuclear charge is the net positive charge that a particular electron experiences from the nucleus after accounting for the screening effect of all other electrons. An electron in the 3s orbital of sodium (Z=11) does not feel the full +11 charge of the nucleus — the 10 inner electrons (in 1s, 2s, 2p) partially shield it, leaving it feeling an effective charge of about +2.5. This explains why sodium's valence electron is easily removed (low first ionisation energy) and why sodium is reactive.

Z* is the single most important factor explaining periodic trends: ionisation energy, atomic radius, electron affinity, and electronegativity all increase with Z* across a period. The Electronegativity Calculator provides Pauling electronegativity values — these directly reflect the Z* trend. The Atom Calculator provides the base proton/neutron/electron counts from which Z* calculations begin.

How to use this Z* calculator

  1. Select Atomic Number Z (1–54) using the slider — covers all main-group and first-row transition elements.
  2. Select Target Shell — the subshell whose effective nuclear charge you want. For valence electron properties: choose the outermost shell for that element.
  3. Read Z* — the effective nuclear charge for an electron in that shell.
  4. Compare Z* across elements in the same period to see the left-to-right increase.
  5. Compare Z* down a group — expect a slow but definite increase despite the addition of inner shells.

Formula & Methodology

Slater's rules — shielding constants:

Z* = Z − σ σ = Σ(contributions from all other electrons)  Target shell 1s:      same-1s electrons contribute 0.30 each Target shell s/p:     same-group → 0.35; (n−1) group → 0.85; all lower → 1.00 Target shell d/f:     same-group → 0.35; all electrons in lower groups → 1.00

Worked example — Chlorine (Z=17), target: 3sp valence electron:

Electron configuration in Slater groups: [1s²] [2s²2p⁶] [3s²3p⁵]

Shielding from [1s²]: 2 electrons × 1.00 = 2.00 Shielding from [2sp⁸]: 8 electrons × 0.85 = 6.80 Shielding from [3sp⁵] (same group, excluding target): 6 electrons × 0.35 = 2.10 σ = 2.00 + 6.80 + 2.10 = 10.90 Z* = 17 − 10.90 = 6.10

Chlorine's Z* of 6.10 is the highest in the third period for an sp electron — explaining why Cl has the highest electronegativity in period 3 and the smallest atomic radius among period 3 non-metals. Chlorine's high Z* and electron affinity make it one of the most powerful oxidising agents used in water treatment in India's municipal water systems (chlorination kills pathogens in drinking water supplied to cities like Mumbai, Delhi, and Bengaluru).

Frequently Asked Questions

Effective nuclear charge (Z* or Z_eff) is the net positive charge experienced by an electron in a multi-electron atom, after accounting for the shielding (screening) by inner-shell electrons. Z* = Z − σ, where Z is the actual nuclear charge (atomic number) and σ is the shielding constant. An electron does not experience the full nuclear charge Z because inner electrons partially cancel the attraction. For chlorine (Z=17) in the 3sp shell: σ ≈ 11.35, so Z* ≈ 5.65 — valence electrons feel only about 1/3 of the full nuclear charge.
Slater's rules (1930) give a systematic way to calculate the shielding constant σ. The rules depend on the electron group of the target electron: (1) Electrons in groups to the right (outer shells) contribute 0 shielding. (2) For s/p electrons in the same group: each contributes 0.35 (except 1s: 0.30). (3) Electrons in the group one step lower (n−1): each contributes 0.85. (4) All electrons in lower groups: each contributes 1.00. (5) For d/f electrons: all electrons in same group contribute 0.35; all electrons in lower groups (including all sp of same n) contribute 1.00. Then Z* = Z − σ.
Select the Atomic Number Z (1–54) and the Target Electron Shell (1s, 2s/2p, 3s/3p, 3d, or 4s/4p). The calculator applies Slater's rules to compute the shielding constant σ and effective nuclear charge Z* = Z − σ. It also shows the electron configuration and interpretation. Default: Cl (Z=17), target shell 3sp — giving Z* ≈ 5.65, σ ≈ 11.35.
Across a period (left to right): Z increases by 1 with each element, but shielding from same-period electrons is only 0.35 per electron — so Z* increases by approximately 0.65 per step. Example: Na (Z=11): Z* ≈ 2.51; Mg (Z=12): Z* ≈ 3.31; Al (Z=13): Z* ≈ 4.07; Cl (Z=17): Z* ≈ 6.12; Ar (Z=18): Z* ≈ 6.75. Down a group: Z increases significantly but the new valence shell is further from the nucleus AND new inner shells add 0.85 shielding each, so Z* increases only slowly. Net result: atomic radius increases down a group (weaker hold on outer electrons).
Ionisation energy increases across a period: higher Z* → stronger hold on valence electrons → more energy needed to remove them. Atomic radius decreases across a period: higher Z* → valence electrons pulled closer. Electronegativity increases across a period: higher Z* → greater ability to attract bonding electrons. These trends are the foundation of NCERT Class 11 Chapter 3 (Classification of Elements and Periodicity in Properties). The [Electronegativity Calculator](/electronegativity-calculator/) tabulates Pauling χ values that reflect these Z* trends.
For Fe (Z=26), 3d electrons: same-group shielding = 5 × 0.35 = 1.75 (for the 6th 3d electron, the other 5 shield it), plus all electrons in 1s, 2s/2p, 3s/3p shells at 1.00 each = (2+8+8) = 18. σ = 1.75 + 18 = 19.75. Z* = 26 − 19.75 = 6.25. High Z* for 3d electrons means they are tightly held, explaining why transition metals have multiple stable oxidation states (Fe²⁺ and Fe³⁺ both exist). It also explains the characteristic colour of transition metal complexes — the partially filled 3d shell with moderate Z* gives d-d transitions in the visible region.
Slater's rules (1930) give approximate Z* values based on simple electron group classification — useful for understanding periodic trends but not accurate enough for modern quantum chemistry. Clementi and Raimondi (1963) computed Z* values by optimising atomic orbitals using Hartree-Fock calculations — these are more accurate but harder to calculate manually. For Cl 3p: Slater Z* ≈ 5.75; Clementi-Raimondi Z* = 6.116. Both correctly predict the trend (increasing across period, slow increase down group) even if the absolute values differ. Slater's rules are what NCERT and JEE cover; Clementi-Raimondi values are used in research-level quantum chemistry.
Lanthanide contraction is the phenomenon that 5d transition metals (Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg) are nearly the same size as the preceding 4d metals (Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd). Reason: the 4f electrons added in the lanthanides (Z=57–71) provide very poor shielding (each f electron contributes ~1.00 shielding per Slater's rules for the 5d, 6s electrons, but this is an approximation — f electrons penetrate poorly, so their actual shielding is less than 1.00). The result is that Z* for 5d and 6s electrons is nearly the same as for 4d and 5s, keeping atomic radii similar.
The concept of effective nuclear charge and Slater's rules are covered qualitatively in NCERT Class 11, Chapter 3 (Classification of Elements and Periodicity in Properties) — the textbook discusses shielding, screening, and effective nuclear charge as explanations for periodic trends in ionisation energy, atomic radius, and electron affinity. Slater's rules are not derived in NCERT but are tested in JEE Advanced problems where candidates may need to compare Z* values for specific electron shells to explain periodic anomalies. The formula Z* = Z − σ is standard JEE content.
Yes — Z* is always positive and can be much greater than 1. For hydrogen (Z=1, only 1 electron, no shielding): Z* = 1 − 0 = 1. For helium (Z=2, 2 electrons in 1s, each shields the other by 0.30): Z* = 2 − 0.30 = 1.70 per 1s electron. For fluorine (Z=9, 2 in 1s shielding at 0.85, 6 others in 2sp at 0.35 each): σ = 2×0.85 + 6×0.35 = 1.70 + 2.10 = 3.80; Z* = 9 − 3.80 = 5.20. Fluorine's valence electrons experience Z* = 5.2 — explaining its extreme electronegativity and small atomic radius (the highest Z* of any first-row element).