# Ionization energy
## Microsim
### Live player
<div class="microsim-player">
<iframe src="https://editor.p5js.org/sciencenibber/full/sTaq4v2KA" width="100%" height="620" frameborder="0" sandbox="allow-scripts allow-same-origin"></iframe>
</div>
<div class="microsim-fallback">
<img src="Microsims/thumbs/Ionization_energy.png" alt="Ionization_energy microsim poster" style="width:100%;border:1px solid #4445;border-radius:6px;">
<p><em>Live microsim (desktop) · <a href="https://editor.p5js.org/sciencenibber/sketches/sTaq4v2KA">open sketch in the p5.js editor</a></em></p>
</div>
**Editor URL:** https://editor.p5js.org/sciencenibber/sketches/sTaq4v2KA
**Description (100 words):**
Two stacked plots driven by one cursor: the selected atomic number Z. The top panel charts first ionization energy IE1 versus Z from hydrogen to krypton, drawn as a connected sawtooth — noble-gas peaks in warm red (helium at the very top, 24.587 eV), alkali-metal troughs in cool blue, the selected element pinned in yellow. Click anywhere on the plot, or use arrow keys, h, n, a, l shortcuts to jump between elements. The bottom panel rebuilds for each Z as a log-scale [[Bar_chart|bar chart]] of successive IE_n for that atom, with violet vertical separators where the next electron belongs to a deeper shell. The shell jumps are dramatic — sodium's IE2 is roughly ten times IE1.
```js
// =====================================================================
// Ionization_energy.js -- Wikitube microsim
// Article: Ionization_energy en.wikitube.io/wiki/Ionization_energy
// Room: Helium Pattern: D (parametric / periodic trend)
// ---------------------------------------------------------------------
// Idea: ionization energy is the energy required to strip the most
// loosely bound electron from a neutral atom (IE1), or successively
// the n-th electron (IE_n). Two facts dominate the physics:
//
// (1) IE1 across the periodic table is NOT monotonic in Z. It rises
// across a row as effective nuclear charge climbs, then drops
// at the start of each new shell. The result is a "sawtooth"
// with peaks at the noble gases (He, Ne, Ar, Kr) and troughs
// at the alkali metals (Li, Na, K). Helium's IE1 = 24.587 eV
// is the highest of any neutral atom -- the central reason the
// Helium room exists as a discrete topic.
//
// (2) Successive IE_n for a single atom show enormous jumps when
// a closed shell is breached. IE_2 / IE_1 for Na is roughly
// tenfold; IE_3 / IE_2 for Mg is even more. These shell jumps
// are the canonical empirical evidence for the shell model of
// the atom -- predating but consistent with the quantum-
// mechanical orbital picture.
//
// The microsim renders both facts in two stacked panels driven by a
// single integer cursor -- the selected atomic number Z:
//
// TOP panel (periodic trend): IE1 vs Z for Z = 1..36 (H..Kr).
// Connected line + dots, peaks (noble gases) in HOT red, troughs
// (alkali metals) in COLD blue, selected element in TRAJ yellow.
// Click on a Z to select; arrow keys nudge; 'h' jumps to helium.
//
// BOTTOM panel (successive IEs): bar chart of IE_n for the selected
// element on a logarithmic y axis, since IE spans roughly 5 eV
// (alkali outer electron) to >2000 eV (last electron of Mg). Bars
// grouped by electron shell with vertical separators showing the
// shell breaks; the giant inter-shell jumps are the visual payoff.
//
// Canonical equation (bottom-right HUD, per Betterfire Standard):
//
// IE_n = R_H * Z_eff^2 / n^2 (hydrogenic limit)
//
// where R_H = 13.6057 eV is the Rydberg unit of energy. The formula
// is *exact* for one-electron ions, including He+ (Z=2, n=1) whose
// IE = 13.606 * 4 = 54.42 eV is the highest IE of any species in the
// periodic table. For neutral atoms IE1 deviates from this hydrogenic
// prediction by inner-shell screening (Slater's rules) and electron-
// electron repulsion, but the *scale* and the *shell structure* are
// set by R_H and Z_eff.
//
// Visual layout (720 x 520 canvas):
// * top: HUD title + en.wikitube.io/wiki/Ionization_energy
// * y = 60-260: IE1 vs Z plot (top panel)
// * y = 290-480: successive IE_n bars (bottom panel, log axis)
// * bottom-left: live readout (element symbol, Z, IE1, IE2, ...)
// * bottom-right: canonical equation IE = R_H * Z^2 / n^2
//
// Conventions (Wikitube Betterfire Standard v0):
// * single ARTICLE constant at the top, single quotes
// * p5.disableFriendlyErrors = true (clean editor console)
// * non-ASCII (sigma, _n, ^2, arrows, accents) in COMMENTS ONLY;
// every text() string literal is ASCII
// * Energy-room palette (P5_JS_EDITOR section 4): dark BG, HOT/COLD
// tones, STRUCT grey, TRAJ yellow accent for the cursor
// * helium highlighted (TRAJ) because this is the Helium room
// =====================================================================
const ARTICLE = 'Ionization_energy';
const TITLE = ARTICLE.replace(/_/g, ' ');
p5.disableFriendlyErrors = true;
// ----- Energy room palette (P5_JS_EDITOR section 4) ------------------
const BG = 18;
const FG = 240;
const DIM = [240, 240, 240, 140];
const HOT = [220, 110, 60]; // noble-gas peaks (high IE)
const COLD = [60, 130, 220]; // alkali troughs (low IE)
const STRUCT = [120, 130, 150]; // grey structural / axis lines
const TRAJ = [240, 220, 80]; // selected element / cursor
const SCRATCH = [120, 120, 120, 80]; // grid / scratch lines
const SHELL = [180, 140, 220]; // shell-break separators
// ----- Constants (top of file per Betterfire Standard) ---------------
const R_H = 13.6057; // Rydberg energy in eV
const Z_MIN = 1;
const Z_MAX = 36; // H..Kr -- four full rows of the periodic table
// ----- Element table: { Z, sym, ie1 (eV), shellBreaks (list of n) } --
// IE1 values from NIST atomic spectra database (CODATA 2018).
// shellBreaks: the n at which a new shell begins for that element.
// He: 1s2 -> [] (no shell break within atom)
// Li: 1s2 2s1 -> [2] (jump after IE1: into 1s shell)
// Na: 1s2 2s2 2p6 3s1 -> [2, 10] (after IE1 and after IE9)
// Convention here: a "shellBreak at k" means an unusually large jump
// happens going from IE_k to IE_{k+1}. We use it to draw separators.
const ELEMENTS = [
{ Z: 1, sym: 'H', ie1: 13.598, shellBreaks: [] },
{ Z: 2, sym: 'He', ie1: 24.587, shellBreaks: [] },
{ Z: 3, sym: 'Li', ie1: 5.392, shellBreaks: [1] },
{ Z: 4, sym: 'Be', ie1: 9.323, shellBreaks: [2] },
{ Z: 5, sym: 'B', ie1: 8.298, shellBreaks: [3] },
{ Z: 6, sym: 'C', ie1: 11.260, shellBreaks: [4] },
{ Z: 7, sym: 'N', ie1: 14.534, shellBreaks: [5] },
{ Z: 8, sym: 'O', ie1: 13.618, shellBreaks: [6] },
{ Z: 9, sym: 'F', ie1: 17.423, shellBreaks: [7] },
{ Z: 10, sym: 'Ne', ie1: 21.565, shellBreaks: [8] },
{ Z: 11, sym: 'Na', ie1: 5.139, shellBreaks: [1, 9] },
{ Z: 12, sym: 'Mg', ie1: 7.646, shellBreaks: [2, 10] },
{ Z: 13, sym: 'Al', ie1: 5.986, shellBreaks: [3, 11] },
{ Z: 14, sym: 'Si', ie1: 8.152, shellBreaks: [4, 12] },
{ Z: 15, sym: 'P', ie1: 10.487, shellBreaks: [5, 13] },
{ Z: 16, sym: 'S', ie1: 10.360, shellBreaks: [6, 14] },
{ Z: 17, sym: 'Cl', ie1: 12.968, shellBreaks: [7, 15] },
{ Z: 18, sym: 'Ar', ie1: 15.760, shellBreaks: [8, 16] },
{ Z: 19, sym: 'K', ie1: 4.341, shellBreaks: [1, 9, 17] },
{ Z: 20, sym: 'Ca', ie1: 6.113, shellBreaks: [2, 10, 18] },
{ Z: 21, sym: 'Sc', ie1: 6.561, shellBreaks: [3, 11, 19] },
{ Z: 22, sym: 'Ti', ie1: 6.828, shellBreaks: [4, 12, 20] },
{ Z: 23, sym: 'V', ie1: 6.746, shellBreaks: [5, 13, 21] },
{ Z: 24, sym: 'Cr', ie1: 6.767, shellBreaks: [6, 14, 22] },
{ Z: 25, sym: 'Mn', ie1: 7.434, shellBreaks: [7, 15, 23] },
{ Z: 26, sym: 'Fe', ie1: 7.902, shellBreaks: [8, 16, 24] },
{ Z: 27, sym: 'Co', ie1: 7.881, shellBreaks: [9, 17, 25] },
{ Z: 28, sym: 'Ni', ie1: 7.640, shellBreaks: [10, 18, 26] },
{ Z: 29, sym: 'Cu', ie1: 7.726, shellBreaks: [11, 19, 27] },
{ Z: 30, sym: 'Zn', ie1: 9.394, shellBreaks: [12, 20, 28] },
{ Z: 31, sym: 'Ga', ie1: 5.999, shellBreaks: [3, 13, 21, 29] },
{ Z: 32, sym: 'Ge', ie1: 7.900, shellBreaks: [4, 14, 22, 30] },
{ Z: 33, sym: 'As', ie1: 9.815, shellBreaks: [5, 15, 23, 31] },
{ Z: 34, sym: 'Se', ie1: 9.752, shellBreaks: [6, 16, 24, 32] },
{ Z: 35, sym: 'Br', ie1: 11.814, shellBreaks: [7, 17, 25, 33] },
{ Z: 36, sym: 'Kr', ie1: 14.000, shellBreaks: [8, 18, 26, 34] }
];
// Noble gas atomic numbers -- drawn in HOT, sawtooth peaks
const NOBLE_GAS = new Set([2, 10, 18, 36]);
// Alkali metals -- drawn in COLD, sawtooth troughs
const ALKALI = new Set([3, 11, 19]);
// ----- Tabulated successive IE_n (eV) for small-Z atoms --------------
// Source: NIST Atomic Spectra Database (eV). Values truncated to 3 dp.
// Keyed by Z; array index i corresponds to IE_{i+1}.
const SUCCESSIVE_IE = {
1: [13.598],
2: [24.587, 54.418],
3: [5.392, 75.640, 122.454],
4: [9.323, 18.211, 153.897, 217.719],
5: [8.298, 25.155, 37.931, 259.375, 340.227],
6: [11.260, 24.383, 47.888, 64.494, 392.090, 489.993],
7: [14.534, 29.601, 47.449, 77.473, 97.890, 552.072, 667.046],
8: [13.618, 35.118, 54.936, 77.414, 113.899, 138.119, 739.290, 871.410],
9: [17.423, 34.971, 62.708, 87.140, 114.243, 157.165, 185.186, 953.911, 1103.117],
10: [21.565, 40.963, 63.450, 97.120, 126.210, 157.930, 207.276, 239.099, 1195.808, 1362.199],
11: [5.139, 47.286, 71.620, 98.910, 138.400, 172.180, 208.500, 264.250, 299.864, 1465.121, 1648.702],
12: [7.646, 15.035, 80.144, 109.265, 141.270, 186.760, 225.020, 265.960, 328.060, 367.499, 1761.805, 1962.665],
13: [5.986, 18.829, 28.448, 119.992, 153.825, 190.490, 241.760, 284.660, 330.130, 398.750, 442.005, 2085.980, 2304.141],
14: [8.152, 16.346, 33.493, 45.142, 166.767, 205.270, 246.500, 303.540, 351.120, 401.380, 476.360, 523.420, 2437.630, 2673.182],
15: [10.487, 19.769, 30.203, 51.444, 65.025, 220.421, 263.570, 309.600, 372.130, 424.400, 479.460, 560.800, 611.740, 2816.910, 3069.842],
16: [10.360, 23.338, 34.790, 47.222, 72.594, 88.053, 280.948, 328.750, 379.550, 447.500, 504.800, 564.440, 652.200, 707.010, 3223.780, 3494.189],
17: [12.968, 23.814, 39.610, 53.465, 67.821, 97.030, 114.196, 348.280, 400.060, 455.630, 529.280, 591.990, 656.710, 749.760, 809.398, 3658.521, 3946.296],
18: [15.760, 27.629, 40.735, 59.580, 74.840, 91.290, 124.410, 143.456, 422.450, 478.690, 538.960, 618.260, 686.100, 755.740, 854.770, 918.030, 4120.886, 4426.224]
};
// ----- Reader's cursor: selected atomic number -----------------------
let zSel = 2; // start at Helium (the room's pin)
// ----- Layout (pixels; set in setup()) -------------------------------
let topX, topY, topW, topH;
let botX, botY, botW, botH;
// ----- IE1 axis limits (top panel y axis) ----------------------------
const IE1_MIN = 0;
const IE1_MAX = 26; // accommodates He at 24.587 with headroom
// ----- IE_n log axis limits (bottom panel y axis) --------------------
const LOG_IE_MIN = Math.log10(3); // ~ 0.477 (covers 5 eV alkali)
const LOG_IE_MAX = Math.log10(5000); // ~ 3.699 (covers Z=18 last shell)
function setup() {
// Fixed canvas per Betterfire Standard -- editor preview stable
createCanvas(720, 520);
pixelDensity(2);
textFont('system-ui');
// Top panel: IE1 vs Z
topX = 60;
topY = 60;
topW = width - 80; // 640 px wide
topH = 200;
// Bottom panel: successive IE_n bars (log axis)
botX = 60;
botY = 290;
botW = width - 80;
botH = 190;
}
function draw() {
background(BG);
drawTopPanel(); // IE1 vs Z sawtooth
drawBottomPanel(); // successive IE_n bar chart
drawHUD(); // title, equation, readout
}
// =====================================================================
// Coordinate transforms
// =====================================================================
function zToPx(z) {
return map(z, Z_MIN, Z_MAX, topX, topX + topW);
}
function ie1ToPx(ie1) {
return map(ie1, IE1_MIN, IE1_MAX, topY + topH, topY);
}
function nToBarX(n, total) {
// Bottom panel: bar n (1..total) maps to x range botX..botX+botW
const slotW = botW / total;
return botX + (n - 0.5) * slotW;
}
function logIEToPx(logIE) {
return map(logIE, LOG_IE_MIN, LOG_IE_MAX, botY + botH - 10, botY + 10);
}
// =====================================================================
// Top panel: IE1 vs Z sawtooth
// =====================================================================
function drawTopPanel() {
push();
// Frame + title
noFill();
stroke(STRUCT);
strokeWeight(1);
rect(topX, topY, topW, topH);
noStroke();
fill(...DIM);
textSize(11);
textAlign(LEFT, BOTTOM);
text('IE1 (eV) vs atomic number Z -- periodic sawtooth, He at peak', topX + 4, topY - 4);
// Y axis ticks every 5 eV
textAlign(RIGHT, CENTER);
textSize(10);
for (let ie = 0; ie <= IE1_MAX; ie += 5) {
const y = ie1ToPx(ie);
stroke(...SCRATCH);
strokeWeight(1);
line(topX, y, topX + topW, y);
noStroke();
fill(...DIM);
text(ie, topX - 6, y);
}
// X axis ticks at noble gases (peak markers)
textAlign(CENTER, TOP);
for (const z of [2, 10, 18, 36]) {
const x = zToPx(z);
stroke(...HOT, 100);
strokeWeight(1);
line(x, topY, x, topY + topH);
}
// The sawtooth line itself
stroke(STRUCT);
strokeWeight(1.5);
noFill();
beginShape();
for (const e of ELEMENTS) {
vertex(zToPx(e.Z), ie1ToPx(e.ie1));
}
endShape();
// Per-element dots
for (const e of ELEMENTS) {
const x = zToPx(e.Z);
const y = ie1ToPx(e.ie1);
let col;
if (e.Z === zSel) col = TRAJ;
else if (NOBLE_GAS.has(e.Z)) col = HOT;
else if (ALKALI.has(e.Z)) col = COLD;
else col = STRUCT;
noStroke();
fill(...col);
const r = (e.Z === zSel) ? 7 : 4;
circle(x, y, r);
// Label noble gases and alkali metals + the selected element
if (NOBLE_GAS.has(e.Z) || ALKALI.has(e.Z) || e.Z === zSel) {
fill(...DIM);
textSize(10);
textAlign(CENTER, BOTTOM);
text(e.sym, x, y - 8);
}
}
// X axis Z labels along bottom edge of plot
noStroke();
fill(...DIM);
textSize(9);
textAlign(CENTER, TOP);
for (let z = Z_MIN; z <= Z_MAX; z += 5) {
text(z, zToPx(z), topY + topH + 3);
}
textSize(10);
fill(...DIM);
textAlign(CENTER, TOP);
text('Z (atomic number)', topX + topW / 2, topY + topH + 16);
pop();
}
// =====================================================================
// Bottom panel: successive IE_n for the selected element
// =====================================================================
function drawBottomPanel() {
push();
// Frame
noFill();
stroke(STRUCT);
strokeWeight(1);
rect(botX, botY, botW, botH);
// Title
noStroke();
fill(...DIM);
textSize(11);
textAlign(LEFT, BOTTOM);
const elem = ELEMENTS[zSel - 1];
text(
'Successive IE_n for ' + elem.sym +
' (Z=' + elem.Z + ') -- log y axis, shell jumps visible',
botX + 4, botY - 4
);
// Y axis log ticks at decades: 10, 100, 1000
textAlign(RIGHT, CENTER);
textSize(10);
for (const tick of [10, 100, 1000]) {
const logT = Math.log10(tick);
if (logT < LOG_IE_MIN || logT > LOG_IE_MAX) continue;
const y = logIEToPx(logT);
stroke(...SCRATCH);
strokeWeight(1);
line(botX, y, botX + botW, y);
noStroke();
fill(...DIM);
text(tick, botX - 6, y);
}
// Pull the successive-IE list. If we don't have data, show a note.
const ieList = SUCCESSIVE_IE[zSel];
if (!ieList) {
fill(...STRUCT);
textAlign(CENTER, CENTER);
textSize(13);
noStroke();
text(
'IE_n table not tabulated for ' + elem.sym +
' in this microsim (Z > 18).',
botX + botW / 2, botY + botH / 2
);
textSize(11);
text(
'IE1 = ' + nf(elem.ie1, 1, 3) + ' eV. Click Z <= 18 above to explore shells.',
botX + botW / 2, botY + botH / 2 + 22
);
pop();
return;
}
const total = ieList.length;
const slotW = botW / total;
// Vertical shell-break separators behind the bars
for (const k of elem.shellBreaks) {
if (k < 1 || k >= total) continue;
const xBreak = botX + k * slotW;
stroke(...SHELL, 180);
strokeWeight(1);
line(xBreak, botY + 4, xBreak, botY + botH - 18);
// Annotation
noStroke();
fill(...SHELL, 200);
textSize(9);
textAlign(CENTER, TOP);
text('shell', xBreak, botY + 4);
}
// Bars
for (let i = 0; i < total; i++) {
const ie = ieList[i];
const cx = botX + (i + 0.5) * slotW;
const yTop = logIEToPx(Math.log10(ie));
const yBot = logIEToPx(LOG_IE_MIN);
const barW = Math.max(slotW * 0.7, 4);
// Colour: HOT for inner-shell electrons (big IE), COLD for outer
let col;
if (ie < 30) col = COLD;
else if (ie < 200) col = STRUCT;
else col = HOT;
noStroke();
fill(...col, 200);
rect(cx - barW / 2, yTop, barW, yBot - yTop);
// n label below the bar
fill(...DIM);
textSize(9);
textAlign(CENTER, TOP);
text(i + 1, cx, botY + botH - 14);
// Value label above the bar -- only when bars are wide enough
if (slotW > 26) {
fill(FG);
textSize(8);
textAlign(CENTER, BOTTOM);
text(nf(ie, 1, 0), cx, yTop - 1);
}
}
// X axis title
noStroke();
fill(...DIM);
textSize(10);
textAlign(CENTER, TOP);
text('ionization step n (IE_n)', botX + botW / 2, botY + botH - 1);
pop();
}
// =====================================================================
// HUD: title, subtitle, equation, readout, hints
// =====================================================================
function drawHUD() {
push();
// Top-left: article title (bright)
noStroke();
fill(FG);
textSize(22);
textAlign(LEFT, TOP);
text(TITLE, 14, 14);
// Top-left: wikitube subtitle (dim)
fill(...DIM);
textSize(12);
text('Wikitube microsim . en.wikitube.io/wiki/' + ARTICLE, 14, 40);
// Top-right: input hints
textSize(10);
textAlign(RIGHT, TOP);
fill(...DIM);
text('click a Z above . arrow keys nudge . h = helium', width - 14, 14);
text('peaks (HOT) = noble gases . troughs (COLD) = alkali metals', width - 14, 30);
// Bottom-left readout: selected element vital statistics
const elem = ELEMENTS[zSel - 1];
textAlign(LEFT, BOTTOM);
textSize(11);
fill(...TRAJ);
text(
'selected: ' + elem.sym + ' Z = ' + elem.Z +
' IE1 = ' + nf(elem.ie1, 1, 3) + ' eV',
14, height - 22
);
fill(...DIM);
textSize(10);
const ieList = SUCCESSIVE_IE[zSel];
if (ieList && ieList.length >= 2) {
const ratio = ieList[1] / ieList[0];
text(
'IE2 = ' + nf(ieList[1], 1, 2) + ' eV . IE2/IE1 = ' +
nf(ratio, 1, 2) + 'x',
14, height - 6
);
} else if (ieList && ieList.length === 1) {
text('only one electron -- no IE2 (hydrogenic ground state).',
14, height - 6);
} else {
text('IE_n table not loaded for Z > 18.', 14, height - 6);
}
// Bottom-right: canonical equation (ASCII only)
textAlign(RIGHT, BOTTOM);
textSize(11);
fill(...DIM);
text('IE_n = R_H * Z^2 / n^2 R_H = 13.6057 eV',
width - 14, height - 6);
pop();
}
// =====================================================================
// Input handlers
// =====================================================================
function mousePressed() {
// Click anywhere inside the top panel to snap zSel to nearest Z.
if (mouseX >= topX && mouseX <= topX + topW &&
mouseY >= topY && mouseY <= topY + topH) {
const zRaw = map(mouseX, topX, topX + topW, Z_MIN, Z_MAX);
zSel = constrain(Math.round(zRaw), Z_MIN, Z_MAX);
}
}
function keyPressed() {
if (keyCode === LEFT_ARROW) {
zSel = constrain(zSel - 1, Z_MIN, Z_MAX);
} else if (keyCode === RIGHT_ARROW) {
zSel = constrain(zSel + 1, Z_MIN, Z_MAX);
} else if (key === 'h' || key === 'H') {
zSel = 2; // helium
} else if (key === 'n' || key === 'N') {
zSel = 10; // neon
} else if (key === 'a' || key === 'A') {
zSel = 18; // argon
} else if (key === 'l' || key === 'L') {
zSel = 3; // lithium (canonical alkali contrast)
}
}
```
## Links (Wikipedia order)
<!-- injected from _registry/childlinks/Ionization_energy.json (2026-07-30T02:09:12Z) -->
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## From the Real GENERATIVE library
*Ionization energy — placed from the Real G.E.N.E.R.A.T.I.V.E. course library (Nuclear room). Source: Wikimedia Commons (via Wikipedia article media). [Details & license](https://commons.wikimedia.org/wiki/File:First_Ionization_Energy_blocks.svg).*
> In physics and chemistry, ionization energy (IE) is the minimum energy required to remove the most loosely bound electron of an isolated gaseous atom, positive ion, or molecule.[1] The first ionization energy is quantitatively expressed as ([Wikipedia](https://en.wikipedia.org/wiki/Ionization_energy))
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## From the vault media library
!Ionization energy thumb.png
*Ionization Energy — from the vault's own media holdings, placed 2026-07-09. MTN / Wikitube.io original · CC BY-SA 4.0.*
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> **Room:** [[Helium]] · **Status:** ✅ shipped
## Overview
**Ionization energy** is the minimum [[Energy|energy]] required to remove the most loosely bound [[Electron|electron]] from a neutral atom, molecule, or ion in its ground state, producing a positive ion and a free electron at rest. It is conventionally expressed in electron volts (eV) per atom or kilojoules per mole, denoted IE1 for the first ionization, with successive removals labeled IE2, IE3, and so on. The first ionization energy follows a characteristic periodic trend: it rises across each row of the periodic table as effective nuclear charge climbs, drops sharply at the start of each new shell, and shows local sawtooth [[Structure|structure]] from the extra stability of half-filled and filled subshells. Helium has the highest first ionization energy of any neutral element — 24.587 eV — a consequence of its compact 1s2 configuration, with the full nuclear charge of 2 felt by each electron and no inner-shell shielding. Its second ionization energy, 54.418 eV, is also the highest of any element, since removing the second electron exposes the bare nucleus and obeys the hydrogenic Rydberg formula exactly. Successive ionization energies of any atom show enormous jumps when a closed shell is breached — IE2 over IE1 for sodium is roughly tenfold — the canonical evidence for the shell model. Koopmans' approximation equates IE1 with the negative of the highest occupied molecular orbital energy from Hartree-Fock theory, anchoring photoelectron spectroscopy. Ionization energy underwrites mass spectrometry, plasma [[Physics|physics]], ionization-chamber radiation detection, photoelectron spectroscopy, helium discharge lamps, and the noble-gas inertness that makes helium an unrivaled [[Shielding_gas|shielding gas]], [[Lifting_gas|lifting gas]], and inert pressurant.
## See also
- Room hub: [[Helium]]
- p5.js Editor conventions: P5 JS EDITOR
- Wiki root: MAIN
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*Scaffolded by `generative-microsim` from row 159 of the Helium sheet on 2026-05-14T19:48:12Z.*
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*Built to the [[WT!P5_js_Microsim_Master_Class|p5.js Master Class]].*
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## Wikipedia : Wikitube
**Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Ionization_energy) : [Wikitube](https://en.wikitube.io/wiki/Ionization_energy)
## Previous hub tags
Tree parents: [[Helium]] · [[Hydrogen]] · [[Oxygen]].
Legacy hubs: none.
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*Sources: 1 legacy note. Minted wave 1, 2026-07-30 (v1.6 order).*