First Ionization Energies of the Elements
| Element | Symbol | Z | First IE (kJ/mol) | Group | Period |
|---|---|---|---|---|---|
| Hydrogen | H | 1 | 1312 | 1 | 1 |
| Helium | He | 2 | 2372 | 18 | 1 |
| Lithium | Li | 3 | 520 | 1 | 2 |
| Beryllium | Be | 4 | 900 | 2 | 2 |
| Boron | B | 5 | 801 | 13 | 2 |
| Carbon | C | 6 | 1086 | 14 | 2 |
| Nitrogen | N | 7 | 1402 | 15 | 2 |
| Oxygen | O | 8 | 1314 | 16 | 2 |
| Fluorine | F | 9 | 1681 | 17 | 2 |
| Neon | Ne | 10 | 2081 | 18 | 2 |
| Sodium | Na | 11 | 496 | 1 | 3 |
| Magnesium | Mg | 12 | 738 | 2 | 3 |
| Aluminum | Al | 13 | 578 | 13 | 3 |
| Silicon | Si | 14 | 786 | 14 | 3 |
| Phosphorus | P | 15 | 1012 | 15 | 3 |
| Sulfur | S | 16 | 1000 | 16 | 3 |
| Chlorine | Cl | 17 | 1251 | 17 | 3 |
| Argon | Ar | 18 | 1521 | 18 | 3 |
| Potassium | K | 19 | 419 | 1 | 4 |
| Calcium | Ca | 20 | 590 | 2 | 4 |
| Scandium | Sc | 21 | 633 | 3 | 4 |
| Titanium | Ti | 22 | 659 | 4 | 4 |
| Vanadium | V | 23 | 651 | 5 | 4 |
| Chromium | Cr | 24 | 653 | 6 | 4 |
| Manganese | Mn | 25 | 717 | 7 | 4 |
| Iron | Fe | 26 | 762 | 8 | 4 |
| Cobalt | Co | 27 | 760 | 9 | 4 |
| Nickel | Ni | 28 | 737 | 10 | 4 |
| Copper | Cu | 29 | 745 | 11 | 4 |
| Zinc | Zn | 30 | 906 | 12 | 4 |
| Gallium | Ga | 31 | 579 | 13 | 4 |
| Germanium | Ge | 32 | 762 | 14 | 4 |
| Arsenic | As | 33 | 947 | 15 | 4 |
| Selenium | Se | 34 | 941 | 16 | 4 |
| Bromine | Br | 35 | 1140 | 17 | 4 |
| Krypton | Kr | 36 | 1351 | 18 | 4 |
| Rubidium | Rb | 37 | 403 | 1 | 5 |
| Cesium | Cs | 55 | 376 | 1 | 6 |
| Barium | Ba | 56 | 503 | 2 | 6 |
| Francium | Fr | 87 | 380 | 1 | 7 |
Values are first ionization energies (IE₁) for the process X(g) → X⁺(g) + e⁻, in kJ/mol, taken from the NIST Atomic Spectra Database and cross-checked against the CRC Handbook (97th ed.). Two structural exceptions to the left-to-right trend: (1) B (801) < Be (900) because removing a 2p electron is easier than a 2s; (2) O (1314) < N (1402) because pairing the fourth 2p electron of O introduces exchange-energy loss and Coulomb repulsion. The same effects recur in periods 3 and 4 (Al < Mg, S < P, Ga < Zn). Transition-metal IEs are nearly flat across the d-block because the added 3d electrons screen the added protons.
Frequently Asked Questions
Why does ionization energy generally increase across a period?
Each step rightward adds a proton and an electron to the same valence shell. Same-shell electrons screen each other poorly (Slater's rules give about 0.35 per electron), so the extra proton wins: effective nuclear charge Z_eff climbs across the period. The valence electron sits in a deeper potential well, atomic radius shrinks, and pulling that electron off costs more energy. Across period 2, IE rises from Li (520) to Ne (2081) — a 4× jump driven almost entirely by Z_eff.
Why is the ionization energy of boron less than beryllium?
Beryllium (900) holds its electron tighter than boron (801) because Be's outermost electron is a filled 2s², while B's is a single 2p¹. The 2p orbital sits higher in energy than 2s and is partially shielded by the underlying 2s² pair, so the 2p electron is both farther out and effectively screened. The same s-versus-p offset reappears at Mg–Al (738 → 578) and Zn–Ga (906 → 579), confirming this is a generic subshell effect, not a one-off.
What information can successive ionization energies reveal about electron configuration?
A sudden jump in successive IE values flags the transition from valence to core electrons. Sodium's IE₁ is 496 kJ/mol (3s), but IE₂ jumps to 4562 kJ/mol because the next electron comes from the filled 2p core. The 9× leap tells you Na has exactly one valence electron. Magnesium shows two relatively low IEs then a big jump at IE₃; aluminum shows three then a jump at IE₄. The position of the discontinuity equals the group number for main-group elements.