First ionisation energy of the elements

The first ionisation energy is the energy needed to remove the most loosely held electron from a neutral atom in the gas phase. It is in electronvolts per atom; multiply by 96.485 for kJ/mol, which the ranked list gives alongside. The values are PubChem’s, published by the US National Institutes of Health, except technetium’s, antimony’s, promethium’s, tantalum’s, tungsten’s, rhenium’s, osmium’s, iridium’s, platinum’s, bismuth’s, astatine’s, radon’s, francium’s, actinium’s, thorium’s, plutonium’s, americium’s, curium’s, berkelium’s, californium’s and einsteinium’s, which the caveats explain. It gives one for 102 of the 118 elements; the rest are hatched in the table below rather than coloured as if they were zero.

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H 13.6, hydrogen He 24.59, helium Li 5.392, lithium Be 9.323, beryllium B 8.298, boron C 11.26, carbon N 14.53, nitrogen O 13.62, oxygen F 17.42, fluorine Ne 21.57, neon Na 5.139, sodium Mg 7.646, magnesium Al 5.986, aluminium Si 8.152, silicon P 10.49, phosphorus S 10.36, sulfur Cl 12.97, chlorine Ar 15.76, argon K 4.341, potassium Ca 6.113, calcium Sc 6.561, scandium Ti 6.828, titanium V 6.746, vanadium Cr 6.767, chromium Mn 7.434, manganese Fe 7.902, iron Co 7.881, cobalt Ni 7.64, nickel Cu 7.726, copper Zn 9.394, zinc Ga 5.999, gallium Ge 7.9, germanium As 9.815, arsenic Se 9.752, selenium Br 11.81, bromine Kr 14, krypton Rb 4.177, rubidium Sr 5.695, strontium Y 6.217, yttrium Zr 6.634, zirconium Nb 6.759, niobium Mo 7.092, molybdenum Tc 7.119, technetium Ru 7.361, ruthenium Rh 7.459, rhodium Pd 8.337, palladium Ag 7.576, silver Cd 8.994, cadmium In 5.786, indium Sn 7.344, tin Sb 8.608, antimony Te 9.01, tellurium I 10.45, iodine Xe 12.13, xenon Cs 3.894, caesium Ba 5.212, barium La 5.577, lanthanum Ce 5.539, cerium Pr 5.464, praseodymium Nd 5.525, neodymium Pm 5.582, promethium Sm 5.644, samarium Eu 5.67, europium Gd 6.15, gadolinium Tb 5.864, terbium Dy 5.939, dysprosium Ho 6.022, holmium Er 6.108, erbium Tm 6.184, thulium Yb 6.254, ytterbium Lu 5.426, lutetium Hf 6.825, hafnium Ta 7.55, tantalum W 7.864, tungsten Re 7.834, rhenium Os 8.438, osmium Ir 8.967, iridium Pt 8.959, platinum Au 9.226, gold Hg 10.44, mercury Tl 6.108, thallium Pb 7.417, lead Bi 7.286, bismuth Po 8.417, polonium At 9.318, astatine Rn 10.75, radon Fr 4.073, francium Ra 5.279, radium Ac 5.38, actinium Th 6.307, thorium Pa 5.89, protactinium U 6.194, uranium Np 6.266, neptunium Pu 6.026, plutonium Am 5.974, americium Cm 5.992, curium Bk 6.198, berkelium Cf 6.282, californium Es 6.368, einsteinium Fm 6.5, fermium Md 6.58, mendelevium No 6.65, nobelium Lr no value, lawrencium Rf no value, rutherfordium Db no value, dubnium Sg no value, seaborgium Bh no value, bohrium Hs no value, hassium Mt no value, meitnerium Ds no value, darmstadtium Rg no value, roentgenium Cn no value, copernicium Nh no value, nihonium Fl no value, flerovium Mc no value, moscovium Lv no value, livermorium Ts no value, tennessine Og no value, oganesson

How it changes across a period and down a group

Across a period it rises, for the same reason atoms contract: the nuclear charge grows by one at each step while the added electrons shield one another poorly. Every period from the first to the sixth ends on its hardest element to ionise, a noble gas, and helium, at 24.587 eV, is the hardest of all.

Down a group it falls. The outermost electron sits in a shell further out, behind more electrons, and is easier to pull away even though the nucleus holds more protons. Caesium, at 3.894 eV, has the lowest value in PubChem’s table.

The rise across a period has two regular breaks. Boron is easier to ionise than beryllium, 8.298 against 9.323 eV, and aluminium than magnesium, because the electron removed is the first in a p subshell, higher in energy and partly screened by the filled s subshell beneath it. Oxygen is easier to ionise than nitrogen, 13.618 against 14.534 eV, and sulfur than phosphorus, because the fourth p electron in each has to share an orbital with another, and the repulsion between the pair makes one of them easier to remove.

Further down the table the second break moves. Tellurium is not easier to ionise than antimony, and in period 6 the dip comes a step earlier, with bismuth below lead. In atoms this heavy, spin-orbit coupling splits the p subshell into a lower level that holds two electrons and a higher one that holds four. Lead’s two p electrons fill the lower level, so the extra stability comes at lead rather than at a half-filled subshell, and bismuth’s third p electron is the first in the higher level.

Extremes and exceptions

Highest and lowest

• Helium, 24.587 eV
• Neon, 21.565 eV
• Fluorine, 17.423 eV
• Caesium, 3.894 eV
• Francium, 4.0727 eV
• Rubidium, 4.177 eV

The three hardest and the three easiest elements to ionise, of those PubChem gives a value for.

Where the rise across a period breaks

• Beryllium to boron: 9.323 to 8.298 eV
• Nitrogen to oxygen: 14.534 to 13.618 eV
• Magnesium to aluminium: 7.646 to 5.986 eV
• Phosphorus to sulfur: 10.487 to 10.36 eV
• Zinc to gallium: 9.394 to 5.999 eV
• Arsenic to selenium: 9.815 to 9.752 eV
• Cadmium to indium: 8.994 to 5.786 eV
• Mercury to thallium: 10.438 to 6.108 eV
• Lead to bismuth: 7.417 to 7.2855 eV

Each pair where the p-block element is easier to ionise than the element before it. In group 13 (boron, aluminium, gallium, indium and thallium) the electron removed is the first in a new p subshell, above a filled s subshell. In group 16 (oxygen, sulfur and selenium) it comes from the one p orbital holding two electrons. Bismuth is the spin-orbit case described above.

Rising down a group instead of falling

• Aluminium to gallium: 5.986 to 5.999 eV
• Indium to thallium: 5.786 to 6.108 eV
• Tin to lead: 7.344 to 7.417 eV
• Caesium to francium: 3.894 to 4.0727 eV
• Barium to radium: 5.212 to 5.279 eV

Gallium follows the first row of the d-block, and thallium and lead follow the lanthanides too. The d and f electrons added before them shield the nucleus poorly, so their outer electrons are held more tightly than the trend down the group predicts. At the foot of groups 1 and 2, relativistic effects in the heaviest atoms contract and stabilise the outer s orbital, with the same result.

The 102 elements with a value, ranked

Equal values share a rank, marked with =.

No value in PubChem’s table, so not ranked: lawrencium, rutherfordium, dubnium, seaborgium, bohrium, hassium, meitnerium, darmstadtium, roentgenium, copernicium, nihonium, flerovium, moscovium, livermorium, tennessine and oganesson.

The values are facts and free to use; this table’s selection and presentation are © 2026 ScienceQuest.

Caveats

• These are first ionisation energies: the energy to remove one electron from a neutral atom. Removing a second always takes more, and the jump is largest once a noble-gas core is reached, which is how successive ionisation energies reveal the shells.
• Electronvolts per atom. For kilojoules per mole multiply by 96.485, the Faraday constant divided by 1000, which is what the kJ/mol column of the ranked list does.
• Lawrencium and every heavier element have no value in PubChem’s table.
• The figures for technetium, antimony, promethium, tantalum, tungsten, rhenium, osmium, iridium, platinum, bismuth, astatine, radon, francium, actinium, thorium, plutonium, americium, curium, berkelium, californium and einsteinium are from the NIST Atomic Spectra Database, in place of older values in PubChem’s table that later measurements have revised: PubChem gives tantalum 7.89 eV, where NIST’s measured figure is 7.5496 eV.

See also

• The interactive periodic table, with every element’s details
• The Faraday constant, which turns eV per atom into kJ/mol
• Photon energy calculator, for the light that could ionise an atom
• All the elements, each with its own page of data