Periodic table with first ionization energies

What first ionization energy means

The first ionization energy is the energy needed to remove the most loosely held electron from one neutral atom in the gas phase: X(g) → X⁺(g) + e⁻. Values here are in electron volts per atom; to convert to the kilojoules per mole used in many textbooks, multiply by 96.485. Each cell shows the value in eV, and lawrencium and the heavier elements, with no value given, show a dash.

Removing a second electron always takes more energy than the first, and the jump is huge once the next electron would come from a full inner shell. Sodium loses one electron easily but almost never a second, which is why it forms Na⁺ and not Na²⁺.

The trend across periods and down groups

Ionization energy increases across a period. The nuclear charge grows while the outer electrons stay in the same shell, so they are held more tightly. Periods 2 to 6 each start with an alkali metal, which has the lowest value in its row, and end with a noble gas, which has the highest.

Ionization energy decreases down a group. Each new shell puts the outer electron farther from the nucleus and behind more inner electrons, so it is easier to remove. This is why the alkali metals react more violently with water the lower they are in group 1.

Exceptions to the trend

Two small dips break the rise across each period. Boron has a lower ionization energy than beryllium (8.298 eV against 9.323 eV) because its outer electron is in a 2p subshell, which is higher in energy and partly shielded by the 2s electrons. Oxygen is lower than nitrogen (13.618 eV against 14.534 eV) because its fourth 2p electron has to share an orbital, and the repulsion between the paired electrons makes one easier to remove. The same dips appear in period 3, from magnesium to aluminum and from phosphorus to sulfur.

Further down, elements that come after a filled d or f subshell hold their electrons more tightly than expected, because d and f electrons shield the nucleus poorly. Gallium is slightly harder to ionize than aluminum, and gold much harder than silver.

Worked example: sodium

Sodium's first ionization energy is 5.139 eV. Multiplying by 96.485 gives about 495.8 kJ/mol, the value found in most textbooks. Compare neon, the noble gas just before it, at 21.565 eV: removing an electron from neon means breaking into a full shell, which takes about four times as much energy.