Electron Configuration Chart of the Elements

How to read the table

• Each cell shows the atomic number, the symbol and the noble gas configuration. The number after a subshell is the number of electrons in it, so 3d6 means six electrons in the 3d subshell.
• Subshells are written in order of shell number first, so 3d comes before 4s, the same order the NIST database uses. Many textbooks write 4s before 3d instead. Both describe the same atom.
• The colors give the block: s (groups 1 and 2, plus helium), p (groups 13 to 18), d (groups 3 to 12) and f (the two rows of 14 below the table). The block follows the position in the table, with lutetium and lawrencium in group 3.

The filling order and its 20 exceptions

Most elements follow the filling order: subshells fill by increasing n + l, and by lower n when n + l is equal, which gives 1s, 2s, 2p, 3s, 3p, 4s, 3d and so on up to 7p. Twenty elements do not. They are chromium, copper, niobium, molybdenum, ruthenium, rhodium, palladium, silver, lanthanum, cerium, gadolinium, platinum, gold, actinium, thorium, protactinium, uranium, neptunium, curium and lawrencium. In each of them the measured ground state has an electron in a different subshell than the rule predicts, usually because two subshells are very close in energy, as 3d and 4s are for chromium and copper. They are marked with a red corner in the table, and the Exceptions view in the list tab and the detail card show the expected value next to the real one. Treat the rule as a good guide, not a law.

How the valence electrons are counted

Valence electrons have no single definition, so this chart states its rule. For hydrogen, helium and the main groups (1, 2 and 13 to 18) it counts the s and p electrons in the outermost shell. For groups 3 to 12 it counts the ns and (n-1)d electrons, which is why iron shows 8 and zinc shows 12, even though zinc usually acts as if it had 2. For the lanthanides and actinides it counts every electron outside the noble gas core, f electrons included, and marks the number with an asterisk because most of them use only 2 to 3 electrons in bonding. Your course may use a different convention, so check it before copying a number into homework.

Where the data comes from, and what is predicted

Configurations for elements 1 to 103 are the ground configurations in the NIST Atomic Spectra Database, checked against the Wikipedia data page for the elements, and the two sources agree. Elements 104 to 118 are shown as predicted. Rutherfordium to hassium appear in NIST with uncertain or theoretical values, and elements 109 to 118 follow relativistic calculations of the kind published by Fricke and by Hoffman, Lee and Pershina. Older tables sometimes give 6d9 7s1 for darmstadtium or 6d10 7s1 for roentgenium by analogy with platinum and gold, while the calculations favor 6d8 7s2 and 6d9 7s2. NIST lists lawrencium as 7s2 7p1, Wikipedia marks that assignment as tentative, and many textbooks still write 6d1 7s2.