Why are the lanthanides and actinides at the bottom of the periodic table?
The lanthanides (elements 57–71) and actinides (elements 89–103) are printed in two rows below the main periodic table only to keep it a practical width. They really belong inside periods 6 and 7, right after barium (56) and radium (88). Putting them in place would stretch the table to 32 columns, too wide to fit comfortably on a page or screen.
Where they really belong
Follow the atomic numbers across period 6: cesium (55), barium (56), then a jump to hafnium (72). The 15 elements from lanthanum (57) to lutetium (71) fill that gap. Period 7 has the same gap between radium (88) and rutherfordium (104), filled by the 15 elements from actinium (89) to lawrencium (103).
Many tables mark the spot in group 3 with a placeholder labeled 57–71 or 89–103. If you slid the two rows into place, periods 6 and 7 would each contain 32 elements, and the table would have 32 columns instead of 18. This wide layout is called the long-form or 32-column periodic table.
The f-block
The lanthanides and actinides make up the f-block. Across these rows, electrons are mainly added to an f subshell two shells below the outermost one: 4f for the lanthanides and 5f for the actinides. An f subshell holds 14 electrons, which is why the f-block is 14 columns wide.
Each row lists 15 elements, one more than the 14 f-block columns. That is because both lanthanum and lutetium count as lanthanides, and both actinium and lawrencium as actinides, while only one of each pair really belongs in group 3. Chemists still debate which pair that should be.
Because the added electrons sit so deep inside the atom, the outermost electrons barely change across each row. Every lanthanide ends in 6s²; neodymium, for example, is [Xe] 4f⁴ 6s². That is why the lanthanides are so similar to one another chemically and so hard to separate.
The lanthanides
The lanthanides, together with scandium and yttrium, are often called the rare earth elements. Despite the name, most are not especially rare in Earth’s crust; they are simply spread out and difficult to separate from one another.
They are mostly soft, silvery metals that typically form +3 ions. Many are vital to modern technology: neodymium is used in strong permanent magnets for motors, headphones and wind turbines, samarium in heat-resistant samarium–cobalt magnets, europium in the phosphors of screens and lamps, and gadolinium in contrast agents for MRI scans. Across the series the atoms shrink slightly, from 240 pm for lanthanum to 221 pm for lutetium in this site’s data, an effect called the lanthanide contraction.
The actinides
All actinides are radioactive. Only thorium and uranium occur in large amounts in nature. Actinium and protactinium exist in tiny traces in uranium and thorium ores, and so do traces of neptunium and plutonium, but almost all of the elements after uranium are made in nuclear reactors or particle accelerators.
Uranium and plutonium are the fuels of nuclear power, and americium-241 is used in many household smoke detectors. Actinides show a wider range of charges than lanthanides: uranium, for example, can be +3, +4, +5 or +6. The heaviest actinides, such as nobelium and lawrencium, have only ever been made in tiny amounts.
Reading them on the table
When you look up a lanthanide or actinide, remember that its period is 6 or 7 even though it is printed in a lower row. Neodymium (60) is in period 6, and uranium (92) is in period 7. Many tables leave the group number blank for these elements, because the f-block columns are not among the 18 numbered groups.
A quick way to remember the ranges: the lanthanide row starts right after barium (56) and the actinide row right after radium (88), the two group 2 metals at the bottom of their column.
Key points
- The lanthanides (57–71) and actinides (89–103) belong in periods 6 and 7, right after barium and radium.
- They are printed below the main table so it stays 18 columns wide instead of 32.
- They make up the f-block, where electrons fill the 4f and 5f subshells.
- Lanthanides are chemically similar to one another and usually form +3 ions; all actinides are radioactive.
- Lanthanides such as neodymium are key to strong magnets, and uranium and plutonium fuel nuclear reactors.
Check yourself
Three quick questions on this lesson.
1. Where do the lanthanides really belong on the periodic table?
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In period 6, right after barium — The lanthanides (57–71) fill the gap in period 6 between barium (56) and hafnium (72).2. Why are the lanthanides so similar to one another chemically?
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Their added electrons go into the deep 4f subshell, so their outer electrons barely change — Across the row, electrons fill the 4f subshell deep inside the atom, so every lanthanide ends in 6s² and reacts much like its neighbors.3. Which statement is true of all the actinides?