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The actinides

The actinides: a chapter of The Periodic Table Book, with each element's symbol, number and story.

16 cards · Science

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Each Zap teaches up to 5 new cards, so you learn the chapter a few at a time.

Checked by the Brain Zap team · reviewed 27 Oct 2026

Sources: Periodic Table of Elements (IUPAC), Periodic Table (Royal Society of Chemistry)

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  • Symbol: Ac

    Show answer: Symbol: Ac

    Actinium

    Learn more: It is so radioactive that it glows a faint blue in the dark.

  • Symbol: Th

    Show answer: Symbol: Th

    Thorium

    Learn more: Named after Thor; it could fuel a new kind of nuclear reactor.

  • Symbol: Pa

    Show answer: Symbol: Pa

    Protactinium

    Learn more: It decays into actinium, hence its name, and it is one of the rarest natural elements.

  • Element 92

    Show answer: Element 92

    Uranium

    Learn more: Named after the planet Uranus; it is the heaviest element found in nature in large amounts.

  • Symbol: Np

    Show answer: Symbol: Np

    Neptunium

    Learn more: It comes after uranium, just as Neptune comes after Uranus in the Solar System.

About this topic

The actinides, elements 89 to 103, sit in the second row below the main table, and every one of them is radioactive. Uranium fuels nuclear power, plutonium powers some spacecraft and americium sits in many smoke alarms. After uranium, the elements were made in reactors and particle accelerators, and their names tell the story: the planets beyond Uranus, the cities and states of the labs that made them, and great scientists.

This is a chapter of The Periodic Table Book. Each card shows a symbol or an atomic number and asks for the element's name, with a hint before you answer and a fact after.

All 16 cards

  1. Symbol: Ac

    Actinium

    Learn more: It is so radioactive that it glows a faint blue in the dark.

  2. Symbol: Th

    Thorium

    Learn more: Named after Thor; it could fuel a new kind of nuclear reactor.

  3. Symbol: Pa

    Protactinium

    Learn more: It decays into actinium, hence its name, and it is one of the rarest natural elements.

  4. Element 92

    Uranium

    Learn more: Named after the planet Uranus; it is the heaviest element found in nature in large amounts.

  5. Symbol: Np

    Neptunium

    Learn more: It comes after uranium, just as Neptune comes after Uranus in the Solar System.

  6. Symbol: Pu

    Plutonium

    Learn more: After uranium and neptunium came plutonium, named after Pluto in 1941; it powers some spacecraft.

  7. Symbol: Am

    Americium

    Learn more: A tiny speck of it in a smoke detector charges the air, and smoke interrupts the current.

  8. Symbol: Cm

    Curium

    Learn more: Named in 1944 after Marie and Pierre Curie; rovers on Mars used it to study rocks.

  9. Symbol: Bk

    Berkelium

    Learn more: Made in 1949 at the University of California, Berkeley, which named it after its home city.

  10. Symbol: Cf

    Californium

    Learn more: Made in 1950 in California; it gives off neutrons used to start up nuclear reactors.

  11. Symbol: Es

    Einsteinium

    Learn more: Found in the fallout of the first hydrogen bomb test in 1952, and named after Albert Einstein.

  12. Element 100

    Fermium

    Learn more: Named after Enrico Fermi, who built the first nuclear reactor.

  13. Symbol: Md

    Mendelevium

    Learn more: Named after Dmitri Mendeleev, who arranged the periodic table in 1869.

  14. Symbol: No

    Nobelium

    Learn more: Named after Alfred Nobel, the inventor of dynamite who founded the Nobel Prizes.

  15. Symbol: Lr

    Lawrencium

    Learn more: Named after Ernest Lawrence, whose particle accelerator made many new elements possible.

  16. Radioactive series from element 89 to 103

    Actinides

    Learn more: All are radioactive; the ones after uranium are made in reactors and accelerators.

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