Name that element! It's the world's most powerful element, fueling both nuclear weapons and power plants. Our mystery element is softer than steel, denser than lead, and has existed for more than 6.6 billion years—longer than Earth itself. Can you name it? For help, grab a periodic table and follow these eight clues. Then turn the page to test your "element IQ."

CLUE 1 A ROCK START

The element's silver-white metal is found nearly everywhere--soil, plants, water. But it's most abundant in rocks, where it combines with other elements to form crystals of naturally occurring chemicals called minerals, or ores. Its most common ore: a marblelike rock called pitchblende.

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CLUE 2 IN THE MIX

In nature it exists as a mixture of three different forms or isotopes. An atom of each isotope shares an equal number of positively charged protons, but a different number of neutrons (uncharged atomic particles). The isotope pictured at right was extracted from an ore and forged into a solid ball. A single atom has 143 neutrons.

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CLUE 3 GET A (HALF) LIFE

It has a half-life of 4.5 billion years. In other words, "If you looked at a chunk of the element today, 4.5 billion years from now, half of it would be gone," says chemist Al Sattelberger at the Los Alamos National Laboratory in New Mexico. That's because it's a radioactive element--an element with an unstable nucleus that decays over time. The reaction is described as nuclear because it occurs inside an atom's nucleus.

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CLUE 4 SHOCKING REACTION

It generates more than 16 percent of the world's electricity. That's because one of its most powerful isotopes yields massive amounts of heat energy, or radiation, under certain conditions. "A single pound can generate as much energy as 1,500 tons of coal," says Sattelberger.

Where does the energy come from? Atoms, the microscopic particles that make up everything in the universe, including elements. When the isotope's nucleus is smashed in two, energy is released. At the same time, it ejects two or three neutrons. They split apart other nearby nuclei, which in turn release more energy. The nuclear process is called a fission chain reaction. Using elaborate machinery, its energy can be harnessed to fuel nuclear power plants.

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CLUE 5 GLOW-IN-THE-DARK GLASSWARE

Over 200 years ago, it was added to glassware to give it a decorative green-yellow gleam. Known as Vaseline glass (it resembles petroleum jelly), it's the only transparent glass that glows beneath an ultraviolet light, a light source that emits energy waves at a higher frequency than visible light. Vaseline glass is mildly radioactive, a property shared by all of the actinide elements, but the mystery element was banned for use as a coloring agent in the 1940s.

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CLUE 6 HEAVY METAL

It's the heaviest element found in nature. In its metallic form, it's 19 times denser than water--a property that makes it useful in yacht keels and as counterweights in airplane rudders. Consider this: Elements are organized on the periodic table according to atomic number and weight, with the lightest element being hydrogen (H).

SCORE 10 POINTS IF YOU NAME THE ELEMENT WITH THE HELP OF SIX CLUES. STILL GUESSING? READ ON.

CLUE 7 DEADLY DEEDS

If a fission chain reaction is uncontrolled--meaning that millions of nuclei are split in less than a fraction of a second with no way to slow them down--it leads to a catastrophic explosion. The explosion creates a towering plume of smoke called a mushroom cloud. The mystery element was used to make the world's first atomic bomb, dropped on Hiroshima, Japan, during World War II. It killed at least 70,000 people.

GUESS THE ELEMENT. SCORE 5 POINTS. IF NOT. TRY YOUR LAST CLUE.

CLUE 8 OUT OF THIS WORLD

The mystery element was named after the third largest planet in the solar system. The giant gas planet has a distinctive green coloring, much like some of the minerals that contain the mystery element.

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ANATOMY OF AN ELEMENT

X--Atomic number

?--Element symbol--Element name

XXXXXXX--Average atomic mass

Key definitions

ATOMIC NUMBER equals the number of protons (positively-charged particles) in an atom's nucleus, or center. Every element contains a different number of protons, and hence has a unique atomic number.

ATOMIC MASS equals the number of protons plus the number of neutrons found in a single atom of an element. The weight of an atom resides in its nucleus, which houses both protons and neutrons.

The Periodic Table

[ILLUSTRATION OMITTED]

The periodic table is a systematic way to organize Earth's elements, which can't be broken down into simpler substances. Today, there are 112 known and named chemical elements. Elements are arranged according to their increasing atomic number. Hydrogen (H), for example, has an atomic number of 1 and is the lightest known natural element on Earth. Scientists have created elements with atomic numbers greater than 92-but they don't exist naturally.

IT'S YOUR CHOICE

Test your chemistry IQ! Answer the following questions about elements and the periodic table.