Chemistry Concepts and Applications

Chapter 8: Periodic Properties of the Elements

Problem of the Week

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Extinction of the Dinosaurs

Hypotheses abound concerning the environmental upheaval that caused the final Cretaceous extinction 65 million years ago. In 1980, Nobel prize winner Luis Alvarez and his colleagues published a paper in Science magazine that proposed the extinction of the dinosaurs and many other creatures was due to the impact of an asteroid or comet with Earth. The element iridium was used as evidence to support this proposal.

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Iridium, the most corrosion-resistant metal known, is rarely found in Earth’s crust. But, it is abundant in Earth’s core and in meteorites and asteroids. Large amounts of iridium have been found in a clay layer that marks the end of Cretaceous-period rock and the beginning of Tertiary rock (the K-T boundary). Researchers have found up to 100 times the amount of iridium in this clay layer than is normally found in the soils around the world.

The Alvarez hypothesis of the final K-T extinction proposes that a 10- to 20-km diameter asteroid collided with Earth and released large quantities of vaporized material (including iridium) into the atmosphere. This material was distributed globally and blocked out the Sun for months causing "impact winter". Chemical reactions between substances in the atmosphere and these pollutants caused the formation of acid rain.

A crater estimated to be 160 to 240 km wide was discovered off the tip of the Yucatan Peninsula in the Gulf of Mexico. Much of the Chicxulub crater lies under the ocean and is hidden under 65 million years of sediment. Research is on-going as is the debate over whether the asteroid that formed this crater could have caused mass extinction.

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Locate iridium on the periodic table and identify the group of elements it belongs to.

 
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