EARTH'S ELEMENTS

Matter is composed of elements, substances that cannot be broken down chemically into simpler or more basic substances. To date, 117 elements have been discovered. Some of these, such as oxygen, silicon, aluminum, and iron, are abundant in Earth's makeup. Though it appears that Earth contains a limited number of naturally occurring elements, the number of compounds formed by the joining of one or more of these elements seems nearly limitless, especially when considering both natural and artificial compounds.

 During the first billion years of the existence of Earth, heat from three sources - meteorite impacts, gravity's compression of magma and other material, and the radioactive decay of some elements - caused melting in the interior. Elements separated into layers based on their density. Heavy elements such as iron and nickel concentrated nearer Earth's center lighter elements such as oxygen and silicon combined and formed surface rocks and minerals. Earth's elements can be displayed in a chart called the periodic table. The periodic table arranges the elements into groups (vertically) of elements sharing common physical and chemical characteristics and into periods (horizontally) based on the atomic configuration of elements. Each element is given a number and a distinctive 115.117. and 118 are reported to have been created experimentally but have not yet received permanent names.

ELEMENTS ESSENTIAL FOR LIFE ON EARTH

When scientists found evidence of simple living cells in rocks they have determined to be 3.5 billion years old, it gave them reason to wonder: How could life have begun on an irradiated, oxygen-starved Earth? In their laboratories, they duplicated Earth's primitive atmosphere. To simulate lightning and harsh sunlight, they seared gas mixtures with high voltage sparks and ultraviolet light—forces that make and break chemical bonds. In the resulting brew they found amino acids, the building blocks of life. Now researchers believe that the same process occurred on Earth 3 to 4 billion years ago. Another experiment showed that amino acids on a hot, dry surface - like that of a cooling rock-form cell-like spheres when splashed with water. If rain washed the spheres into a tidal pool, a place safe from ultraviolet radiation, more complex molecules could form. At last one appeared with the ability to reproduce itself: That molecule was similar to deoxyribonucleic acid, or DNA, the complex organic molecule that exists today in every living cell. And with the appearance of DNA, life on Earth began.

DMITRY MENDELEYEV

-INVENTOR OF PERIODIC TABLE

Born in Siberia, the youngest of 14 children, Dmitry I. Mendeleyev (1834–1907) was working as a chemist in St. Petersburg in the early 1860s when he devised a visual scheme by which to organize Earth's elements. At the time, 63 elements were known, and Mendeleyev arranged them in a table by atomic weight, making groupings based on shared properties. He also used his principles to predict the existence of new elements. During his lifetime three new elements were recognized gallium, scandium, and germanium. They fit his table according to plan. These discoveries went far to validate Mendeleyev's periodic table, which has become a fixture in the field of chemistry. Many other scientists have contributed to it since its invention, and it continues to help us understand how our planet and universe are put together.

"The elements, if arranged according to their atomic weights, exhibit an apparent periodicity of properties."

-DMITRY MENDELEYEV

FAST FACTS

• The periodic table of elements contains placeholders for element not yet officially recognized.