Elements

What Are Elements?
Elements are pure substances made of only one kind of atom.
Atoms are tiny structures found in all matter.
Most substances contain many different atoms.
Only the elements contain only one kind of atom.
Salt
Sodium and Chlorine
ELEMENTS
1. One kind of atom
2. Pure
3. Separated in nuclear reactions
Gold
Jewelry
Aluminum
Soda Can
COMPOUNDS
1. Two or more kinds of atoms chemically bonded
2. Pure
3. Separated in chemical reactions
Water
Hydrogen and Oxygen
MIXTURES
1. Two or more elements or compounds physically together
2. Not pure
3. Separated in physical reactions
Italian Dressing
The Periodic Table of the Elements
The periodic table was first constructed in 1869 to organize the elements by their properties. The periodic table in this book shows 112 elements. It organizes them by their families, atomic numbers, and many other properties.


Ancient Times
Some elements were known and used by ancient civilizations. These elements were:
Iron (-2500 BCE) Tin (-2100 BCE) Antimony (-1600 BCE) Lead (-1000 BCE)
Carbon (pre-history) Sulfur (pre-history) Copper (-5000 BCE) Silver (-3000 BCE) Gold (-3000 BCE)

Alchemy to the First Periodic Table (1000-1869)
During this time, 52 elements were discovered. Many of these elements were discovered by alchemists. Alchemists were people who tried to combine science and magic. They tried to change lead into gold. They discovered and used the scientific method. A few elements that were discov­ered during this period were:
Nitrogen (1755) Oxygen (1774) Chlorine (1774) Aluminum (1825)
Arsenic (-1250) Zinc (-1500) Phosphorus (1669) Platinum (-1700) Nickel (1751)

Enter the Periodic Table
The first periodic table was drawn by a Russian scientist named Dimitry Mendeleyev in 1869. Mendeleyev put all 62 known elements on the first periodic table, and even allowed enough space for over 20 elements that had not yet been discovered. Mendeleyev's first periodic table closely resembles the table we use today. He also arranged the elements in the order of their atomic numbers, just as we do today.


Rare and Radioactive Elements (1869-1899)
Many rare and radioactive elements were discovered during this modern period. Rare ele­ments are elements that occur in very small amounts on earth. Radioactive elements are ele­ments that give off small particles. A few elements discovered during this period include:
A.4
Fluorine (1886) Argon (1894) Helium (1895) Krypton (1898)
Neon (1898) Polonium (1898) Radium (1898) Actinium (1899)

Rare, Radioactive, and Synthetic Elements (1900-present)
During this modern period in chemistry, 30 elements have been created or discovered so far. These 20th-century elements are very rare on earth. Some of them are very radioactive. Many of them are not found on earth at all, but were created in a laboratory. These are called syn­thetic elements. Some of these rare, radioactive, and synthetic elements include:
Radon (1900) Francium (1939) Plutonium (1940)
Americium (1944) Dubnium (1970) Ununbium(1996)
The Future

More new elements may be created in the future. Some periodic tables today provide space for elements up to #118. Many scientists are wondering why we should create new elements. Synthetic elements are very radioactive, they decay very quickly, and they cannot be studied or used. Other scientists are creating more elements because they believe that a stable, usable element may be discovered. Some scientists have even predicted that element #114 may be stable. Who knows? Perhaps this stable element will be discovered in your lifetime.