the periodic table: an introduction. invention of the periodic table elements vary widely in their...

The Periodic Table:

An Introduction

Invention of the Periodic Table Elements vary widely in their properties, but

some elements have similarities in their properties.

The first to notice that these properties varied in a regular pattern was John Newlands. He arranged the first 16 elements known at the time

(except hydrogen) in order of increasing mass and then placed them in two rows.

He noticed that the two sets of elements had similar properties. Because the properties repeated with the eighth element, he called his pattern the law of octaves.

John Newland’s octaves

Li Be B C N O F

Na Mg Al Si P S Cl

Mendeleev and the Periodic Table

In 1870, the Russian chemist Dmitri Mendeleev arranged all 63 known elements in such a way that: They were in order of atomic mass. They were arranged in a table in such a way that

elements with similar properties were arranged in vertical columns.

When this was done, the patterns of properties clearly repeated themselves in a periodic fashion.

Exceptions to the pattern

Mendeleev had to reverse the order of two elements, tellurium and iodine. He assumed that the atomic weights were in error. When isotopes were discovered, it was found that he

had correctly placed these elements in the order of their atomic numbers.

Mendeleev also predicted that additional elements would be discovered to fill in gaps that he left to make the properties line up.

The Modern Periodic Table

http://serc.carleton.edu/images/usingdata/nasaimages/periodic-table.gif

Horizontal Rows are called periods.Vertical columns are called groups.

Groups have similar properties.

The elements are divided into three main groups:MetalsSemi-conductors or metalloidsNonmetals

Metals vs. Nonmetals

Metals are good conductors of electricity. except for mercury, are solids at room temperature.

Nonmetals are poor conductors of electricity. exhibit a wide variety of properties. may be solids, liquids or gases at room temperature.

The alkali metals are very reactive, having only oneelectron in their outer shells. They tend to be very soft.

The alkaline-earth metals are reactive, but not as reactive as thealkali metals. They are harder and have higher melting pointsthan the alkali metals.

Transition metals have outer electrons in d orbitals.They are generally not as reactive as the alkali oralkaline-earth metals.

Semiconductors or metalloids conduct electricity betterthan nonmetals but not as well as metals.

Halogens are nonmetals which are very reactive. They tend to form salts with metals.

The noble gases are nonmetals which are very inert, or unreactive. Their full outer shells account for this property.