The Periodic Table

Even before we had this nice understanding of the atomic structure, scientists had identified certain substances as elements. So there were many attempts to arrange the known elements so that there were some correlations between their known properties. The first reasonably successful attempt was made by Dimitri Mendeleev in 1869. He had the idea of arranging elements in order of increasing atomic mass , and, most importantly, found that elements with similar chemical and physical properties occured periodically. He placed these similar elements under each other in columns.

In 1914, Henry Moseley determined that a better arrangement was in order of increasing atomic number, giving us the periodic table we have today. We can define the periodic table as an arrangement of elements in order of increasing atomic number, placing those with similar chemical and physical properties in columns.

Features of the Periodic Table

Groups

Vertical columns are called groups. Elements within a group have similar chemical and physical properties. Groups are designated at the top by the numbers 1-8 and by the letters A and B. (Note: Group labeling is somewhat arbitrary, so watch out for other designations, particularly with A and B)

A group elements- Representative or main group elements

B group elements- Transition elements

In addition to the number-letter designation, some groups have their name.

Periods

Horizontal rows are called periods. The numbers on the left designate periods in the periodic table. The two long rows placed just below the main body of the table are the inner transition elements

Elements 58-71 are the Lanthanide Series

Elements 90-103 are the Actinide Series

The Three Categories of Elements

There are three broad categories of elements called

1. Metals
2. Non-metals
3. Metalloids

To separate the metals and non-metals, we draw a stairstep line to the left of and below B, Si, As, Te, and At.

This classification or group is useful because certain properties are associated with each category.

Metals

• solids at room temperature (except Hg)
• metallic luster
• malleable and ductile
• good conductors of heat and electricity

Non-metals

• gases or solids at room temperature (except Br₂)
• variety of color and appearance
• brittle solids
• insulators (poor conductors)

Metalloids

• intermediate in properties between metals and non-metals
• solids at room temperature
• many have more than one structure (one metallic, the other non-metallic)
• some are semi-conductors