Periodic Table History

From Dobereiner to the Present

J.W. Dobereiner

• Early 1800s, German

• Elements classified into groups of 3: triads

• Elements in triad have similar properties

• Properties of middle element are average of the first and third elements in triad

J.A.R. Newlands

• 1800s, British

• Elements arranged in order of increasing atomic mass – pattern repeats every 8 elements

• Law of octaves

Dmitri Mendeleev

• Russian, 1800s; Lothar Meyer developed concurrently

• Organized elements by increasing atomic mass

• Elements with similar properties grouped together

• Had some discrepancies, but predicted properties of undiscovered elements

Henry Moseley

• Early 20th century, British

• Organized elements by atomic number

• This solved discrepancies in Mendeleev’s periodic table

Groups

• Vertical columns in table

• Elements in a group have similar chemical properties

Periods

• Horizontal rows in table are called periods

• Represent energy level or principal quantum number, n

Organization of Elements in Modern Periodic Table

• Metallic Character

• Metals• Nonmetals• Metalloids

Organization (Continued)

• Blocks

• Main Group Elements• Transition Metals• Inner Transition

Metals

Numbering

IUPAC and American Numbering Systems