Triads In 1817 Johann Dobereiner a German Chemist grouped elements with similar chemical properties. Most groups had three members TRIADS Ex. Lithium, Sodium and Potassium

In 1863 John Newlands an English Chemist arranged elements by increasing mass properties repeated every eighth element similar to Music THE LAW OF OCTAVES

Periodic Law 1869 – Lother Meyer and Demitri Mendeleev discover the Periodic Law.

KNOWN ELEMENTS H Li soft silvery metal that reacts vigorously with water to form hydrogen gas and LiOH Be B C N O F highly reactive deadly gas yellow in color Na soft silvery metal that reacts vigorously with water to form hydrogen gas and NaOH Mg Al Si P S Cl highly reactive deadly gas yellow-green in color K soft silvery metal that reacts vigorously with water to form hydrogen gas and KOH

Periodic Law- If the elements are arranged in order of increasing atomic weight, there is a periodic repetition of properties.

Mendeleev’s First Periodic Table, 1869

Figure 7.28: The periodic table with atomic symbols, atomic numbers, and partial electron configurations.

Special names for groups in the periodic table. Groups and Periods The elements in columns comprise groups. The elements in rows comprise periods.

Figure 7.31: The values of first ionization energy for the elements in the first six periods.

Figure 7.32: Trends in ionization energies (kJ/mol) for the representative elements.

Figure 7.33: The electron affinity values for atoms among the first 20 elements that form stable, isolated X- ions.

Figure 7.34: The radius of an atom (r) is defined as half the distance between the nuclei in a molecule consisting of identical atoms.

Figure 7.35: Atomic radii (in picometers) for selected atoms.

Potassium reacts violently with water.