History of the Periodic TableThe scientists behind the

organization

Dmitri Mendeleev 1834 – 1907, Russian Arranged elements in

order of increasing atomic mass

Noticed patterns in chemical and physical properties

Arranged elements with the most similar properties side by side

Mendeleev continued There were blank spots in

the table because not all elements had been discovered

Periodic Law: Elements arranged according to their atomic masses present a clear periodicity of properties

Henry Moseley 1887 – 1915, British 1913 – determined

atomic number of elements

Rearranged periodic table according to atomic number

Went to WWI, killed at age 28

British response to Moseley’s death British government no

longer assigns scientists to combat duty in times of war

We will never know what his brilliant mind could have accomplished had he lived longer

Terms explained Atomic Mass: mass relative to carbon,

approximately the number of protons + neutrons

Atomic Number: the number of protons in a single atom of an element. In a neutral atom, the number of electrons is the same as the number of protons.