1. Organizing Elements – 5.1

2. INTRODUCTION A. Until 1750, scientists had identified only 17 elements
Mainly metals, such as copper and iron
B. In 1789, Antoine Lovoisier grouped elements into 4 categories: metals, nonmetals, gases, and earths

3. MENDELEEV A. He organized elements according to their atomic mass
B. He recognized patterns of repeated properties
C. Created a periodic table – arrangement of elements in columns based on similar properties that repeat

4. MENDELEEV, CONT.
D – published his table of elements with horizontal rows of increasing atomic masses. Elements with similar properties were in vertical columns.
- atomic masses increased from left to right and from top to bottom
- elements were in columns according to their reactivity to oxygen and hydrogen

6. MENDELEEV, CONT.
E. By using his periodic table, Mendeleev was able to predict the properties of several undiscovered elements.
F. He predicted the properties of both gallium (Ga) and germanium (Ge) before they were both discovered. They were discovered in his lifetime, so he was able to prove his theories correct.

7. Prediction of Gallium (Ga)
PROPERTY
PREDICTED
ACTUAL
Atomic mass
70 u
69.72 u
Color
Dark gray
Gray-white
Density
5.9 g/mL
Melting point
~ 25°C
29.7°C

8. MENDELEEV, CONT.
F. Mendeleev’s ability to predict properties of undiscovered elements led scientists to accept his periodic table.