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Discovering the Elements  By the year 1869, sixty three elements had been discovered.  A Russian scientist named Dmitri Mendeleev discovered that the elements followed certain patterns.

Mendeleev’s Role  Mendeleev noticed that some elements shared physical and chemical properties. Example: fluorine and chlorine are both gases that irritate your lungs. Example: copper and silver are both metals that tarnish when exposed to air.

Patterns  Mendeleev tried to find patterns in the elements using Melting point Density Color Atomic mass Chemical bonds

Patterns  He noticed that if he put the elements in order of increasing atomic mass, patterns appeared.  He also noticed that the properties of the elements repeated.  He put these elements in order by atomic mass, but also grouped them according to properties.

Each color is a different “group” on the Periodic Table. The elements in these groups share common properties and will react in similar ways.

Groups on the Periodic Table  Elements in the same group have similar chemical and physical properties!! (Mendeleev did that on purpose.)  Why?? They have the same number of valence electrons. They will form the same kinds of ions.

How Did He Do It?  For example: After fluorine, the next heaviest known element was sodium. Sodium reacted with water the same way that lithium did. So he placed those elements into the same group.

Predicting New Elements  When Mendeleev began placing the elements in order based on common properties and atomic masses, he noticed that there were a few empty spaces.  Mendeleev predicted that the blank spaces would be filled by elements that had not yet been discovered.  He could even predict the properties of the unknown elements!

Finding the Missing Elements  Within 16 years of Mendeleev’s Periodic Table, the missing three elements were discovered.  They were called scandium, gallium, and germanium.  Their properties were very close to what he had predicted.

The Modern Periodic Table  The Periodic Table that we use today is very similar to Mendeleev’s first version.  New elements have been added as they have been discovered.  Look in your pink sheets to see the modern Periodic Table of Elements.

Atomic Number  The only major change occurred in 1913, when a scientist named Henry Moseley discovered how to measure an atom’s atomic number.  The Periodic Table was rearranged in order of atomic number instead of atomic mass.

Review 1. What was the name of the scientist who first developed the Periodic Table? 2. How many known elements were there at that time? 3. How did he set up his Periodic Table? 4. What was he able to predict about unknown elements using his Periodic Table? 5. How has the Periodic Table changed since then?

Answers 1. Dmitri Mendeleev elements 3. He put the elements in order of increasing atomic mass. 4. He was able to predict the existence of unknown elements and the chemical properties of those unknown elements. 5. The elements are now in order by atomic number and there are now more than 100 known elements.