1. Organizing the Elements Essential Question: What determines where an element will be located in the periodic table?

3. Element A pure substance that cannot be broken down into any other substances.

4. Molecule A group of two ore more atoms held together by chemical bonds.

5. Atom The basic particle from which all elements are made

6. Atomic Mass The average mass of all the different forms of the same element. The atomic mass equals the number of protons and neutrons in the nucleus of an atom. So if you need to find out the number of neutrons you subtract the atomic # from the atomic mass.

7. Dmitri Mendeleev In 1869 a total of 63 elements had been discovered. Scientists wondered if the properties of elements followed a pattern. Dmitri Mendeleev discovered a set of patterns that applied to all the known elements.

8. Mendeleev’s Work Mendeleev knew that some elements had similar chemical and physical properties. Mendeleev thought these similarities were important clues to a hidden pattern. To find this pattern Mendeleev wrote each element’s melting point, density, color, and atomic mass on an individual card. Then, he tried arranging the cards in different ways.

9. Mendeleev’s Work Mendeleev noticed that a pattern of properties appeared when he arranged the elements in order of increasing atomic mass. He also found that the properties repeated regularly. He lined up the cards with similar properties into groups.

10. Periodic Table An arrangement of elements showing the repeating pattern of their properties.