1. Describe or read the following summary of research on the structure of the atom:

• By the end of the nineteenth century, most scientists agreed on the existence of atoms and, thanks to the work of a physicist named J. J. Thomson, knew that these structures contained particles called electrons. Thomson proposed that the atom is a sphere filled with positively charged matter that contains negatively charged electrons embedded in it like plums in a pudding.

  Today's "nuclear model" of the atom, first proposed by physicist Ernest Rutherford, is very different from Thomson's model. In it, the nucleus, which carries all of the positive charge and 99.9 percent of the atom's mass, is at the center of the atom. Orbiting at a distance of up to 100,000 times the diameter of the nucleus are the electrons. These tiny particles are anything but inconsequential, however. We now know that their arrangement and behavior is responsible for each element's unique set of chemical characteristics.

2. Distribute the Periodic Table of the Elements essay handout and have students read it. When they have finished, lead a discussion about how it was possible for Mendeleyev to develop a periodic table of elements before atomic structure, the essence of the table, was even known. (You might relate Mendeleyev's work to that of Gregor Mendel, who worked out the rules of simple genetics, or Charles Darwin, who described evolution by natural selection, both without specific knowledge of genes.)

3. Ask students to work in pairs as they explore the Periodic Table of the Elements interactive activity. Direct students to several sample elements within the interactive and identify the following information specific to each:

• The atomic number refers to the number of protons in the nucleus of the element's atoms. (Gallium, for example, has 31 protons.)
• The atomic mass increases as the atomic number increases, but unevenly. This is because the number of protons does not always equal the number of neutrons. The listed atomic mass is the average mass for all forms, or isotopes, of each element.
• The number of neutrons shown is the number for the most common isotope of the element.
• Electron configuration is the number of electrons found in each energy level of the element's atoms, starting from the innermost, or lowest, energy level.

4. Distribute copies of the Chemistry Scavenger Hunt (PDF) handout and allow 10 minutes for pairs of students to locate the missing information. You may create additional scavenger hunt entries -- this time with less starting information. Just be sure that at least one of the shaded columns is filled in to allow students to locate each element on the table.