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California Standards Chemistry

Standard 3a Preknowledge

3a) Students know how to use the periodic table to determine the number of electrons available for bonding.

CALIFORNIA FRAMEWORKS SUMMARY: Only electrons in the outermost energy levels of the atom are available for bonding; this outermost bundle of energy levels is often referred to as the valence shell or valence shell of orbitals. All the elements in a group have the same number of electrons in their outermost energy level. Therefore, alkali metals (Group 1) have one electron available for bonding, alkaline earth metals (Group 2) have two, and elements in Group 13 (once called Group III) have three. Unfilled energy levels are also available for bonding. For example, Group 16, the chalcogens, has room for two more electrons; and Group 17, the halogens, has room for one more electron to fill its outermost energy level.

To find the number of electrons available for bonding or the number of unfilled electron positions for a given element, students can examine the combining ratios of the elementís compounds. For instance, one atom of an element from Group 2 will most often combine with two atoms of an element from Group 17 (e.g., MgCl2) because Group 2 elements have two electrons available for bonding, and Group 17 elements have only one electron position open in the outermost energy level. (Note that some periodic tables indicate an elementís electron configuration or preferred oxidation states. This information is useful in determining how many electrons are involved in bonding.)