B The Periodic Table and Electron Configurations

Sample Problem B An element has the electron configuration [Kr]4d 5s Without looking at the periodic table, identify the period, block, and group in which this element is located. Then, consult the periodic table to identify this element and the others in its group. 

The number of the highest occupied energy level is 5, so the element is in the fifth period. There are five electrons in the d sublevel, which means that it is incompletely filled. The d sublevel can hold 10 electrons. Therefore, the element is in the d-block. For d-block elements, the number of electrons in the ns subievel (1) plus the number of electrons in the n- )d subievel (5) equals the group number, 6. This Group 6 element is molybdenum. The others in Group 6 are chromium, tungsten, and seaborgium. 

Without looking at the periodic table, write the outer electron configuration for the Group 12 element in the fifth period. 

For atoms of p-block elements, the total number of electrons in the highest occupied level is equal to the group number minus 10. For example, bromine is in Group 17. It has 17-10 = 7 electrons in its highest energy level. Because atoms of p-block elements contain two electrons in the ns sublevel, we know that bromine has five electrons in its outer p sublevel. The electron configuration of bromine is [Ar]3d 4s 4p.  

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