Electron Configurations, Valence Electrons, and the Periodic Table

An atom s valence electrons are those that are important in chemical bonding. For main-group elements, the valence electrons are those in the outermost principal energy level. For transition elements, we also count the outermost d electrons among the valence electrons (even though they are not in an outermost principal energy level). The chemical properties of an element depend on its valence electrons, which are instrumental in bonding because they are held most loosely (and are therefore the easiest to lose or share). We can now see why the elements in a column of the periodic table have similar chemical properties they have the same number of valence electrons. 

Valence electrons are distinguished from all the other electrons in an atom, which are called core electrons. The core electrons are those in complete principal energy levels and those in complete d and / sublevels. For example, silicon, with the electron configuration has four valence electrons (those in the = 3 principal 

A FIGURE 8.6 Outer Electron Configurations of the First 18 Elements in the Periodic Table. 

EXAMPLE 8.3 Valence Electrons and Core Electrons Write the electron configuration for Ge. Identify the valence electrons and the core electrons. SOLUTION  

Write the electron configuration for Ge by determining the total number of electrons from germanium s atomic number (32) and then distributing them into the appropriate orbitals. Ge s 2s 2p 3s 3p 4s 3d °4p  

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Electron Configurations, Valence Electrons, and the Periodic Table 345... 

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