![]() ![]() For example, after filling the 3 p block up to Ar, we see the orbital will be 4 s (K, Ca), followed by the 3 d orbitals. The filling order simply begins at hydrogen and includes each subshell as you proceed in increasing Z order. Since the arrangement of the periodic table is based on the electron configurations, Figure 4 provides an alternative method for determining the electron configuration. Figure 3 illustrates the traditional way to remember the filling order for atomic orbitals. Electrons enter higher-energy subshells only after lower-energy subshells have been filled to capacity. Each added electron occupies the subshell of lowest energy available (in the order shown in Figure 1), subject to the limitations imposed by the allowed quantum numbers according to the Pauli exclusion principle. This procedure is called the Aufbau principle, from the German word Aufbau (“to build up”). Beginning with hydrogen, and continuing across the periods of the periodic table, we add one proton at a time to the nucleus and one electron to the proper subshell until we have described the electron configurations of all the elements. To determine the electron configuration for any particular atom, we can “build” the structures in the order of atomic numbers. The diagram of an electron configuration specifies the subshell ( n and l value, with letter symbol) and superscript number of electrons. The notation 3 d 8 (read “three–d–eight”) indicates eight electrons in the d subshell ( l = 2) of the principal shell for which n = 3.
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