In order to develop interesting new compounds inorganic chemists often start by noting the location of an element in the periodic table and its valence electron configuration. Suppose you are working in a laboratory developing catalysts for an industrial process. Predict the ground-state electron configuration of

(a) A vanadium atom and

(b) A lead atom.

ANTICIPATE Because vanadium is a member of the d-block, you should expect its atoms to have a partially filled set of d-orbitals. Because lead is in the same group as carbon, you should expect the configuration of its valence electrons to be similar to that of carbon (ns² np²).

PLAN Follow the procedure in Toolbox 1E.1.

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Toolbox 1E.1 HOW TO PREDICT THE GROUND-STATE ELECTRON CONFIGURATION OF AN ATOM CONCEPTUAL BASIS Electrons occupy orbitals in such a way as to minimize the total energy of an atom by maximizing attractions and minimizing repul- sions in accord with the Pauli exclusion principle and Hund's rule. 3 If more than one orbital in a subshell is available, add electrons to different orbitals of the subshell before doubly occupying any of them (Hund's rule). PROCEDURE Use the following rules of the building-up principle to assign a ground-state configuration to a neutral atom of an element with atomic number Z: 1 Note from the periodic table in which period and group the element is found. Its core configuration will be that of the preceding noble-gas configuration together with any completed d- and f-subshells. The period number gives the value of the principal quantum number of the valence shell and the group number is used to find the number of valence electrons. 2 Add Z electrons, one after the other, to the orbitals in the order shown in Figs. 1E.1 and 1E.4 but with no more than two electrons in any one orbital (the Pauli exclusion principle). 4 Write the labels of the orbitals in order of increasing energy, with a superscript that gives the number of electrons in that orbital. The configuration of a filled shell is represented by the symbol of the noble gas having that configuration, as in [He] for 1s². In most cases this procedure gives the ground-state electron configuration of an atom. Any arrangement other than the ground state corresponds to an excited state of the atom. Note that the structure of the periodic table can be used to predict the electron configurations of most elements by noting which orbitals are being filled in each block of the periodic table (see Fig. 1E.4). Example 1E.1 shows how these rules are applied.