The Gold atom (Au), with an atomic number of 79, possesses 79 electrons orbiting its nucleus. The chemical behavior of any atom, including Gold, is governed by its outermost electrons, known as valence electrons. These electrons determine how an atom interacts with other elements, forming the basis of chemical reactions.
Defining Valence Electrons
Valence electrons are the electrons located in the outermost shell or energy level of an atom. They participate in the formation of chemical bonds. For many elements, the number of valence electrons can be easily determined by the element’s group number on the periodic table. For instance, an element in Group 1, like Sodium, has one valence electron.
The concept of electron shells describes the general regions where electrons are found, labeled by a principal quantum number, \(n\). Electrons in the highest \(n\) value are generally the farthest from the nucleus and most accessible for bonding. This simple rule, however, becomes less straightforward when discussing the heavy atoms found in the transition metal section of the periodic table, such as Gold.
The Electron Configuration of Gold (Au)
Gold is a heavy transition metal, and its electron configuration is an exception to the standard filling rules. The configuration for a neutral Gold atom (79 electrons) is condensed using the noble gas core of Xenon (Xe): \([Xe] 4f^{14} 5d^{10} 6s^1\).
If Gold followed the expected pattern for its position in the periodic table, its configuration would end in \(6s^2 5d^9\). Instead, one electron promotes from the \(6s\) orbital to complete the \(5d\) orbital, resulting in the more stable \(5d^{10}\) configuration with a half-filled \(6s^1\) orbital. This unusual arrangement is primarily influenced by relativistic effects, which become significant in heavy atoms like Gold. The speed of the innermost electrons approaches the speed of light, causing them to increase in mass and contract their orbitals, which subsequently affects the energies of the outer \(6s\) and \(5d\) orbitals.
Determining the Number of Valence Electrons in Gold
The calculation of Gold’s valence electrons is complicated by its unique electron configuration and its nature as a transition metal. Based on the traditional definition, which only counts electrons in the outermost principal quantum shell (\(n=6\)), Gold has 1 valence electron. This single electron resides in the \(6s^1\) subshell.
However, for transition metals, valence electrons are defined as any electron outside the noble gas core that can participate in bonding. The \(5d\) subshell, although technically an inner shell, is very close in energy to the \(6s\) subshell due to relativistic effects. Because the \(5d\) electrons can also be involved in chemical reactions, Gold is often said to have a maximum of 3 valence electrons when considering its chemical states.
How Gold Uses Its Valence Electrons in Chemical Reactions
The dual nature of Gold’s valence electrons explains its common chemical behaviors, defined by stable oxidation states of +1 and +3. The most prevalent chemical state, Gold(I), corresponds to the loss of the single \(6s\) electron. This leaves the atom with the stable, fully-filled \(5d^{10}\) shell, which is why the +1 state is common.
The Gold(III) state involves a total loss of three electrons, demonstrating the involvement of the \(d\)-orbital electrons. This state is achieved by the loss of the single \(6s\) electron and two additional electrons from the \(5d\) subshell. The ability of Gold to utilize these inner \(5d\) electrons confirms why the count of 3 valence electrons is a practical answer for its maximum chemical reactivity.