Scandium (Sc) is a silvery-white metallic element with an atomic number of 21. It is the first element in the series known as the transition metals. Determining the number of electrons available for bonding requires understanding its atomic structure, as its valence electron count is interpreted differently than those of main-group elements.
Defining Valence Electrons
Valence electrons are defined as the electrons that reside in the outermost shell, or highest energy level, of an atom. These electrons are directly involved in chemical reactions and the formation of bonds, governing an element’s reactivity. For main-group elements (s-block and p-block), identifying the valence shell is simple, corresponding to the highest principal quantum number (‘n’). However, this traditional definition becomes complex for d-block elements like Scandium, where inner-shell electrons can also participate in chemical bonding due to energy considerations.
Scandium’s Electron Configuration
Scandium is element number 21 on the periodic table, meaning a neutral atom contains 21 protons and 21 electrons. Because it is the first element in the fourth period, its electrons follow a specific organizational pattern known as its electron configuration. Chemists often use noble gas notation to simplify this description by representing the inner, non-reactive electrons with the symbol of the preceding noble gas, Argon.
The configuration for Scandium is written as \([Ar] 3d^1 4s^2\). This notation shows that beyond the stable core of 18 electrons held by the Argon configuration, there are three additional electrons. Two of these electrons occupy the \(4s\) orbital, and one electron occupies the \(3d\) orbital.
Electrons fill the \(4s\) orbital before they enter the \(3d\) orbital, even though \(4s\) is typically written last in the configuration. This is dictated by the Aufbau principle, which states that electrons occupy the lowest available energy levels first. The \(4s\) orbital possesses a slightly lower energy state than the \(3d\) orbital in the neutral atom. The placement of these electrons defines Scandium as a transition metal.
Determining Scandium’s Valence Count
Based on its electron configuration and chemical behavior, Scandium typically has three valence electrons. This count includes both the two electrons in the \(4s\) orbital and the single electron in the \(3d\) orbital. While the \(4s\) electrons are in the outermost shell, the inner \(3d\) electrons are counted as valence because their energy level is extremely close to that of the \(4s\) electrons.
Because the inner \(d\)-orbital electrons are readily available for bonding, the traditional definition of valence electrons is insufficient for transition metals. The total valence electron count is therefore the sum of the \(n\)s electrons and the \((n-1)d\) electrons. This inclusion of the \(d\)-orbital electrons is a defining characteristic of transition metal chemistry.
The strongest evidence for this three-electron count is found in Scandium’s chemical reactivity. Scandium almost exclusively forms compounds in a positive three (+3) oxidation state. To achieve this stable ion, the neutral atom must lose all three valence electrons: the two \(4s\) electrons and the one \(3d\) electron. Losing these three electrons leaves the Scandium ion (\(Sc^{3+}\)) with the stable electron configuration of the noble gas Argon, confirming that the \(4s^2\) and \(3d^1\) electrons collectively function as the valence electrons.