Mercury (\(\text{Hg}\)) is a unique element. A neutral mercury atom has precisely 80 electrons, a count fixed for the elemental form and directly related to the atom’s identity. This number is determined by consulting the Periodic Table. However, the electron count can change when the atom forms chemical bonds.
The Atomic Basis of Electron Count
The electron count of any neutral atom is fundamentally determined by its atomic number, which is a fixed property of the element. Mercury is defined by the atomic number 80, meaning every mercury atom contains 80 protons within its nucleus. For an atom to exist in a neutral, uncharged state, the total positive charge from the protons must be exactly balanced by an equal amount of negative charge provided by the electrons. Therefore, a neutral atom of mercury must contain 80 electrons to perfectly neutralize the charge. This simple rule provides the initial answer to the question of mercury’s electron count, though this balance can be temporarily disrupted when mercury interacts with other substances.
How Mercury’s Electrons Are Arranged
The specific arrangement of the 80 electrons determines mercury’s chemical properties. These electrons are organized into distinct energy levels, or shells, labeled numerically from 1 to 6. The electron configuration maps where these electrons reside in specific subshells (\(s\), \(p\), \(d\), and \(f\) orbitals). Mercury’s complete configuration is \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6 4d^{10} 5s^2 5p^6 4f^{14} 5d^{10} 6s^2\).
Valence Electrons
The 80 electrons fill six main shells. The inner shells, including the \(4f\) and \(5d\) subshells, are completely filled, accounting for 78 electrons. The outermost, or valence, shell is the \(6s\) orbital, which contains the final two electrons. These two valence electrons are the most loosely held and are the ones involved in forming chemical bonds.
Electron Counts in Mercury Ions
The count of 80 electrons applies only to the neutral mercury atom. When mercury forms ions, it loses valence electrons, resulting in a positive charge and a change in the total electron count. Mercury commonly exhibits two positive oxidation states: \(+1\) and \(+2\).
The Mercuric Ion (\(\text{Hg}^{2+}\))
The \(\text{Hg}^{2+}\) ion (mercuric ion) is formed when the neutral atom loses its two outermost \(6s\) valence electrons. The loss of two electrons means the resulting ion has 78 electrons. This count matches the electron configuration of the noble gas xenon.
The Mercurous Ion (\(\text{Hg}_2^{2+}\))
The \(+1\) oxidation state exists as the mercurous ion. Mercury in this state does not form a simple \(\text{Hg}^+\) ion in isolation. Instead, two \(\text{Hg}^+\) units bond together to form a diatomic ion, \(\text{Hg}_2^{2+}\). Each mercury atom in this structure has lost one electron from the \(6s\) orbital, leaving 79 electrons per atom in the bonded state.