Bismuth (Bi) has an atomic number of 83, meaning every neutral bismuth atom contains exactly 83 protons in its nucleus. The defining rule of a neutral atom is that the number of negatively charged electrons must precisely balance the number of positively charged protons. Therefore, a neutral atom of bismuth is characterized by having 83 electrons.
The Total Electron Count
The atomic number, symbolized as \(Z\), is the foundational count that dictates the identity of an element. For bismuth, \(Z=83\), which confirms the presence of 83 protons. Since atoms in their standard state carry no net electrical charge, this positive charge from the nucleus must be counterbalanced by an equal number of electrons.
This electrical balance defines the atom’s neutral state. The specific count of 83 electrons is a fixed numerical property of bismuth, regardless of whether the atom is isolated or part of a chemical compound. This precise numerical identity is the first step before exploring how these electrons are spatially organized around the nucleus.
Arrangement in Energy Levels
The 83 electrons in a bismuth atom are not randomly scattered; they are arranged in specific regions of space known as principal energy shells and subshells. Because bismuth is a heavy element located in the sixth row of the periodic table, its electrons occupy six main energy levels. The full electron configuration shows how these particles are distributed across the various orbitals.
The first five shells are completely filled, accounting for 78 of the 83 electrons. This massive core structure includes electrons filling the \(s\), \(p\), \(d\), and \(f\) subshells, resulting in the simplified shell structure of \(2, 8, 18, 32, 18\). The capacity of the fourth shell to hold 32 electrons is due to the inclusion of the \(4f\) subshell.
The filling order of these orbitals is governed by rules like the Pauli Exclusion Principle and Hund’s rule. These rules minimize electron-electron repulsion and ensure the most stable energy state for the entire atom. The innermost shells are held tightly to the nucleus, while the outermost electrons determine the atom’s chemical behavior.
The Reactive Electrons (Valence Shell)
The chemical identity of bismuth is determined by the electrons residing in its outermost, partially filled energy level, the sixth shell. These five electrons are known as the valence electrons, and their configuration is \(6s^2 6p^3\). This arrangement of two electrons in the \(s\) subshell and three electrons in the \(p\) subshell places bismuth in Group 15 of the periodic table.
These five valence electrons are the only ones that participate in chemical bonding and reactions with other atoms. Bismuth commonly exhibits two distinct oxidation states: +3 and +5. The +5 state occurs when all five valence electrons are involved in bonding, but this state is relatively unstable for bismuth.
A phenomenon known as the inert pair effect significantly influences bismuth’s chemistry. For heavy elements like bismuth, the two electrons in the outermost \(s\) orbital (\(6s^2\)) are held much more tightly by the nucleus and become reluctant to participate in chemical reactions. Consequently, the +3 oxidation state, which involves only the three \(p\) electrons (\(6p^3\)), is the more stable and frequently observed state for bismuth compounds.