Elemental sulfur, the form most commonly found in nature, exists as a molecule with eight sulfur atoms chemically bonded together, represented by the formula \(S_8\). \(S_8\) is a nonpolar molecule, a direct consequence of its unique chemical structure.
The Crown Shape of Elemental Sulfur
Elemental sulfur forms a closed, eight-membered ring structure. Each sulfur atom in the \(S_8\) molecule is covalently bonded to two other sulfur atoms, forming a continuous cycle. All atoms are identical and linked only to other sulfur atoms.
The molecule adopts a distinctive conformation often described as a “crown” or a “puckered ring.” This shape arises because the bond angles between the sulfur atoms are bent, specifically around \(107.8^\circ\), instead of lying flat in a single plane. This three-dimensional structure is highly symmetrical, which is a significant factor in determining its properties.
How to Determine Molecular Polarity
Determining the polarity of any molecule involves analyzing bond polarity and molecular geometry. Bond polarity arises from the difference in electronegativity between the two atoms sharing electrons in a covalent bond. If this difference is significant, the shared electrons are pulled closer to the more electronegative atom, creating a partial negative charge \((\delta^-)\) and a partial positive charge \((\delta^+)\) at opposite ends of the bond.
This uneven sharing establishes a bond dipole moment, which is a vector quantity possessing both magnitude and direction. The overall molecular polarity is determined by the molecule’s net dipole moment, which is the vector sum of all bond dipoles acting within the molecule’s specific three-dimensional geometry. Even if a molecule contains polar bonds, if those bond dipoles are arranged symmetrically and pull equally in opposing directions, they will cancel out, resulting in a nonpolar molecule.
Why \(S_8\) is Nonpolar
The nonpolar nature of \(S_8\) is established by examining its bond polarity and molecular symmetry. First, the molecule is composed exclusively of sulfur-sulfur \((S-S)\) single covalent bonds. Since all atoms involved are identical sulfur atoms, the electronegativity difference between them is zero.
This zero difference means that the electron density is shared equally between the bonded atoms, making every individual \(S-S\) bond a nonpolar covalent bond. Consequently, no bond dipole moments are generated within the ring structure.
Even if the individual \(S-S\) bonds were slightly polar due to minor structural distortions, the overall molecular structure would still lead to nonpolarity. The characteristic crown shape of the \(S_8\) ring exhibits a high degree of symmetry. This symmetrical arrangement ensures that any potential minor bond dipoles would be geometrically oriented to perfectly oppose and cancel one another out. The combination of nonpolar bonds and a highly symmetrical molecular structure results in a net dipole moment of zero.
What Nonpolarity Means for \(S_8\) Solubility
The nonpolar nature of the \(S_8\) molecule dictates its solubility, following the chemical principle known as “like dissolves like.” Since \(S_8\) is nonpolar, it is highly insoluble in polar solvents, such as water, because it lacks the ability to form the strong dipole-dipole or hydrogen bonds necessary to interact effectively with water molecules.
Conversely, \(S_8\) is readily soluble in nonpolar solvents, such as carbon disulfide \((CS_2)\) or toluene. The dissolution occurs because the primary intermolecular forces present in both \(S_8\) and these solvents are the weak, temporary attractions known as London Dispersion Forces (LDFs). The similar strength and nature of these forces allow the solvent molecules to effectively separate and surround the \(S_8\) molecules, resulting in a homogeneous solution.