Chemical bonds are the forces that hold atoms together to form molecules and compounds. The nature of these bonds determines the physical and chemical properties of the resulting substance. When considering whether two elements, such as sulfur and bromine, will join, chemists must determine the type of bond that will form. The interaction between sulfur and bromine does not result in an ionic compound; instead, these two elements combine through electron sharing known as a covalent bond.
The Difference Between Ionic and Covalent Bonds
An ionic bond involves the complete transfer of one or more valence electrons from one atom to another. This transfer typically occurs between a metal, which readily gives up electrons, and a nonmetal, which readily accepts them. The result is the formation of oppositely charged particles, called ions, that are held together by strong electrostatic attraction.
Covalent bonds are formed when atoms share electrons to achieve a stable electron configuration. This type of bonding usually happens between two nonmetals, both of which have a strong tendency to hold onto their own electrons. Because neither atom is powerful enough to completely strip electrons from the other, the atoms end up sharing one or more pairs of electrons. This sharing creates a strong, directional bond between the atoms.
Identifying Sulfur and Bromine as Nonmetals
The first step in predicting the bond type involves looking at the location of the elements on the periodic table. Sulfur (\(\text{S}\)) is found in Group 16, while Bromine (\(\text{Br}\)) is a halogen in Group 17. Both elements reside on the upper-right side of the table, a region that is exclusively occupied by nonmetallic elements.
Nonmetals are characterized by having high ionization energies, meaning they require a large amount of energy to lose an electron. They also possess a strong affinity for electrons. Because both sulfur and bromine exhibit reluctance to give up electrons, the conditions necessary for a complete electron transfer and the formation of an ionic bond are not present. Their natural tendency is to attract electrons, setting the stage for a sharing arrangement.
Using Electronegativity to Determine Bond Type
Chemists use the concept of electronegativity to predict the nature of a bond. Electronegativity is a measure of an atom’s ability to attract a shared pair of electrons toward itself within a chemical bond. The difference in electronegativity between two bonding atoms reveals the degree of electron sharing or transfer that will occur.
A large difference in this value, typically exceeding 1.7, indicates a bond with enough electron transfer to be classified as ionic. Bromine has an electronegativity value of approximately 2.96 on the Pauling scale, and sulfur has a value of about 2.58. Calculating the difference between these two values yields a result of only 0.38.
This small difference confirms the bond is overwhelmingly covalent, as the electron sharing is nearly equal. Since the difference is not zero, the bond is further categorized as a polar covalent bond. This means the shared electrons are pulled slightly closer to the more electronegative bromine atom, giving the atoms partial positive and negative charges, but not the full charges found in an ionic compound.
The Characteristics of the Resulting Compound
The compound formed between sulfur and bromine is typically sulfur dibromide (\(\text{SBr}_2\)), which is characterized by its covalent properties. This substance is known to be a toxic gas that is highly unstable at standard conditions. The instability and gaseous state are hallmarks of a covalent compound, which lack the rigid, strong lattice structure of an ionic solid.
In terms of structure, the sulfur dibromide molecule adopts a bent molecular geometry, similar to that of water. The molecule’s bent shape, combined with the polar \(\text{S-Br}\) bonds, results in the overall molecule having polarity. Covalent compounds like this also exhibit low melting and boiling points, and they typically do not conduct electricity when dissolved in water.
This behavior stands in sharp contrast to ionic compounds, such as table salt (sodium chloride), which are hard solids with extremely high melting points. Ionic compounds also readily dissociate into ions when dissolved, allowing the solution to conduct electricity. The properties of sulfur dibromide therefore provide physical confirmation of its covalent structure.