Question

Boric acid has a polymeric layer structure in which planar \(\mathrm{BO}_{3}\) units are joined by (a) covalent bonds (b) two centre - two electron bonds (c) coordinate bonds (d) hydrogen bonds.

Short answer

The planar \(\mathrm{BO}_{3}\) units in boric acid are joined by hydrogen bonds.

Step-by-step solution

Understanding Boric Acid Structure

Consider the structure of boric acid, which consists of planar \(\mathrm{BO}_{3}\) units. These units must be connected in a way that maintains the stability of the polymeric layer structure of the compound.

Considering Bond Types

Next, consider each type of bond given in the options. Covalent bonds would generally form individual molecules, not a polymeric layer structure. Two centre - two electron bonds are essentially another type of covalent bond and would not contribute to a layered structure. Coordinate bonds are a type of covalent bond where both electrons come from the same atom; they are more typical in complex ions or coordination compounds, not layer structures. Hydrogen bonds, however, are known for creating networks, as in ice or water, and they can facilitate a layer structure without the need for direct covalent bonding between the \(\mathrm{BO}_{3}\) units.

Identifying the Correct Bond Type

Considering the ability of hydrogen bonds to create a polymeric layer structure and the known structure of boric acid in which hydrogen atoms connect planar \(\mathrm{BO}_{3}\) units, one can conclude that the \(\mathrm{BO}_{3}\) units in boric acid are joined by hydrogen bonds.

Key concepts

Hydrogen Bonds

The concept of hydrogen bonds is essential in understanding the structure of various compounds, including boric acid. Hydrogen bonds are a type of weak chemical bond that forms when a hydrogen atom, which is covalently bonded to one electronegative atom, like oxygen or nitrogen, experiences an attraction to another electronegative atom.

In the context of boric acid, hydrogen bonds act as the glue that holds the planar BO3 units together in a polymeric layer structure. Unlike stronger covalent bonds, hydrogen bonds are easily broken and reformed, which contributes to substances having unique properties like high boiling points and the ability to form extended network structures. These bonds are also responsible for the characteristic three-dimensional polymeric lattice of boric acid, which is critical to its stability and properties.

Polymeric Layer Structure

The polymeric layer structure is a specific type of molecular arrangement that is commonly found in certain types of crystalline materials, such as boric acid. In boric acid, the BO3 units are linked into a two-dimensional polymeric layer structure.

This structure is significant because it confers unique properties such as resistance to heat and dissolution in water, which can be attributed to the strength and extensive nature of the hydrogen bonding network. Each layer in the structure is relatively stable and exhibits a certain degree of flexibility due to the weaker nature of hydrogen bonds compared to covalent bonds. The concept of a polymeric layer structure is crucial when discussing the solid state chemistry and the interaction between molecules within a compound.

Chemical Bonding

Chemical bonding is foundational to molecular structure and directly influences the physical and chemical properties of substances. There are several types of chemical bonds, including ionic, covalent, metallic, and hydrogen bonds.

In the structure of boric acid, while covalent bonds create the rigid BO3 units, hydrogen bonds are responsible for the larger structure, binding these units into a cohesive polymeric layer structure. The hydrogen bonds are not as strong as covalent bonds, yet they are crucial for maintaining the integrity of the extended lattice in boric acid. This exemplifies the importance of understanding various bond types and their roles in building compound structures.

BO3 Units

BO3 units are the building blocks of boric acid's structure, comprising a central boron atom covalently bonded to three oxygen atoms in a trigonal planar arrangement. Each oxygen atom carries a lone pair which can participate in hydrogen bonding.

These BO3 units do not exist in isolation; rather, they are chemically bonded to each other through hydrogen bonds to form the greater polymeric layer structure of boric acid. The significance of the BO3 unit lies in its ability to form a stable, planar geometry, which is conducive to forming extended networks of hydrogen bonds with neighboring BO3 units within the crystal lattice.