Question

\(\mathrm{BCl}_{3}\) has an incomplete valence shell. Use Lewis structures to show how it could form a coordinate covalent bond with a water molecule.

Short answer

Boron in \text{BCl}_{3} has six valence electrons after bonding with Chlorine, while Oxygen in H2O has two lone pairs. One of Oxygen's lone pairs can be donated to Boron to form a coordinate covalent bond, resulting in a complete octet for Boron.

Step-by-step solution

Identify Valence Electrons

Boron (B) has 3 valence electrons, and chlorine (Cl) has 7 valence electrons. Since there are three chlorine atoms, this gives a total of 3 + (3*7) = 24 valence electrons for the BCl3 molecule. Oxygen (O) in water (H2O) has 6 valence electrons, and each hydrogen (H) has 1 valence electron, for a total of 6 + (2*1) = 8 valence electrons in the water molecule.

Draw the Lewis Structure for BCl3

Place Boron (B) in the center and distribute the three Chlorine (Cl) atoms around it. Boron will share three of its electrons with each Chlorine atom to form three covalent bonds. Since Chlorine atoms have seven valence electrons, each Cl atom will have three lone pairs after bonding.

Draw the Lewis Structure for H2O

Oxygen (O) is in the center with two Hydrogen (H) atoms forming a bent structure. Oxygen will share two of its six valence electrons to form covalent bonds with the two Hydrogens. The remaining four valence electrons on Oxygen will form two lone pairs.

Identify the Incomplete Octet

Boron in BCl3 only has six electrons around it after forming bonds with the three Chlorines, falling short of the octet rule.

Form Coordinate Covalent Bond

Since Oxygen in water has two lone pairs, it can donate one pair to Boron, creating a coordinate covalent bond. This will complete Boron's octet without changing the number of valence electrons around Oxygen.

Key concepts

Lewis Structures

Understanding how atoms connect in a molecule is fundamental in the study of chemistry, and Lewis structures provide a visual representation of these connections. To illustrate this, we use dots to symbolize the valence electrons of atoms, which are the electrons found in the outermost shell. These dots are placed around the chemical symbols of the elements to show potential bonds and lone pairs.

For instance, in our exercise, we see that Boron is surrounded by three Chlorine atoms in the molecule BCl3, each sharing an electron pair to form a covalent bond. Lewis structures are vital for predicting the shape and reactivity of molecules, making them an essential tool in chemistry.

Valence Electrons

A core concept in chemistry is the role of valence electrons. These are the electrons that reside in the outermost shell of an atom and are responsible for the chemical properties of the element. They are mainly involved in the formation of chemical bonds. To determine the number of valence electrons, one can refer to the element's group number on the periodic table, except for transition metals which have varying valence electrons.

In our exercise, Boron has three valence electrons while each Chlorine atom has seven. Oxygen has six, and Hydrogen has one. Knowing the count of valence electrons is a stepping-stone to predicting how atoms will bond, and this is why it's crucial for mapping out Lewis structures.

Octet Rule

The octet rule is a chemical rule of thumb that reflects the observation that atoms of main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, thereby obtaining a noble gas configuration. This tendency is a driving force for atoms when they form covalent bonds.

In our example, the Boron atom doesn't satisfy the octet rule after forming regular covalent bonds with three Chlorine atoms, as it ends up with only six electrons. It requires two more electrons to complete its octet, which is why it seeks a coordinate covalent bond with a water molecule, using the lone pair of electrons from Oxygen.

Molecular Bonding

To comprehend how molecules take shape and remain together, we explore the concept of molecular bonding. Covalent bonds form when two atoms share a pair of valence electrons, but when one atom provides both electrons for a shared pair, the bond is a special type of covalent bond known as a coordinate covalent bond.

In the exercise's final step, a coordinate covalent bond is formed when Oxygen from the water molecule donates a lone pair of electrons to the electron-deficient Boron in BCl3, thereby completing Boron's octet. Molecular bonding dictates not only the stability of the molecules formed but also their 3D shape and, as a result, how molecules interact with each other.