Question

Write the Lewis structure for each molecule (octet rule not followed). \begin{equation}\mathrm{ a.} {BCl}_{3} \quad \text { b. } \mathrm{NO}_{2} \quad \text { c. } \mathrm{BH}_{3}\end{equation}

Short answer

BCl3: Boron central atom with single bonds to three Cl; NO2: Nitrogen central atom with one single bond and one double bond to two O atoms, with one unpaired electron; BH3: Boron central atom with single bonds to three H atoms.

Step-by-step solution

Drawing Lewis Structure for BCl3

Start by arranging the valence electrons. Boron is the central atom with 3 valence electrons, and each chlorine has 7 valence electrons. Draw a single bond between Boron and each Chlorine atom. Distribute the remaining valence electrons to satisfy the octet for each chlorine. Note that in BCl3, Boron does not follow the octet rule—it has only 6 electrons in its valence shell.

Drawing Lewis Structure for NO2

Nitrogen is the central atom with 5 valence electrons, and each oxygen has 6 valence electrons. Form a single bond between Nitrogen and each Oxygen atom. One oxygen must form a double bond with Nitrogen to give it a formal charge closest to zero; thus, Nitrogen has an unpaired electron and does not follow the octet rule.

Drawing Lewis Structure for BH3

Boron is again the central atom with 3 valence electrons, and each hydrogen has 1 valence electron. Connect each Hydrogen to Boron with a single bond. Boron, with only three bonds to Hydrogen, does not follow the octet rule, as it only has 6 valence electrons.

Key concepts

Octet Rule Exceptions

Understanding the octet rule and its exceptions is vital in chemistry, especially when dealing with molecules that don’t conform to the traditional rule. The octet rule states that atoms tend to form bonds in such a way that they have eight electrons in their valence shell, giving them the same electronic arrangement as a noble gas. This leads to stability in the molecule. But not all elements follow this rule. Boron, for example, is often found with less than eight electrons in its valence shell. In the case of BCl₃ and BH₃, boron forms only three bonds, resulting in six valence electrons instead of the usual eight.

Similarly, odd-electron molecules like NO₂ have an odd number of total valence electrons accounting for their failure to conform to the octet rule. NO₂ has 17 valence electrons, which makes complete pairing impossible and leads to the presence of an unpaired electron in the Lewis structure. It is important to note that these exceptions occur primarily with elements in period two of the periodic table, such as boron and nitrogen, which are small enough to accommodate fewer than eight electrons.

Valence Electrons

Valence electrons play a pivotal role in chemical bonding and molecular structure. These are the electrons located in the outermost shell of an atom. They are significant because they participate in chemical reactions and bond formation. In the process of drawing Lewis structures, the correct arrangement of valence electrons is critical for representing molecular structure.

Starting with the central atom, the valence electrons are distributed to form bonds with the surrounding atoms. Each bond, whether single, double, or triple, usually contains two electrons; one from each of the bonding atoms. In our exercises, for example, boron has three valence electrons which it uses to form single bonds with three chlorine atoms in BCl₃, or with three hydrogen atoms in BH₃. This leaves boron with less than an octet. Nitrogen, having five valence electrons, can form a total of four bonds (as in NH₄) but in NO₂ it forms two bonds and retains an unpaired electron, illustrating the flexibility and complexity of valence electron distribution.

Molecular Structure

When analyzing molecular structure, we look at the way in which atoms are arranged and bonded within a molecule. The Lewis structures highlight the bonding between atoms as well as any non-bonding valence electrons. In BCl₃, the boron atom forms a trigonal planar structure with 120-degree bond angles, which is typical for a BCl₃ molecule with an incomplete octet.

Nitrogen dioxide (NO₂), meanwhile, presents a bent geometry due to the presence of an unpaired electron causing repulsion that influences the molecular shape. BH₃ is another unique molecule; it typically exists in dimer forms, such as B₂H₆, because the simple BH₃ does not fulfill the stable octet. These differences in molecular structure have profound effects on the properties of the substances, including reactivity, polarity, and the types of reactions they participate in.