Question

Decide whether the indicated atom obeys the octet rule. If not, indicate how the octet rule is broken. (a) \(\mathrm{B}\) in \(\mathrm{BCl}_{3}\) (c) \(\mathrm{F}\) in \(\mathrm{F}_{2}\) (b) \(\mathrm{Cl}\) in \(\mathrm{ClF}_{2}{ }^{-}\)ion (d) \(\mathrm{S}\) in \(\mathrm{SF}_{6}\)

Short answer

Boron in BCl3 does not obey the octet rule, it lacks 2 electrons. Fluorine in F2 obeys the octet rule. Chlorine in ClF2- ion does not obey the octet rule, it exceeds the octet by 2 electrons. Sulfur in SF6 also does not obey the octet rule; it exceeds the octet by 4 electrons.

Step-by-step solution

Check Boron in BCl3

Boron (B) has three valence electrons. It forms three covalent bonds with Chlorine (Cl) in BCl3, in which each Chlorine atom provides 1 electron. So, Boron has six (3 original and 3 from Chlorine atoms) electrons instead of eight. Conclusion: Boron in BCl3 does not obey the octet rule. It lacks 2 electrons.

Check Fluorine in F2

Fluorine (F) has seven valence electrons. In F2, Fluorine forms a single covalent bond with another Fluorine atom, that contributes one electron. So, Fluorine now has eight valence electrons. Conclusion: Fluorine in F2 obeys the octet rule.

Check Chlorine in ClF2- ion

Chlorine (Cl) has seven valence electrons. In ClF2-, Chlorine forms 2 single covalent bonds with two Fluorine atoms and also has an additional electron from negative charge. This accounts for 10 valence electrons to Chlorine (7 original, 2 from Fluorine atoms and 1 extra). Conclusion: Chlorine in ClF2- does not obey the octet rule. It exceeds the octet by 2 electrons.

Check Sulfur in SF6

Sulfur (S) has six valence electrons. In SF6, Sulfur forms 6 covalent bonds with 6 Fluorine atoms that contribute a total of six electrons. Thus, Sulfur now has twelve valence electons. Conclusion: Sulfur in SF6 does not obey the octet rule. It exceeds the octet by 4 electrons.

Key concepts

Valence Electrons

Valence electrons are the electrons that reside in the outermost shell of an atom's electron configuration. These are the electrons involved in forming bonds with other atoms. Most elements follow a pattern called the octet rule, which states that atoms tend to form bonds until they have eight valence electrons, thereby achieving a stable electron configuration similar to that of noble gases.

For example, in the exercise, Fluorine (F) in F₂ obeys the octet rule by sharing one valence electron with another Fluorine atom to form a covalent bond, thus achieving a full octet. On the other hand, Boron in BCl₃ only has six valence electrons after bonding, which indicates that not all elements obey the octet rule rigidly. Some, particularly those in the third row of the periodic table or beyond, such as Sulfur (S) in SF₆, can accommodate more than eight electrons due to the availability of d-orbitals.

Covalent Bonds

Covalent bonds are a type of chemical bond where pairs of electrons are shared between atoms. These shared electrons count as part of each atom's valence shell. A single covalent bond consists of one shared pair of electrons. Covalent bonding typically occurs between nonmetal atoms that have similar electronegativities. When atoms share electrons, they're better able to reach the stable octet configuration that's energetically favorable.

In the context of our exercise, the Boron atom in BCl₃ forms three single covalent bonds with three Chlorine atoms. However, even with these bonds, Boron doesn't achieve an octet, showcasing that there are exceptions to the octet rule. In contrast, each Fluorine atom in F₂ reaches an octet by sharing two electrons through a single bond, demonstrating an ideal covalent bond in action.

Electron Configuration

Electron configuration refers to the distribution of electrons among the various orbitals around the nucleus of an atom. This distribution follows specific rules, such as the Pauli Exclusion Principle and Hund's Rule, to minimize the energy of the atom. The electron configuration can significantly impact an atom's chemical properties and its ability to interact with other atoms.

The valence shell is the most involved in chemical bonding. In the exercise, Chlorine (Cl) in the ClF₂^- ion goes against the typical electron configuration that would result from following the octet rule. Instead of eight valence electrons, it possesses ten. This is an example of an expanded valence shell, which is permissible for elements located below Period 2 on the periodic table due to the presence of vacant d orbitals that can be occupied. Similarly, Sulfur in SF₆ can expand beyond the octet due to its position on the periodic table and its available d orbitals. These exceptions highlight the flexible nature of electron configuration for specific elements.