Question

Which of the following covalent molecules in an exception of octet rule? (a) \(\mathrm{CH}_{4}\) (b) \(\mathrm{CO}_{2}\) (c) \(\mathrm{BeCl}_{2}\) (d) \(\mathrm{H}_{2} \mathrm{O}\)

Short answer

 ext{BeCl}_2 is the exception to the octet rule.

Step-by-step solution

Understanding the Octet Rule

The octet rule states that atoms tend to form bonds so that each atom has eight electrons in its valence shell. This is similar to the electron configuration of a noble gas.

Analysis of  ext{CH}_4

In  ext{CH}_4 (methane), carbon forms four bonds with hydrogen atoms. Carbon has four valence electrons that it shares with four hydrogen atoms (each contributing one), filling its valence shell to satisfy the octet rule.

Analysis of  ext{CO}_2

In  ext{CO}_2 (carbon dioxide), carbon forms double bonds with each of the two oxygen atoms. Carbon shares four of its electrons (two with each oxygen), allowing it to satisfy the octet rule.

Analysis of  ext{BeCl}_2

In  ext{BeCl}_2 (beryllium chloride), beryllium forms two single bonds with each chlorine atom. Beryllium only has two valence electrons, which it shares with chlorine atoms. This gives beryllium four electrons in its valence shell, which does not satisfy the octet rule.

Analysis of  ext{H}_2 ext{O}

In  ext{H}_2 ext{O} (water), oxygen forms two single bonds with two hydrogen atoms. Oxygen shares two of its six valence electrons with hydrogen atoms and retains additional electrons to satisfy the octet rule.

Conclusion

Among the given options, only  ext{BeCl}_2 does not satisfy the octet rule for beryllium. Other molecules follow the octet rule for their central atoms.

Key concepts

Covalent Bonding

Covalent bonding is a type of chemical bond where two atoms share electrons. This sharing allows each atom to attain the electron configuration similar to noble gases, fulfilling the octet rule in many cases. For instance, in a molecule like octane, two carbon atoms share four electrons (two from each atom) to form a double bond, giving them both a filled outer shell.

• These bonds typically occur between nonmetal atoms.
• An example is the bond between two hydrogen atoms in \(\text{H}_{2}\), where they share a single pair of electrons.
• They can be single, double, or triple bonds, depending on the number of shared electron pairs.

Covalent bonds are characterized by the sharing of one or more pairs of valence electrons, which leads us to the importance of understanding valence electrons.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining how atoms bond with each other. They participate directly in the formation of chemical bonds. Most atoms strive to achieve a stable configuration similar to noble gases through these electrons.

• The number of valence electrons dictates how many bonds an atom can form.
• For example, carbon has four valence electrons and can form four bonds, as seen in methane (\(\text{CH}_{4}\)).

Beryllium in \(\text{BeCl}_{2}\) is a good example of how an element might not satisfy the typical tendency to achieve eight valence electrons, making it an exception to the octet rule. Understanding the role of valence electrons is fundamental in predicting molecular structure.

Molecular Structure

Molecular structure refers to the three-dimensional arrangement of atoms within a molecule. The structure determines how a molecule interacts with other molecules and its overall stability. Generally, molecules adopt shapes that enable them to maintain optimal distances between electronegative areas.

• Predicting the structure involves understanding both the covalent bonds and lone pairs of electrons.
• Water (\(\text{H}_{2}\text{O}\)) adopts a bent shape due to its two lone pairs of electrons, affecting interactions like hydrogen bonding.
• Linear structures, such as those found in \(\text{CO}_{2}\), occur when there are no lone pairs on the central atom, causing the bonds to be at maximum angles from one another.

The shape can significantly affect the chemical reactivity and physical properties of the molecule.

Exceptions to Octet Rule

While the octet rule is a useful guideline indicating that atoms tend to complete eight electrons in their valence shell, there are notable exceptions. Some molecules do not follow this rule due to the nature of the atoms involved.

• Elements like beryllium and boron have fewer valence electrons and thus form stable compounds without fulfilling the octet rule.
• For example, beryllium forms \(\text{BeCl}_{2}\) with just four electrons in its valence shell.
• Other exceptions include expanded octets often found in elements in the third period or higher, such as phosphorus and sulfur, which can hold more than eight electrons.
• Molecules with an odd number of total valence electrons can also defy the octet rule.

These exceptions highlight the versatility and complexity of chemical bonding. Understanding them is key to mastering the concept of molecular structure and reactions.