Question

Which of the following substances contains an atom that does not follow the octet rule? (a) \(\mathrm{AlCl}_{3}\) (b) \(\mathrm{PCl}_{3}\) (c) \(\mathrm{PCl}_{5}\) (d) \(\mathrm{SiCl}_{4}\)

Short answer

(a) \(\mathrm{AlCl}_{3}\)

Step-by-step solution

Recall the Octet Rule

The octet rule states that atoms tend to form compounds in ways that give them eight valence electrons, completing their outer shell. Elements that typically follow this rule are found in the second period of the periodic table.

Evaluate Each Substance

Let's go through each compound to see which element might not follow the octet rule:- (a) \(\mathrm{AlCl}_{3}\): Aluminum (Al) in \(\mathrm{AlCl}_{3}\) typically has only six electrons in its valence shell, and thus it does not complete an octet.- (b) \(\mathrm{PCl}_{3}\): Phosphorus (P) has five valence electrons and forms three bonds with chlorine, completing the octet.- (c) \(\mathrm{PCl}_{5}\): In this case, phosphorus exceeds the octet by having ten electrons in its valence shell.- (d) \(\mathrm{SiCl}_{4}\): Silicon (Si) forms four bonds with chlorine and satisfies the octet rule with eight electrons.

Identify the Exception

Identify which atom in the compounds deviates from having eight electrons. In \(\mathrm{AlCl}_{3}\), aluminum only has six valence electrons, whereas in \(\mathrm{PCl}_{5}\), phosphorus exceeds its octet.

Conclusion

Given the options, \(\mathrm{AlCl}_{3}\) contains aluminum which does not follow the octet rule because it does not achieve eight electrons. Hence, \(\mathrm{AlCl}_{3}\) is the substance where an atom definitively does not follow the octet rule.

Key concepts

Valence Electrons

Valence electrons are the electrons found in the outermost shell of an atom. These electrons play a crucial role in chemical reactions and bonding. Knowing the number of valence electrons helps predict how an element will interact with others in forming bonds.

• Each element in a group of the periodic table has the same number of valence electrons.
• For example, the group 1 elements, such as sodium and lithium, have one valence electron.
• Elements in the noble gas group have a complete set of eight valence electrons, making them generally unreactive.

The concept of valence electrons is essential for understanding why and how atoms bond. The tendency of atoms to reach a stable electron configuration similar to noble gases often drives chemical reactions.

Chemical Bonding

Chemical bonding involves the interaction of atoms to achieve stability through electron exchange or sharing. These interactions result in the formation of molecules and compounds.

• Covalent bonds occur when atoms share pairs of valence electrons.
• Ionic bonds form when electrons are transferred from one atom to another, resulting in charged ions that attract each other.
• Metallic bonds involve delocalized electrons shared across many atoms, typical in metals.

Understanding chemical bonds helps explain molecular structure and properties. For instance, the shape and arrangement of molecules govern properties like boiling point, solubility, and electrical conductivity.

Exceptions to the Octet Rule

While the octet rule is a useful guideline to understand chemical bonding, there are several exceptions. Certain elements either do not fill their valence shell with eight electrons or can accommodate more.

• Some elements like hydrogen and helium are stable with two valence electrons.
• Elements such as aluminum, found in \(\mathrm{AlCl}_{3}\), can be stable with fewer than eight electrons. Aluminum often has only six valence electrons.
• Phosphorus, when forming \(\mathrm{PCl}_{5}\), can exceed the octet rule by utilizing d orbitals to hold more than eight electrons.
• In some molecules, elements can have an odd number of electrons or form complex structures like radicals.

Noting these exceptions is crucial for understanding real-world chemical reactions and the behaviors of different substances.