Question

Doping Agent Material scientists are studying the properties of polymer plastics doped with \(\mathrm{AsF}_{5}\). Explain why the compound \(\mathrm{AsF}_{5}\) is an exception to the octet rule.

Short answer

The compound AsF5 is an exception to the octet rule because the central atom, arsenic (As), has an expanded valence shell with 10 electrons rather than the usual 8 electrons. This occurs due to arsenic having five valence electrons available for bonding and forming bonds with five fluorine atoms, each contributing one electron. The resulting electron configuration of AsF5 shows that As has 10 electrons in its valence shell.

Step-by-step solution

Review the octet rule

The octet rule states that elements tend to combine with other elements in a way that allows each atom to achieve a stable electron configuration, typically by having eight electrons in their outermost (valence) shell. This is similar to the electron configuration of noble gases, which are the most stable elements.

Identify the elements in the compound AsF5

In the compound AsF5, there are two elements involved: arsenic (As) and fluorine (F).

Determine the electron configuration of As and F

Arsenic (As) has an atomic number of 33, so it has 33 electrons. Its electron configuration is: \[1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^{3}\] Fluorine (F) has an atomic number of 9, so it has 9 electrons. Its electron configuration is: \[1s^{2}2s^{2}2p^{5}\]

Investigate the electron configuration in AsF5

In AsF5, arsenic is the central atom, and it forms chemical bonds with five fluorine atoms. Each fluorine atom is going to share one electron with arsenic in order to complete its octet in the 2p orbital. The valence electron configuration of As is 4s2 4p3, which means it has five valence electrons available for bonding. When it bonds with five fluorine atoms, the valence electrons in the 4s and 4p orbitals are shared with the fluorine atoms, resulting in the following electron configuration: \[As: \underline{4s^{2}4p^{3}} \to 4s^{0}4p^{10}\] This configuration implies that the arsenic atom is surrounded by 10 electrons in its valence shell, which is an exception to the octet rule.

Describe why AsF5 is an exception to the octet rule

The compound AsF5 is an exception to the octet rule because the central atom, arsenic (As), is surrounded by 10 electrons in its valence shell, rather than the usual 8 electrons. This occurs because arsenic has five valence electrons available for bonding, and it forms bonds with five fluorine atoms, each of which contributes one electron. The resulting electron configuration of AsF5 shows that As has an expanded valence shell with 10 electrons.

Key concepts

Electronic Configuration

Electronic configuration refers to the distribution of electrons in an atom or molecule across different energy levels and orbitals. The electronic configuration of an element is key to understanding its chemical behavior, particularly its bonding capabilities.

For example, arsenic (As) has the electron configuration listed in the solution, with the valence electrons being in the 4s and 4p orbitals. Valence electrons are the outermost electrons and are crucial to the formation of chemical bonds. An expanded valence shell occurs when elements in periods 3 and beyond in the periodic table use available d-orbitals to accommodate more than eight electrons, as seen in the case of AsF₅.

In this molecule, arsenic's d-orbitals come into play, allowing it to hold more than eight valence electrons, a phenomenon well-explained by considering its complete electron configuration. In-depth understanding of electronic structures is essential, as exceptions to the octet rule are typically associated with elements that have these available d-orbitals.

Chemical Bonding

Chemical bonding is the force that holds atoms together in a molecule or compound, and understanding the electronic configuration is vital for grasping why and how atoms bond. There are different types of chemical bonds, including ionic, covalent, and metallic bonds.

In the case of AsF₅, covalent bonds are formed between the arsenic and fluorine atoms. Covalent bonding involves the sharing of valence electrons between atoms, and the number of valence electrons dictates how many bonds an atom can make. With As being capable of expanding its electron capacity beyond the octet, it bonds with five fluorine atoms rather than the expected three, thereby forming a sometimes unexpected bond geometry.

When students visualize these bonding patterns and how the orbitals overlap to share electrons, it helps clarify why the compound AsF₅ exhibits an exception to the octet rule, emphasizing the interconnectedness of electronic configuration and bonding concepts.

Valence Electrons

Valence electrons are the electrons that reside in an atom's outermost shell and participate in chemical bonding. They determine the atom's ability to gain, lose, or share electrons, and are hence central to an atom's chemical properties.

For elements that follow the octet rule, achieving a complete set of eight valence electrons is the goal, mirroring the noble gas electron configuration. However, arsenic in AsF₅ surpasses this by having ten valence electrons after bonding. By acknowledging and analyzing valence electrons simplifies predicting how an atom will chemically react and whether it will obey or break the octet rule.

In the exercise, understanding that the valence electron configuration for As is 4s² 4p³ before bonding, and recognizing how these electrons pair with those from the fluorine atoms is crucial for a deep comprehension of why AsF₅ doesn't conform to the octet norm. This insight brings high significance to valence electrons when discussing chemical reactions and bonding exceptions.