Problem 76 - Chemistry: The Central Science in SI Units, Global Edition

Question

(a) Why does xenon react with fluorine, whereas neon does not? (b) Using appropriate reference sources, look up the bond lengths of Xe-F bonds in several molecules. How do these numbers compare to the bond lengths calculated from the atomic radii of the elements?

Step-by-Step Solution

Answer

Xenon reacts with fluorine because of its ability to form "d" or "f" orbitals and its larger atomic size compared to neon, which cannot form such orbitals and is unreactive to fluorine. The bond lengths of Xe-F bonds found in reference sources (e.g., 197.5 pm for XeF₂, 196.1 pm for XeF₄, and 193.5 pm for XeF₆) are generally longer than those calculated using atomic radii (172 pm). This difference could result from factors like electron repulsion and chemical bonding properties not considered in a simple atomic radii calculation.

Step 1: Understanding the properties of inert gases

Inert gases, or noble gases, are elements in Group 18 of the periodic table. They have a full outer electron shell, which makes them relatively non-reactive. Xenon (Xe) and neon (Ne) are both members of this group. However, we need to delve into the differences in their electron configurations to understand why xenon can form compounds with fluorine while neon cannot.

Step 2: Electron Configurations

Neon, with an atomic number of 10, has an electron configuration of 1s² 2s² 2p⁶. This configuration gives neon a complete outer electron shell, making it very stable and resistant to reactions with other elements. On the other hand, xenon has an atomic number of 54, and its electron configuration is [Kr] 5s² 4d¹⁰ 5p⁶. Although xenon also has a full outer electron shell, it has additional energy levels and electron shells compared to neon. This makes xenon less resistant to forming bonds with highly electronegative elements such as fluorine.

Step 3: Reactivity of Xenon and Neon with Fluorine

Xenon reacts with fluorine because it can form "d" or "f" orbitals and accommodate additional electrons due to its larger atomic size and more complex electron configuration. On the other hand, neon cannot form these orbitals and therefore remains unreactive to fluorine.

Step 4: Looking Up Xe-F Bond Lengths

Using reference sources such as scientific journals or online databases, you can look up the bond lengths of Xe-F bonds in several molecules. As an example, let us consider these three molecules: XeF₂, XeF₄, and XeF₆. According to reference sources, their Xe-F bond lengths are: 1. XeF₂: 197.5 pm 2. XeF₄: 196.1 pm 3. XeF₆: 193.5 pm

Step 5: Calculating Bond Lengths Using Atomic Radii

To calculate the bond lengths from the atomic radii of xenon and fluorine elements, we can use the sum of their atomic radii. The atomic radius of xenon is 108 pm, and the atomic radius of fluorine is 64 pm. Adding these together, we get: Bond length(Xe-F) = Atomic radius(Xe) + Atomic radius(F) = 108 pm + 64 pm = 172 pm

Step 6: Comparison of Bond Lengths

Now that we have the bond lengths from both reference sources and the calculation using atomic radii, we can compare them: 1. XeF₂: 197.5 pm (reference) vs. 172 pm (calculated) 2. XeF₄: 196.1 pm (reference) vs. 172 pm (calculated) 3. XeF₆: 193.5 pm (reference) vs. 172 pm (calculated) As seen from the comparison, the bond lengths found in reference sources for Xe-F bonds are generally longer than those calculated from atomic radii. This difference could be due to various factors, such as electron repulsion and chemical bonding properties that are not considered in a simple atomic radii calculation.

Key Concepts

Xenon

Xenon is a noble gas, part of the Group 18 elements on the periodic table. Unlike most noble gases, xenon can form chemical bonds under certain conditions. This is surprising because noble gases are typically unreactive due to their complete electron shells.
Xenon, with an atomic number of 54, has a larger atomic structure compared to lighter noble gases like neon. Its electron configuration is more complex:

• It consists of the inner core electron configuration of krypton, \( [Kr] \),
• along with its own electrons: \( 5s^2 4d^{10} 5p^6 \).

This unique configuration means xenon can interact with other elements, like fluorine, under conditions that involve a high-energy input, allowing reaction despite its complete shell.

Fluorine

Fluorine is the most electronegative element in the periodic table, which means it has a strong tendency to attract electrons towards itself. This characteristic makes it highly reactive and capable of forming stable bonds with elements that can provide electrons.
Fluorine's high reactivity is due to:

• A small atomic radius that allows for a strong attraction to bonding electrons.
• Its electron configuration, \( 1s^2 2s^2 2p^5 \), missing just one electron to complete its outer shell.

This need to gain an electron leads to the formation of strong covalent bonds with other elements, even those like xenon that are typically nonreactive.

Electron Configuration

Electron configuration is the distribution of electrons of an atom in its atomic or molecular orbitals. It helps explain an element's chemical characteristics, including its reactivity and bonding behavior.
For noble gases like xenon and neon:

• Neon has a simpler electron configuration, \( 1s^2 2s^2 2p^6 \), with a full outer shell, making it very stable and unreactive.
• Xenon, however, has additional electron shells: \( [Kr] 5s^2 4d^{10} 5p^6 \), which provide more space for potential bonding.

The ability to use 'd' or 'f' orbitals in xenon's configuration allows it to interact with highly electronegative elements like fluorine under specific conditions.

Atomic Radii

Atomic radii refer to the size of an atom from its nucleus to the outer boundary of the surrounding cloud of electrons. This size influences how atoms interact chemically. Typically, atomic radius decreases across a period. For xenon and fluorine, understanding atomic radii helps in calculating bond lengths:

• Xenon's atomic radius: 108 pm. Such a large size compared to other noble gases allows it to interact as it has more available space for bonding.
• Fluorine's atomic radius: 64 pm. Its small size enables it to get closer to other nuclei, facilitating strong bonds due to its electronegativity.

Knowing these radii lets us estimate expected bond lengths, although real bonds can be affected by additional factors not accounted for by simple radius addition alone, such as electron density or repulsion.

Chemical Bonding

Chemical bonding is the process of atoms combining to form molecules and compounds. It explains how elements combine to create various substances. In the context of xenon and fluorine, bonding occurs despite xenon being a noble gas, known for having a full valence shell.Xenon reacts with fluorine through a type of chemical bond called a covalent bond, where electrons are shared between atoms. It can do this because xenon:

• Has space to expand its valence shell into available \( d \) orbitals,
• Can stabilize itself while sharing electrons with highly electronegative fluorine.

The resulting Xe-F bonds vary in length due to differences in molecular structure, electron pair repulsion, and the specific environmental conditions surrounding the bond formation.