Question

a When Xe and Pt are treated wath fuorine gas under pressure at \(200^{0} \mathrm{C}\), the ionic solid \(\mathrm{XeF}_{5}+\mathrm{PtF}_{6}^{-}\)is formed. Write the Lewis structure for \(\mathrm{XeF}_{5}+\) and, from VSEPR theory, predict its shape. (b) When two molecules of XeF_ react with one molecule of AsF 5 , the resulting compound has the formala \(\mathrm{Xe}_{2} \mathrm{~F}_{3}{ }^{+} \mathrm{AsF}_{6}{ }^{-}\). Predict the Lewis structure of the \(\mathrm{Xe}_{2} \mathrm{~F}_{3}{ }^{*}\) ion.

Short answer

The Lewis structure for XeF5+ ion has Xe in the center with five F atoms around it and one lone pair, having a square pyramidal shape. For the Xe2F3+ ion, two Xe atoms are bonded, with three F atoms surrounding one of the Xe atoms, forming a complex structure that is not symmetric.

Step-by-step solution

Counting Valence Electrons for XeF5+

For the XeF5+ ion, calculate the total number of valence electrons. Xenon (Xe) has 8 valence electrons as a noble gas. Each fluorine (F) atom has 7 valence electrons, with 5 fluorine atoms that would account for 35 valence electrons. Also, since the molecule has a +1 charge, one electron is removed. Total valence electrons = 8 (Xe) + 5*7 (for F) - 1 (positive charge) = 42 electrons.

Draw the Lewis Structure for XeF5+

Draw the Lewis structure with Xe in the center and arrange five F atoms around it. Make sure each F atom gets 8 valence electrons to complete their octets. Connect each F to the Xe with single bonds, and assign the remaining valence electrons to complete the octet on Xe. After doing this, Xe will have a 10-electron configuration, which is acceptable for elements in the third period and beyond due to their ability to have expanded octets.

Predicting the Shape Using VSEPR Theory

With five bonding pairs and one lone pair of electrons around xenon, the VSEPR theory predicts a square pyramidal shape for the XeF5+ ion.

Counting Valence Electrons for Xe2F3+

Xenon has 8 valence electrons and fluorine has 7. Each Xe2F3+ ion has 2 xenon atoms and 3 fluorine atoms. Total valence electrons = 2*8 (for Xe) + 3*7 (for F) – 1 (positive charge) = 47 valence electrons.

Draw the Lewis Structure for Xe2F3+

Draw two Xe atoms bonded together. Arrange three F atoms around one of the Xe atoms. Distribute the valence electrons, ensuring each F has a complete octet and the xenon atoms share electrons through a bonding pair. With 47 electrons, there will be some lone pairs on the xenon atoms.

Predicting the Shape of Xe2F3+ Ion

For the Xe2F3+ ion, due to the presence of three bonded pairs and multiple lone pairs on the xenon atoms, the precise shape prediction requires considering both steric effects and repulsion between lone pairs and bonding pairs. This can lead to a less symmetrical shape.

Key concepts

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. The premise of this theory is that electron pairs in the valence shell will arrange themselves to be as far apart as possible to minimize repulsion. This is due to the negative charges of electrons repelling each other. VSEPR theory helps us understand the three-dimensional structure of molecules, which, in turn, influences the compound's reactivity, polarity, phase of matter, color, magnetism, and biological activity.

The VSEPR model assigns a geometrical shape to molecules depending on the number of bonding electron pairs (groups where electrons are shared between atoms) and lone pairs (pairs of electrons that are not shared between atoms) around the central atom. For example, a central atom with four bonding pairs of electrons would be predicted to have a tetrahedral shape, as seen in the VSEPR theory's prediction of the shape for the XeF5+ ion as square pyramidal based on its five bonding pairs and one lone pair.

Chemical Bonding

Chemical bonding is the process where atoms combine to form molecules. This occurs by sharing or transferring valence electrons, the electrons that are found in the outermost shell of an atom. There are three primary types of chemical bonds:

• Ionic bonds — formed when one atom donates an electron to another, resulting in positive and negative ions that attract each other.
• Covalent bonds — occur when atoms share pairs of electrons, creating a stronger connection between the atoms.
• Metallic bonds — where electrons are shared among a lattice of metal atoms.

For example, in the XeF5+ ion, xenon and fluorine form covalent bonds, where electrons are shared, and this sharing of valence electrons allows the completion of the octet for the fluorine atoms while xenon adopts an expanded octet. Understanding chemical bonding is crucial for predicting molecular structure and stability.

Valence Electrons

Valence electrons play a critical role in chemical bonding as they are the ones that interact with other atoms. An atom's chemical properties are defined by its number of valence electrons, and elements in the same group of the periodic table typically have the same number of valence electrons. For instance, elements in Group 1 have one valence electron, leading to their reactivity and tendency to form +1 ions.

In the exercises provided, valence electrons are counted for XeF5+ and Xe2F3+. The noble gas xenon has 8 valence electrons, while each fluorine has 7. The charge on the ions, such as the +1 on XeF5+, requires the removal of one valence electron from the total count. Correctly counting valence electrons is the first step in predicting molecular structure through Lewis structures and VSEPR theory.

Molecular Geometry

Molecular geometry, also referred to as molecular shape, describes the three-dimensional arrangement of atoms within a molecule. Understanding molecular geometry is crucial as it determines many physical and chemical properties of a substance, including reactivity, polarity, phase, color, and biological activity. The geometry is influenced by the number of electron pairs in the valence shell of the central atom, bonded atoms, and lone pairs of electrons.

The various possible shapes include linear, bent, tetrahedral, trigonal planar, trigonal bipyramidal, octahedral, and square pyramidal, with deviations and adjustments for lone pairs of electrons which cause different repulsion than bonding pairs. For example, the XeF5+ ion is predicted to have a square pyramidal shape because of its five bonding pairs and one lone pair of electrons. The Xe2F3+ is more complex due to multiple lone pairs, impacting the final shape of the molecule. Accurate prediction of molecular geometry is a key component in understanding the nature of chemical substances and their interactions.