Question

Draw the Lewis structure, and then specify the electron-pair and molecular geometries for each of the following molecules or ions. Identify the hybridiration of the central atom. (a) \(\operatorname{SiF}_{6}^{2-}\) (b) \(\mathrm{SeF}_{4}\) (c) \(1 \mathrm{Cl}_{2}^{-}\) (d) \(\mathrm{XeF}_{4}\)

Short answer

(a) Octahedral; sp^3d^2. (b) Seesaw; sp^3d. (c) Linear; sp^3d. (d) Square planar; sp^3d^2.

Step-by-step solution

Determine Total Valence Electrons for SiF6^2-

Silicon (Si) has 4 valence electrons and each fluorine (F) has 7. The molecule has 6 fluorine atoms and a -2 charge, adding 2 more electrons. Total valence electrons = 4 (Si) + 6 * 7 (F) + 2 (-2 charge) = 48 electrons.

Draw Lewis Structure for SiF6^2-

Place the Si atom in the center surrounded by the six F atoms. Distribute the 48 valence electrons, forming a single bond between Si and each F. Each F ends with 6 valence electrons as lone pairs, achieving an octet, and Si shares one electron with each F.

Determine Electron Pair Geometry of SiF6^2-

The electron pair geometry is octahedral because there are 6 regions of electron density around the central Si atom.

Determine Molecular Geometry of SiF6^2-

With 6 bonded pairs and no lone pairs, the molecular geometry is also octahedral.

Identify Hybridization of Si in SiF6^2-

The central Si atom is sp^3d^2 hybridized due to six areas of electron density forming an octahedral shape.

Determine Total Valence Electrons for SeF4

Selenium (Se) has 6 valence electrons and each fluorine (F) has 7. The molecule has 4 fluorine atoms. Total valence electrons = 6 (Se) + 4 * 7 (F) = 34 electrons.

Draw Lewis Structure for SeF4

Place the Se atom in the center and form single bonds with the four F atoms, using 8 electrons. The remaining 26 valence electrons are used to complete the octets of F atoms (24 electrons) and place a lone pair on the central Se (2 more electrons).

Determine Electron Pair Geometry of SeF4

The electron pair geometry is trigonal bipyramidal, with 4 bond pairs and 1 lone pair.

Determine Molecular Geometry of SeF4

The molecular geometry is seesaw due to the presence of one lone pair in a trigonal bipyramidal electron pair geometry.

Identify Hybridization of Se in SeF4

The central Se atom is sp^3d hybridized as there are five regions of electron density.

Determine Total Valence Electrons for ICl2^-

Iodine (I) has 7 valence electrons and each chlorine (Cl) has 7. The -1 charge adds 1 electron. Total valence electrons = 7 (I) + 2 * 7 (Cl) + 1 (-1 charge) = 22 electrons.

Draw Lewis Structure for ICl2^-

Place the I atom in the center and form single bonds with two Cl atoms, using 4 electrons. Place 3 lone pairs around I to use the remaining 18 electrons, completing the octets.

Determine Electron Pair Geometry of ICl2^-

The electron pair geometry is trigonal bipyramidal with 2 bond pairs and 3 lone pairs.

Determine Molecular Geometry of ICl2^-

The molecular geometry is linear due to the arrangement of lone pairs arranging themselves to minimize repulsion.

Identify Hybridization of I in ICl2^-

The central I atom is sp^3d hybridized as there are five regions of electron density.

Determine Total Valence Electrons for XeF4

Xenon (Xe) has 8 valence electrons and each fluorine (F) has 7. The molecule has 4 fluorine atoms. Total valence electrons = 8 (Xe) + 4 * 7 (F) = 36 electrons.

Draw Lewis Structure for XeF4

Place Xe in the center and form single bonds with the four F atoms, using 8 electrons. Place lone pairs on each F to use 24 more electrons, and place two lone pairs on Xe with remaining 4 electrons.

Determine Electron Pair Geometry of XeF4

The electron pair geometry is octahedral, with 4 bond pairs and 2 lone pairs.

Determine Molecular Geometry of XeF4

The molecular geometry is square planar, as lone pairs are opposite each other and the F atoms form a square around Xe.

Identify Hybridization of Xe in XeF4

The central Xe atom is sp^3d^2 hybridized because there are six regions of electron density.

Key concepts

Electron Geometry

Electron geometry refers to the arrangement of all regions of electron density around the central atom in a molecule, which includes both bond pairs and lone pairs. This is important because it influences the overall shape and properties of the molecule.

Consider \(\text{SiF}_6^{2-}\). Here, we find an octahedral geometry since there are six regions of electron density around the silicon atom, each corresponding to one of the fluorine atoms. These regions spread out as much as possible, following the principle of minimizing electron pair repulsions, which leads to an octahedral shape.

In the case of \(\text{SeF}_4\), the electron geometry is trigonal bipyramidal, with five regions of electron density (four bonds and one lone pair). Similarly, \(\text{ICl}_2^-\) and \(\text{XeF}_4\) show variations due to the presence of lone pairs. \(\text{ICl}_2^-\) also has a trigonal bipyramidal arrangement but differs in actual shape because of the lone pairs. For \(\text{XeF}_4\), the electron pair geometry remains octahedral even with two lone pairs around the xenon atom.

• Electron geometry patterns are based on the number of total regions of electron density.
• Octahedral: Six directions of electron density
• Trigonal bipyramidal: Five directions
• Tetrahedral: Four directions

Understanding the electron geometry helps in predicting the molecular shape and chemical behavior.

Molecular Geometry

Molecular geometry is concerned with the arrangement of atoms in a molecule and does not count lone pairs. It's crucial in determining how molecules interact with each other and with light.

For a molecule like \(\text{SiF}_6^{2-}\), both electron pair geometry and molecular geometry are octahedral because there are no lone pairs to cause any difference. However, in the case of \(\text{SeF}_4\), the presence of a lone pair on the central selenium atom pushes the \(\text{F}\) atoms into a seesaw molecular shape.

Similarly, in \(\text{ICl}_2^-\), the three lone pairs create a linear molecular shape even though the electron geometry is trigonal bipyramidal. \(\text{XeF}_4\) forms a square planar molecular shape, contrasting the octahedral electron geometry due to its two lone pairs acting oppositely, permitting the four fluorines to adopt a perfect square.

Understanding molecular geometry aids in visualizing the spatial arrangement and can predict molecular interactions.

• Molecular geometry considers only bonding atoms.
• Linear, bent, or angular shapes often result due to lone pair-bond pair interactions.
• The difference between molecular and electron pair geometries arises mainly due to lone pairs.

Hybridization

Hybridization is the concept of mixing atomic orbitals into new hybrid orbitals to facilitate bond formation. It is crucial to understand how atoms bond in complex molecules.

In \(\text{SiF}_6^{2-}\), six electron regions indicate \(sp^3d^2\) hybridization. These orbitals originate from mixing one \(s\), three \(p\), and two \(d\) orbitals. Each hybrid orbital forms a \(\sigma\) bond with a fluorine atom.

For \(\text{SeF}_4\), with five regions of electron density, \(sp^3d\) hybridization occurs. It possesses one \(s\), three \(p\), and one \(d\) orbital, accommodating bonding and lone pair interactions.

Similarly, \(\text{ICl}_2^-\) exhibits \(sp^3d\) hybridization from five regions of density. On the other hand, \(\text{XeF}_4\) shows \(sp^3d^2\) hybridization due to its six electron domains.

• Hybridization types include \(sp, sp^2, sp^3, sp^3d\), and \(sp^3d^2\).
• Number of hybrid orbitals equals the number of atomic orbitals mixed.
• Hybridization decides molecular shape's foundational structure.

Valence Electrons

Valence electrons are the outer shell electrons involved in forming chemical bonds. The number of valence electrons determines how atoms bond and the molecule's shape.

Taking \(\text{SiF}_6^{2-}\) as an example, the total count includes all valence electrons from silicon and fluorine atoms, along with the extra two electrons due to charge, resulting in 48. Knowing this helps establish the correct Lewis structure and anticipate the molecule's behavior.

In \(\text{SeF}_4\), selenium's 6 valence electrons combine with those of fluorine to create bonds and lone pair arrangements, totaling 34 electrons. Similarly, \(\text{ICl}_2^-\) has 22 electrons thanks to the iodine and chlorine contributions. Lastly, \(\text{XeF}_4\) consists of xenon's 8 plus fluorine's, making 36 in total.

• Valence electrons dictate bond types and molecular geometry.
• Total electrons include all from constituent atoms and charges.
• Counting valence electrons is crucial for Lewis structure drawing.