Question

Selenium tetrafluoride reacts with fluorine to form selenium hexafluoride. (a) Draw Lewis structures for both selenium fluorides, and predict any deviations from ideal bond angles. (b) Describe the change in orbital hybridization of the central Se during the reaction.

Short answer

SeF4 is a see-saw shape with bond angles deviating from 90° and 120°. SeF6 is octahedral with 90° bond angles. Se changes hybridization from sp^3d in SeF4 to sp^3d^2 in SeF6.

Step-by-step solution

- Draw Lewis Structure for Selenium Tetrafluoride (SeF4)

To draw the Lewis structure for SeF4:1. Determine the total number of valence electrons: Se (6) + 4F (7 each) = 34 electrons.2. Place selenium (Se) in the center and arrange the four fluorine (F) atoms around it.3. Connect each F atom to Se with a single bond.4. Distribute the remaining electrons to complete each F atom's octet.5. Place any remaining electrons on the Se atom.6. The final structure should show Se with two lone pairs and four Se-F bonds, creating a see-saw shape, with bond angles deviating from the ideal 90° and 120° due to the lone pairs.

- Draw Lewis Structure for Selenium Hexafluoride (SeF6)

To draw the Lewis structure for SeF6:1. Determine the total number of valence electrons: Se (6) + 6F (7 each) = 48 electrons.2. Place selenium (Se) in the center and arrange the six fluorine (F) atoms around it.3. Connect each F atom to Se with a single bond.4. Distribute the remaining electrons to complete each F atom's octet.5. There are no remaining electrons to place on Se.6. The final structure forms an octahedral shape, with 90° bond angles between all Se-F bonds.

- Predict Bond Angle Deviations in SeF4

In SeF4, the ideal bond angles for a see-saw shape derived from a trigonal bipyramidal electron-domain geometry would be 90°, 120°, and 180°. However, due to the lone pairs on Se, the bond angles will be slightly less in actual structure as lone pairs occupy more space and repel adjacent bonds, reducing bond angles.

- Predict Bond Angles in SeF6

In SeF6, the molecule forms an ideal octahedral shape. All bond angles are 90° or 180°, as lone pairs do not affect the structure. Therefore, no deviations from the ideal bond angles are expected.

- Describe Orbital Hybridization Change

For Se in SeF4, the central atom is sp^3d hybridized as there are five electron pairs around Se (four bonding pairs and one lone pair). In SeF6, the central atom is sp^3d^2 hybridized as there are six bonding electron pairs. The change from sp^3d to sp^3d^2 hybridization occurs during the reaction of SeF4 with additional F2.

Key concepts

Lewis Structures

Lewis structures help us understand molecule structure by showing the arrangement of electrons around atoms in a compound. To draw the Lewis structure for SeF4, we start by calculating the total number of valence electrons: Selenium (Se) contributes 6 electrons, and each of the four fluorine (F) atoms contributes 7, totaling 34 electrons. We place Se in the center and arrange the fluorine atoms around it. Connecting each F to Se with a single bond uses up 8 electrons, leaving 26 to complete the octets of the F atoms and Se itself. Se in SeF4 has two lone pairs, resulting in a 'see-saw' shape due to electron repulsion effects.
The Lewis structure for SeF6 is calculated similarly: Se (6) + 6F (7) total 48 electrons. We again place Se centrally and surround it with six F atoms, connecting each with a single bond. The remaining electrons complete the octets of the F atoms. In this structure, all pairs of electrons are bonding pairs, forming an octahedral shape.

Orbital Hybridization

Orbital hybridization is a concept where atomic orbitals mix to form new hybrid orbitals suitable for the pairing of electrons to form chemical bonds in valence bond theory. In SeF4, selenium is surrounded by five electron pairs: four bonding pairs and one lone pair. These electron pairs require an sp^3d hybridization on the Se atom, forming one s orbital, three p orbitals, and one d orbital to generate five hybrid orbitals.
When SeF4 reacts with fluorine to form SeF6, the selenium needs to accommodate six bonding pairs. This requires sp^3d^2 hybridization, involving one s orbital, three p orbitals, and two d orbitals to produce six hybrid orbitals. This shift from sp^3d to sp^3d^2 hybridization accounts for the structural changes that occur during the reaction.

Bond Angles

Bond angles are the angles between adjacent bonds within a molecule, indicative of its shape and electron pair repulsion. In SeF4, the molecule adopts a 'see-saw' structure due to the lone pairs on Se. Ideally, a 'see-saw' derived from trigonal bipyramidal geometry would feature 90°, 120°, and 180° angles. However, the actual bond angles deviate due to lone pair repulsions compressing the F-Se-F angles.
On the other hand, SeF6 has an ideal octahedral shape. All bond angles are either 90° or 180° because no lone pairs are present to distort the geometry. The regular octahedral structure of SeF6 ensures that there are no deviations from the ideal bond angles, maintaining symmetry and uniformity.