Question

Use the VSEPR method to predict the geometries of the (d) \(\mathrm{SF}_{4}\). following species: (a) \(\mathrm{I}_{3}^{-}\), (b) \(\mathrm{SiCl}_{4}\), (c) \(\mathrm{PF}_{5}\),

Short answer

The geometry of \(\mathrm{SF}_4\) is see-saw.

Step-by-step solution

Determine the Valence Electrons

For \(\mathrm{SF}_4\), sulfur (S) has 6 valence electrons and each fluorine (F) has 7, giving us a total of \(6 + 4 \times 7 = 34\) electrons.

Identify the Central Atom and Arrange Electrons

Place the sulfur atom at the center and arrange the four fluorine atoms around it. Connect each fluorine to sulfur with a single bond, using one electron pair for each bond. This uses up 8 of the 34 electrons, leaving us with 26 electrons.

Distribute Remaining Electrons

Distribute the remaining 26 electrons to give each fluorine atom a full octet of electrons (6 more electrons per fluorine), using up \(4 \times 6 = 24\) electrons. This leaves us with 2 electrons, which will be added to the sulfur as a lone pair.

Apply VSEPR Theory

Using VSEPR theory, we note that \(\mathrm{SF}_4\) has 4 bonded pairs and 1 lone pair. This gives a total of 5 regions of electron density, which corresponds to a trigonal bipyramidal electron-pair geometry.

Determine Molecular Geometry

The molecular geometry of \(\mathrm{SF}_4\) is 'see-saw' due to the presence of the lone pair on one of the equatorial positions in the trigonal bipyramidal electron-pair geometry.

Key concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. Understanding this concept helps us predict and explain the shape of molecules. However, it's crucial not to confuse this with electron-pair geometry, which considers the spatial arrangement of all electron pairs around the central atom. In ul>

The type and number of bonds (single, double, triple) and the presence of lone pairs influence the molecular geometry.

In the case of SF₄, the molecular geometry is described as a 'see-saw' shape.

This unique geometry results from the sulfur atom, acting as the central atom, being surrounded by four fluorine atoms and one lone pair of electrons. The lone pair slightly alters the angular placement of bonded atoms, giving it the see-saw characteristic.

Electron-Pair Geometry

Electron-pair geometry is a broader concept than molecular geometry. It includes both bonding electron pairs and lone pairs in its consideration. This concept is fundamental in the VSEPR theory, which predicts the shape based on electron pairs repelling to minimize energy.

• VSEPR stands for Valence Shell Electron Pair Repulsion, emphasizing the repulsion between electron pairs as the guiding principle in determining the shape.
• For \(\mathrm{SF}_4\), the electron-pair geometry is trigonal bipyramidal.
• This geometry arises because there are five regions of electron density around the sulfur atom: four bonded pairs to fluorine and one lone pair.

The lone pair occupies an equatorial position in the trigonal bipyramidal arrangement, which influences the ultimate molecular geometry.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in forming chemical bonds. They determine how atoms interact with each other and are key in predicting molecular formation and properties.

• Sulfur, in SF₄, has six valence electrons.
• Each fluorine atom contributes seven valence electrons.

The total number of valence electrons in SF₄ is thus calculated by adding the valence electrons from sulfur and the four fluorine atoms (6 + 4×7 = 34 electrons). These electrons are arranged to prefer minimal repulsion, influencing both molecular and electron-pair geometry.

Lone Pairs

Lone pairs, also known as non-bonding pairs, are pairs of valence electrons that are not shared with another atom in a covalent bond. Even though they do not directly participate in bonding, they greatly influence the shape and properties of a molecule.

• In SF₄, there is one lone pair on the sulfur atom.
• These electrons occupy more space than bonding pairs, because they are only attracted to one nucleus rather than being shared between two.
• This spatial requirement causes deviations in bond angles and affects overall molecular geometry.

The presence of a lone pair in SF₄ alters the angles between the fluorine atoms, contributing to its distinctive see-saw molecular shape. Understanding lone pairs is crucial for predicting deviations from idealized geometrical shapes.