Question

Predict the shape and polarity of each of the following molecules: a. A central atom with four identical bonded atoms and no lone pairs. b. A central atom with four bonded atoms that are not identical and no lone pairs.

Short answer

a) Tetrahedral, nonpolar; b) Tetrahedral, polar.

Step-by-step solution

Understanding VSEPR Theory for Molecule A

For a central atom with four identical bonded atoms and no lone pairs, apply the Valence Shell Electron Pair Repulsion (VSEPR) theory. The shape is determined to minimize repulsion between electron pairs.

Determining the Molecular Shape for Molecule A

With four bond pairs and no lone pairs, the electron geometry is tetrahedral. This also makes the molecular geometry tetrahedral.

Determining the Polarity for Molecule A

Since the four bonded atoms are identical and arranged symmetrically, the dipoles cancel out. This means the molecule is nonpolar.

Understanding VSEPR Theory for Molecule B

For a central atom with four bonded atoms that are not identical and no lone pairs, apply VSEPR theory. The shape is still set to minimize repulsion between electron pairs, but the nature of the atoms affects dipole moments.

Determining the Molecular Shape for Molecule B

The scenario still involves four bond pairs and no lone pairs, so the electron geometry is tetrahedral. This also makes the molecular geometry tetrahedral.

Determining the Polarity for Molecule B

With four different bonded atoms, the dipoles do not cancel out due to asymmetrical distribution of electron density. This makes the molecule polar.

Key concepts

molecular geometry

Molecular geometry is a key concept to understand the shape of molecules. It tells us how atoms are spatially arranged around a central atom. The arrangement helps in predicting the molecule's behavior and properties.

To determine the molecular geometry, we use the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory states that electron pairs around a central atom will orient themselves as far apart as possible to minimize repulsion. This general rule helps predict the 3D structure of a molecule.

For example:

• If a molecule has four identical atoms bonded to a central atom with no lone pairs, like in molecule A, the shape is tetrahedral, meaning the atoms are positioned at the corners of a tetrahedron.
• Even if the bonded atoms are not identical, like in molecule B, the VSEPR theory will still predict a similar tetrahedral shape.

This geometric configuration is crucial because it affects the molecule’s properties, including its polarity and how it interacts with other molecules.

electron pairs repulsion

Electron pairs, whether they are in bonds (called bond pairs) or unshared (called lone pairs), repel each other. This repulsion shapes the structure of the molecule. According to VSEPR theory, the geometry around a central atom is determined by the repulsion between all electron pairs in the valence shell.

In molecule A, where there are four identical bonded atoms and no lone pairs, all bond pairs repel each other equally, leading to a perfect tetrahedral shape.

In molecule B, the repulsion is still guided by the VSEPR theory. However, because the bonded atoms are not identical, the dipole moments are different and do not cancel each other out, leading to different molecular properties like polarity.

• This repulsion principle is why molecules adopt specific shapes and is fundamental in determining molecular geometry.
• It accounts for why lone pairs occupy more space than bonding pairs, affecting molecular angles and shapes.

Understanding this concept helps predict and explain the structure and behavior of molecules accurately.

molecule polarity

Molecule polarity depends on both the shape of the molecule and the distribution of electrical charge over the atoms forming the molecule. Polarity determines many properties of molecules including their solubility, boiling/melting points, and interactions with other molecules.

Polarity arises when atoms with different electronegativities form bonds, resulting in a dipole moment.

For molecule A, where the atoms bonded to the central atom are identical and symmetrically arranged, the individual dipoles cancel each other out, making the molecule nonpolar.

In molecule B, the different atoms cause an uneven distribution of electron density. This asymmetry prevents dipole cancellation, making the molecule polar.

• Nonpolar molecules have an even distribution of charge, often leading to uniform interactions.
• Polar molecules, having an uneven charge distribution, tend to interact strongly with other polar substances and have higher boiling/melting points.

Knowing the polarity of a molecule helps predict and explain its chemical properties and reactivity.