Question

Estimate the bond angle between atoms in these molecules: (a) \(\mathrm{H}_{2} \mathrm{~S}\) (b) \(\mathrm{NH}_{3}\) (c) \(\mathrm{NH}_{4}^{+}\) (d) \(\mathrm{SiCl}_{4}\)

Short answer

(a) Less than 109.5 degrees, (b) 107.3 degrees, (c) 109.5 degrees, (d) 109.5 degrees

Step-by-step solution

Understanding Bond Angles

Bond angles are determined by the geometrical arrangement of atoms around a central atom. The Valence Shell Electron Pair Repulsion (VSEPR) theory helps predict molecular geometries.

Determine the Molecular Geometry of \(\require{mhchem} \ce{H2S} \)

The central atom is sulfur (S) which has 6 valence electrons. With two hydrogen atoms bonded, there are two lone pairs of electrons. This gives a bent shape similar to water (\require{mhchem} \- \ce{H2O}), resulting in a bond angle slightly less than 109.5\(^\backslashcirc\).

Determine the Molecular Geometry of \(\require{mhchem} \ce{NH3} \)

The central atom is nitrogen (N) which has 5 valence electrons. Nitrogen forms three bonds with hydrogen atoms and has one lone pair of electrons. This gives a trigonal pyramidal shape, resulting in a bond angle of approximately 107.3\(^\backslashcirc\).

Determine the Molecular Geometry of \(\require{mhchem} \ce{NH4+} \)

The central atom is nitrogen (N) which uses all five valence electrons to form four bonds with four hydrogen atoms, with no lone pairs. This gives a tetrahedral shape with bond angles of exactly 109.5\(^\backslashcirc\).

Determine the Molecular Geometry of \(\require{mhchem} \ce{SiCl4} \)

Silicon (Si) is the central atom with 4 valence electrons. It forms four single bonds with chlorine atoms and no lone pairs. This also gives a tetrahedral shape with bond angles of 109.5\(^\backslashcirc\).

Key concepts

VSEPR theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a fundamental concept in chemistry that predicts the shape of molecules. The principle behind VSEPR is that electron pairs around a central atom will arrange themselves to minimize repulsion. This results in specific, predictable shapes for molecules.
Whether the electron pairs are involved in bonding (bonding pairs) or not (lone pairs), they all repel one another. The geometry of the molecule depends on the number of bonding pairs and lone pairs.
For example, methane (CH4) has no lone pairs, just four bonding pairs, and forms a tetrahedral shape with bond angles of 109.5°. If lone pairs are present, they will alter the shape. For instance, water (H2O) has two lone pairs and two bonding pairs, resulting in a bent shape.

Molecular geometry

Molecular geometry refers to the three-dimensional arrangement of atoms around a central atom in a molecule. This shape determines the molecule's properties and reactivity.
According to VSEPR theory, the geometries can be classified into several shapes:

• Linear - 180° bond angles (e.g., CO2)
• Trigonal planar - 120° bond angles (e.g., BF3)
• Tetrahedral - 109.5° bond angles (e.g., CH4)
• Trigonal bipyramidal - 90° and 120° bond angles (e.g., PCl5)
• Octahedral - 90° bond angles (e.g., SF6)

The presence of lone pairs on the central atom can alter these geometries, making the shapes less symmetric. For instance, the geometry of ammonia (NH3) is trigonal pyramidal due to one lone pair, rather than the trigonal planar shape it would have with no lone pairs.

Valence electrons

Valence electrons are the outermost electrons of an atom that are involved in chemical bonding. These electrons occupy the highest energy level of an atom and are crucial for determining how atoms interact with each other.
The number of valence electrons can be identified using the periodic table. For instance, oxygen is in group 16 and has six valence electrons, while hydrogen, in group 1, has one valence electron.
In bonding, these valence electrons can be shared (covalent bonds) or transferred (ionic bonds) to meet the octet rule, aiming for eight electrons in the outer shell. In VSEPR theory, knowing the number of valence electrons helps predict the shape of the molecule. Lone pairs and bonding pairs derive from the total number of valence electrons available.

Tetrahedral shape

A tetrahedral shape is one of the most common molecular geometries, where four atoms are symmetrically arranged around a central atom. This shape has a bond angle of 109.5°.
The tetrahedral structure arises when there are four bonding pairs and no lone pairs around the central atom. An example of a tetrahedral molecule is methane (CH4), where carbon is at the center, bonded to four hydrogen atoms.
The VSEPR theory explains this shape: with four bonding pairs repelling each other, they spread out to the corners of a tetrahedron to minimize repulsion. This structure is vital in many organic compounds and significantly affects the molecule's chemical properties and reactions.