Question

An element M reacts with chlorine to form a compound \(\mathrm{X}\). The bond angle in \(\mathrm{X}\) is \(120^{\circ} .\) What is \(\mathrm{M}\) ? (a) \(\mathrm{B}\) (b) \(\mathrm{N}\) (c) \(\mathrm{Be}\) (d) \(\mathrm{Mg}\)

Short answer

M is B (boron).

Step-by-step solution

Identify the Characteristics of the Compound

The compound \( \mathrm{X} \) has a bond angle of \( 120^{\circ} \). This bond angle suggests that the compound has a trigonal planar shape. Trigonal planar shapes are associated with compounds that use \( sp^2 \) hybridization.

Determine the Hybridization Requirements

To achieve a \( sp^2 \) hybridization and a trigonal planar shape, the element \( M \) should have three electron domains around it. This could occur if the element forms three bonds with chlorine without any lone pairs on it.

Evaluate Each Element Option

- Option (a), \( \mathrm{B} \), can form \( \mathrm{BCl}_3 \), which is trigonal planar with \( 120^{\circ} \) bond angles.- Option (b), \( \mathrm{N} \), typically forms \( \mathrm{NCl}_3 \), which is not planar due to lone pairs on nitrogen.- Option (c), \( \mathrm{Be} \), forms \( \mathrm{BeCl}_2 \), which is linear with \( 180^{\circ} \) bond angles.- Option (d), \( \mathrm{Mg} \), forms \( \mathrm{MgCl}_2 \), which is also typically linear in the gas phase.

Conclude the Correct Element

Based on the bond angle of \( 120^{\circ} \) in the compound \( \mathrm{X} \), the element that forms this type of compound is \( \mathrm{B} \), because \( \mathrm{BCl}_3 \) is trigonal planar with \( 120^{\circ} \) bond angles.

Key concepts

Trigonal Planar Geometry

When we talk about trigonal planar geometry, imagine arranging three identical balloons attached at a single point. In chemistry, this translates to a central atom bonded to three other atoms, all lying in a single two-dimensional plane. These three atoms spread out as far as possible around the central atom, creating angles of exactly 120° between them. This specific layout minimizes repulsion between the atoms' bonding pairs, making it a very stable structure.

Trigonal planar geometry is often observed in compounds where the central atom is surrounded by three electron-rich areas, which could all be bonding pairs. Because there are no lone electron pairs pushing the bonds closer together, the 120° angle is maintained. This geometry is a telltale sign of compounds using the specific hybridization known as "sp² hybridization." Understanding this geometry immensely helps in predicting the shape and bond angles of simple molecular compounds.

Hybridization

Hybridization is a key concept in chemistry that describes how atomic orbitals mix to form new hybrid orbitals. These hybrid orbitals have unique shapes and energy levels suitable for bonding.

For a compound to display trigonal planar geometry, it typically involves sp² hybridization. Here's how it works:

• In sp² hybridization, one s orbital combines with two p orbitals from the same atom.
• This results in three equivalent hybrid orbitals, all lying in the same plane.
• The angles between the orbitals are 120°, matching the trigonal planar arrangement.

Thus, when an atom undergoes sp² hybridization, it can form three identical bonds in a planar structure, leading to the formation of compounds with 120° bond angles, like boron trifluoride (\( ext{BCl}_3 \)). Understanding hybridization helps in visualizing how atoms bond and the shapes they form.

Molecular Geometry

Molecular geometry defines the three-dimensional arrangement of atoms in a molecule and greatly influences its physical and chemical properties. The geometry is determined by the number of bonds and lone pairs around the central atom.

In cases where a molecule forms a trigonal planar shape, like in the compound \( \ ext{BCl}_3 \), the molecular geometry is entirely dictated by the bonding pairs, with no lone electron pairs altering the shape. This leads to a symmetric structure with equal bond lengths and angles. The shape affects how the molecule will interact with light and other molecules, playing a crucial role in reactivity and polarity.

Molecular geometry predictions provide meaningful insights when analyzing chemical reactions and designing compounds, making it an essential part of understanding chemical bonding.