Question

Why is \(\mathrm{BeCl}_{2}\) molecule linear, but \(\mathrm{BF}_{3}\) is trigonal planar?

Short answer

Answer: BeCl2 has a linear shape because there are two electron pairs with no lone pairs on the central atom (Be), which creates a linear arrangement. On the other hand, BF3 has a trigonal planar shape because there are three electron pairs with no lone pairs on the central atom (B), which creates a trigonal planar arrangement.

Step-by-step solution

Determine the electron configurations of the central atoms

Both BeCl2 and BF3 have a central atom with multiple bonded atoms around. In BeCl2, the central atom is Be (beryllium) and in BF3, the central atom is B (boron). Let's determine each central atom's electron configuration: For Be (atomic number 4): 1s^2 2s^2. For B (atomic number 5): 1s^2 2s^2 2p^1.

Calculate the number of valence electrons for each molecule

To understand the shape of the molecules, we need to determine the number of valence electrons they have. We will calculate the valence electrons considering the number of shared electrons in each bond: For BeCl2: Be (2 valence electrons) + 2 Cl atoms (each has 7 valence electrons) = 2 + (2 × 7) = 16 valence electrons. For BF3: B (3 valence electrons) + 3 F atoms (each has 7 valence electrons) = 3 + (3 × 7) = 24 valence electrons.

Use VSEPR theory to determine the molecular shape of BeCl2

In BeCl2, there are 16 valence electrons. The central atom Be is bonded with two Cl atoms using two electron pairs to form two shared bonds. Thus, there are no lone pairs of electrons on the central atom. According to the VSEPR theory, if there are two electron pairs and no lone pairs on the central atom, they will arrange themselves linearly to minimize repulsions. Therefore, BeCl2 has a linear molecular shape.

Use VSEPR theory to determine the molecular shape of BF3

In BF3, there are 24 valence electrons. The central atom B is bonded with three F atoms using three electron pairs to form three shared bonds. Similar to BeCl2, there are no lone pairs of electrons on the central atom. According to the VSEPR theory, if there are three electron pairs and no lone pairs on the central atom, they will arrange themselves in a trigonal planar shape to minimize repulsions. Therefore, BF3 has a trigonal planar molecular shape. To summarize: - BeCl2 has a linear shape because there are two electron pairs with no lone pairs on the central atom (Be), which creates a linear arrangement. - BF3 has a trigonal planar shape because there are three electron pairs with no lone pairs on the central atom (B), which creates a trigonal planar arrangement.

Key concepts

Molecular Geometry

Molecular geometry describes the three-dimensional shape that molecules form. The shape of a molecule is crucial because it affects how the molecule behaves and interacts with others. This shape is determined by the angles between bonds and the distances between atoms. Scientists use VSEPR (Valence Shell Electron Pair Repulsion) theory to predict and explain the geometry of molecules. VSEPR theory states that electron pairs around a central atom will space out as much as possible to minimize repulsion. This concept helps us understand why different molecules have different shapes, including linear, trigonal planar, tetrahedral, and more.
Understanding molecular geometry can explain a molecule's physical properties, like boiling and melting points, and its chemical reactivity, which is vital in fields such as chemistry and biology.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are important because they are involved in forming chemical bonds. The number of valence electrons determines how atoms interact with each other. These electrons fill the outermost electron shell of an atom and play a key role in bonding, either by sharing with other atoms or by being transferred.
For example, the beryllium atom in BeCl₂ has 2 valence electrons, meaning it can form bonds with two chlorine atoms. Chlorine has 7 valence electrons, and when two chlorine atoms pair with beryllium, the number of valence electrons shared totals to 16. Similarly, boron in BF₃ has 3 valence electrons, allowing it to form bonds with three fluorine atoms, adding up to 24 valence electrons shared in the molecule.

Electron Configuration

Electron configuration describes the arrangement of electrons in an atom. It is written as a series of numbers and letters that represent the energy levels, subshells, and number of electrons. Understanding electron configuration helps predict an element's properties and how it will bond with other elements.
For instance, beryllium has an electron configuration of 1s² 2s², indicating that all of its valence electrons reside in the 2s subshell. Boron has an electron configuration of 1s² 2s² 2p¹, showcasing one more electron in the 2p subshell, contributing to its ability to form three bonds, as seen in BF₃. By analyzing electron configurations, scientists can infer the most likely chemical reactions and bond formations of elements.

Linear Molecular Shape

A linear molecular shape occurs when a molecule's atoms are arranged in a straight line. This shape typically arises when there are two atoms bonded to a central atom, with no lone pairs of electrons on the central atom to alter the shape. Linear shapes allow bond angles to be spread out at 180° to minimize repulsion.
For example, in BeCl₂, the central beryllium atom forms two bonds with chlorine atoms. There are no additional lone electron pairs influencing the arrangement, resulting in a linear shape according to VSEPR theory. This shape is simple and can significantly impact the physical properties of the molecule, such as its polarity and interaction with other molecules.

Trigonal Planar Shape

The trigonal planar shape is characterized by three atoms bonded symmetrically around a central atom in a flat, triangular formation. This shape arises when there are three bonding pairs and no lone electron pairs on the central atom. Each bond angle is approximately 120° to efficiently minimize electron pair repulsion.
For instance, the molecule BF₃ has a trigonal planar shape. With boron as the central atom, three fluorine atoms bond symmetrically around it. The absence of lone pairs on boron allows the electron pairs to arrange themselves in this geometric configuration as per VSEPR theory. Trigonal planar shapes are common in molecules where elements like boron play a central role and significantly influence the chemical properties and reactions of the molecules.