The molecular shape of boron trifluoride (BF₃) can be determined by considering the electronic structure and applying the Valence Shell Electron Pair Repulsion (VSEPR) theory. Let’s break this down step by step:
Lewis Structure of BF₃:
- Boron (B) is in Group 13 of the periodic table, so it has 3 valence electrons.
- Each fluorine (F) atom, being in Group 17, has 7 valence electrons.
- In BF₃, boron forms three single bonds with three fluorine atoms. Each fluorine contributes one electron to form the bond, while boron shares one electron with each fluorine atom.
- There are no lone pairs of electrons on the boron atom because it only needs three bonds to achieve a stable electron configuration (though it does not achieve an octet).
- Each fluorine atom has three lone pairs of electrons, as fluorine needs one more electron to complete its octet.
So the Lewis structure of BF₃ is:
F
|
F - B - F
Electron Domain Count:
- The electron domains are regions of electron density that are either bonding pairs or lone pairs around the central atom.
- In BF₃, boron has three bonding pairs (one bond to each fluorine) and no lone pairs of electrons.
- According to the VSEPR theory, each bonding pair counts as an electron domain.
Application of VSEPR Theory:
- The VSEPR theory states that electron pairs around an atom will arrange themselves in such a way that they minimize repulsion, achieving the most stable structure.
- Since BF₃ has three electron domains (all bonding pairs) and no lone pairs, the most stable arrangement will be one that maximizes the distance between these three electron domains.
- The ideal geometry for three electron domains is trigonal planar.
Trigonal Planar Geometry:
- In this geometry, the three fluorine atoms are positioned at the corners of an equilateral triangle with the boron atom at the center.
- The bond angles between the fluorine atoms are 120°, as this is the angle that minimizes the repulsion between the bonding pairs.
- Since there are no lone pairs of electrons to distort the shape, the structure remains perfectly flat, in the plane.
Electron and Molecular Geometry:
- Electron geometry refers to the arrangement of all electron domains (bonding and lone pairs) around the central atom. In BF₃, this is trigonal planar.
- Molecular geometry refers to the arrangement of the atoms in space. Since there are no lone pairs on boron, the molecular geometry is also trigonal planar.
Bonding and Polarity:
- The bonds in BF₃ are polar because fluorine is more electronegative than boron. However, due to the symmetrical trigonal planar arrangement, the individual dipoles cancel each other out, and BF₃ is a nonpolar molecule overall.
Summary of Key Points:
- Molecular Shape: Trigonal planar
- Electron Geometry: Trigonal planar
- Bond Angles: Approximately 120°
- Polarity: Nonpolar molecule (despite polar bonds)
- No lone pairs on central atom (boron)
Thus, BF₃ adopts a trigonal planar molecular shape, with three bonding pairs around the boron atom arranged symmetrically.
