Consider the Lewis structure for NBr3 There are a total of electrons, valence electrons. There are non-bonding electrons, and bonding lone pairs of electrons.
Using VSEPR theory, a molecule with the general formula AX#E# (A is the central atom, X is the bonded atom and E is a lone pair of electrons), how many regions of electron density around the central atom in AXzE? [Select] AX3E will have what kind of electronic geometry? [Select] AX3E will have what kind of molecular geometry? (Select
The Correct Answer and Explanation is:
Let’s walk through the Lewis structure of NBr₃ and apply VSEPR theory to determine the regions of electron density, electronic geometry, and molecular geometry.
Step 1: Lewis Structure of NBr₃
Total Valence Electrons:
- Nitrogen (N) has 5 valence electrons.
- Each bromine (Br) has 7 valence electrons.
- There are 3 bromine atoms.
So,
Total valence electrons = 5 (N) + 3 × 7 (Br) = 5 + 21 = 26 electrons
Bonding & Non-Bonding Electrons:
- Nitrogen forms 3 single bonds with 3 Br atoms (each bond has 2 electrons → 3 bonds × 2 = 6 bonding electrons).
- That leaves 20 non-bonding electrons (26 total − 6 bonding = 20 non-bonding).
- Each Br atom gets 3 lone pairs (6 electrons × 3 = 18 electrons), and nitrogen gets 1 lone pair (2 electrons).
So:
Bonding pairs = 3
Non-bonding electrons = 20 (or 10 lone pairs total: 3 on each Br, 1 on N)
Step 2: Using VSEPR Theory for NBr₃ (AX₃E)
- General formula is AX₃E (3 bonding pairs (X) and 1 lone pair (E) around the central atom N).
Regions of Electron Density:
Each bond and lone pair counts as one region of electron density.
So, AX₃E has:
3 bonding + 1 lone pair = 4 regions of electron density
Electronic Geometry:
- With 4 regions of electron density, the electronic geometry is tetrahedral.
Molecular Geometry:
- In AX₃E, one of the tetrahedral positions is occupied by a lone pair.
- That gives a molecular geometry of trigonal pyramidal.
Summary:
- Total valence electrons: 26
- Non-bonding electrons: 20
- Bonding pairs (lone pairs on atoms): 10 lone pairs (3 on each Br, 1 on N)
- Regions of electron density: 4
- Electronic geometry: Tetrahedral
- Molecular geometry: Trigonal pyramidal
Explanation (~300 words):
The structure and shape of a molecule can be understood by drawing its Lewis structure and applying VSEPR (Valence Shell Electron Pair Repulsion) theory. For nitrogen tribromide (NBr₃), we begin by calculating the total number of valence electrons. Nitrogen contributes 5 valence electrons, and each of the three bromine atoms contributes 7, totaling 26 valence electrons. These electrons are distributed to form three single covalent bonds between the nitrogen and each bromine atom. Each bond uses 2 electrons, consuming 6 electrons. The remaining 20 electrons are used as lone pairs: 18 go to the three Br atoms (6 each), and 2 remain as a lone pair on the nitrogen atom.
In VSEPR theory, both bonding and lone pairs of electrons around the central atom are treated as regions of electron density that repel each other and adopt a geometry that minimizes repulsion. In NBr₃, nitrogen has three bonded atoms (Br) and one lone pair, which fits the AX₃E model. The four regions of electron density result in a tetrahedral electron geometry.
However, the lone pair does not contribute to the molecular shape as visible from the outside. This leads to a trigonal pyramidal molecular geometry, where the nitrogen is at the center and the three bromine atoms form the base of a pyramid. The lone pair occupies one vertex of the tetrahedron, causing slight repulsion that makes the Br–N–Br bond angles slightly less than the ideal 109.5° of a perfect tetrahedron.
Thus, NBr₃ is best described as a molecule with tetrahedral electron geometry and trigonal pyramidal molecular geometry, due to one lone pair on the nitrogen.