Write the Lewis structure of di methyl amine, (CH3)2NH. part 2). Choose the correct variant of its geometry.
a) Each inner atom has SN = 5, so the electron group geometry about each inner atom is octahedral.
b) Each inner atom has SN = 4, so the electron group geometry about each inner atom is tetrahedral. The shape about the N atom is tetrahedral also.
c) Each inner atom has SN = 4, so the electron group geometry about each inner atom is tetrahedral. The shape about the N atom is trigonal pyramidal, like ammonia.
d) Each inner atom has SN = 3, so the electron group geometry about each inner atom is trigonal pyramidal. The shape about the N atom is tetrahedral, like ammonia.
The Correct Answer and Explanation is:
Let’s analyze the Lewis structure and geometry of dimethylamine, (CH3)2NH, and then evaluate the correct option regarding its geometry.
Lewis Structure of (CH3)2NH
- The molecule consists of:
- Two methyl groups (CH3)
- One nitrogen atom (N)
- One hydrogen atom (H) attached to nitrogen
- Nitrogen (N) has 5 valence electrons.
- Each carbon (C) has 4 valence electrons.
- Hydrogen (H) has 1 valence electron.
In dimethylamine:
- Nitrogen forms three sigma bonds:
- Two bonds with two methyl carbons (C)
- One bond with one hydrogen (H)
- Nitrogen also holds one lone pair of electrons.
Thus, the nitrogen has four electron groups around it: three bonding pairs and one lone pair.
Step 2: Determine the Steric Number (SN) and Electron Geometry
Steric Number (SN) is the total number of bonded atoms plus lone pairs on the central atom.
- For the nitrogen in (CH3)2NH:
- Number of bonded atoms = 3 (two carbons and one hydrogen)
- Number of lone pairs = 1
So, SN = 3 + 1 = 4.
Step 3: Electron Group Geometry and Molecular Shape
- Electron group geometry considers all electron groups (bonding and lone pairs).
- For SN = 4, the electron group geometry is tetrahedral.
- However, the molecular shape (actual geometry of atoms) excludes lone pairs but is influenced by them.
Because nitrogen has one lone pair, the molecular shape around nitrogen is trigonal pyramidal — this is the same shape as ammonia (NH3).
Now, review the options:
a) Each inner atom has SN = 5, so the electron group geometry about each inner atom is octahedral.
- Incorrect. SN = 5 corresponds to trigonal bipyramidal, not octahedral.
- Also, nitrogen does not have 5 electron groups.
b) Each inner atom has SN = 4, so the electron group geometry about each inner atom is tetrahedral. The shape about the N atom is tetrahedral also.
- Partially correct on electron group geometry (tetrahedral) but incorrect on molecular shape.
- Shape is not tetrahedral; it is trigonal pyramidal due to lone pair.
c) Each inner atom has SN = 4, so the electron group geometry about each inner atom is tetrahedral. The shape about the N atom is trigonal pyramidal, like ammonia.
- Correct.
- SN = 4 means electron geometry is tetrahedral.
- Molecular shape about N is trigonal pyramidal (like ammonia), because of the lone pair.
d) Each inner atom has SN = 3, so the electron group geometry about each inner atom is trigonal pyramidal. The shape about the N atom is tetrahedral, like ammonia.
- Incorrect.
- SN = 3 corresponds to trigonal planar electron geometry, not trigonal pyramidal.
- Ammonia is trigonal pyramidal shape but with SN = 4.
Final answer:
(c) Each inner atom has SN = 4, so the electron group geometry about each inner atom is tetrahedral. The shape about the N atom is trigonal pyramidal, like ammonia.
Explanation (300+ words)
Dimethylamine, (CH3)2NH, contains a nitrogen atom bonded to two methyl groups (CH3) and one hydrogen atom (H). In addition to these three bonded atoms, the nitrogen atom has one lone pair of electrons. This gives nitrogen a total of four regions of electron density: three bonding pairs and one lone pair. The steric number (SN), which is the sum of bonded atoms and lone pairs around the central atom, is therefore 4.
When an atom has four electron groups, the electron group geometry is tetrahedral. This tetrahedral geometry is based on the positions of all electron pairs around the central atom, including both bonding and lone pairs. The ideal tetrahedral arrangement minimizes electron pair repulsions, positioning the electron groups approximately 109.5° apart.
However, molecular shape (or molecular geometry) describes the arrangement of only the bonded atoms, ignoring lone pairs. Lone pairs occupy more space than bonding pairs because their electron density is concentrated closer to the central atom, causing repulsion that slightly distorts bond angles. In the case of dimethylamine, the presence of the lone pair on nitrogen pushes the bonded atoms slightly closer together, resulting in a trigonal pyramidal shape around the nitrogen, similar to ammonia (NH3).
Therefore, the electron group geometry of nitrogen in dimethylamine is tetrahedral, while the actual shape of the molecule around nitrogen is trigonal pyramidal. This geometry influences many properties of the molecule, including its polarity and reactivity.
The other options are incorrect either because they misidentify the steric number or confuse electron group geometry with molecular shape. For example, a steric number of 5 corresponds to trigonal bipyramidal geometry (not octahedral), and a steric number of 3 corresponds to trigonal planar geometry, not trigonal pyramidal.
In conclusion, option (c) correctly describes the geometry of dimethylamine by recognizing the tetrahedral electron group geometry around nitrogen and the trigonal pyramidal molecular shape caused by the lone pair.