VSEPR theory and molecular shapes
The order is trigonal pyramidal, tetrahedral, trigonal planar, bent.
NI₃
NI₃ is trigonal pyramidal (see Figure 1).
Think of a trigonal pyramid as a three-legged stool, with the “central” atom (N) as the seat. The three iodine atoms form the feet of the stool.
If you close in all the sides of the figure, you have a trigonal (triangular) pyramid, i.e., a pyramid with a triangular base).
CCl₄
CCl₄ is tetrahedral (see Figure 2).
A tetrahedron is also a pyramid with a three-cornered base. The difference is that the central atom (C) is buried in the middle of the figure and all sides have the same length.
The three sides plus the base make four identical sides (tetra = four + hedra = faces) that are equilateral triangles.
BF₃
BF₃ is trigonal planar (see Figure 3).
It is flat, sort of like a slice of pizza. The three bonds point to the corners of an equilateral triangle.
NO₂⁻
NO₂⁻ is a bent molecule.
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Unit conversions
You want to convert miles per hour ⟶ kilometres per hour.
Convert miles to kilometres.
1 mi = 1.61 km Do the conversion.
99 × 1.61/1 = 160 km/h
Convert hours to minutes.
1 h = 60 min Do the conversion
159 × 1/60 = 2.7 km/min
Note: The answers can have only two significant figures because they all depend on the original speed (99 mph = two significant figures).
