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Square planar has 6 electron dense regions but two of those are lone pairs, making the formula AX4E2 whereas tetrahedral only has 4 electron dense regions, all of which are bonds to another atom, making the formula AX4. The two lone pairs in a square planar molecule repulse the 4 bonds to form a square shape and have their angles be 90 degrees.
Using the VSEPR formula, AX4 is tetrahedral because it has no lone pairs (E). However, if it is AX4E2, the shape is square planar because the two lone pairs of electrons cause the bonded electrons to be arranged in a square shape, which allows them to be farthest from one another.
I found it extremely helpful to make a chart with the steric number as the “y-axis” and the number of lone pairs as the “x-axis” for the molecular shapes. This would make the difference between shapes more obvious.
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