Which of the following is not tetrahedral in shape?

The correct answer is option 3. \(SF_4\).

Among the options provided, the molecule that is not tetrahedral in shape is \(SF_4\) (sulfur tetrafluoride).

Let's analyze the shape of each molecule:

1. \(NH_4^+\) (ammonium ion): It has four electron pairs around the central nitrogen atom, resulting in a tetrahedral shape with bond angles close to \(109.5^\circ\).

2. \(SiCl_4\) (silicon tetrachloride): Silicon has four bonding pairs of electrons with no lone pairs, leading to a tetrahedral molecular geometry.

3. \(SF_4\) (sulfur tetrafluoride): It has one lone pair and four bonding pairs of electrons around the central sulfur atom, resulting in a seesaw or distorted tetrahedral shape, not a perfect tetrahedron.

4. \(CH_4\) (methane): It has four bonding pairs of electrons around the central carbon atom with no lone pairs, resulting in a tetrahedral molecular geometry.

Therefore, the correct answer is 3. \(SF_4\).