How is the formal charge on an atom in a molecule ion calculated? Explain taking the example of molecule.

Predict the shapes of the following molecule using the valence shell electron pair repulsion model.

${\mathrm{SiF}}_4$

Predict the shapes of the following molecule using the valence shell electron pair repulsion model.

${\mathrm{BF}}_3$

Predict the shape of the following molecule using the valence shell electron pair repulsion model.

${\mathrm{NH}}_3$