Question

How does Lewis structure help predict the shape of the compound?

Answer 1

Lewis structure alone cannot determine the shape of the molecule but with help of VSEPR theory the shape of the molecules can be known. According to the VSEPR theory the valence electrons whether bonding or nonbonding will arrange themselves in order to minimize electron-electron repulsion between electron pairs.

Complete step by step answer:
A Lewis Structure is the representation of the valence electrons in the molecule. The valence electrons are the electrons present in the outermost shell of the chemical element. The Lewis structure is used to show how the electrons are arranged around the individual atoms in a molecule. Electrons are shown as “dots” or the bonding electrons are shown as line between the two atoms. The Lewis structure should satisfy the octet rule and the formal charges.
With the help of the VSEPR Theory, the Lewis structure helps to predict the shape of the compound from the electron pair surrounding the central atom of the molecule. The valence shell electron pair repulsion theory was first given by Sidgwick and Powell in 1940. The VSEPR theory is based on the assumption that the molecule will tend to take a shape so that the electronic repulsion in the valence shell of that atom is minimized.
When two electron pairs are present around the central atom, it will be $180^\circ$ with respect to each other and as result forms linear shape, when three electron pairs are present around the central atom, then the atoms will be $120^\circ$ which forms trigonal planar shape, when four electrons pairs, than the overall structure becomes 3-dimensional, tetrahedral, and the electron pairs (bonding or not) form angle of $109^\circ$ with respect to each other; 5 electron pairs forms a trigonal bipyramidal shape and 6 electron pairs forms a octahedral shape.

Note: Lewis structure does not try to explain the geometry of molecules, how the bonds are formed between the atoms, or how the electrons are shared between the atoms. It is the simplest theory used for electronic structure.