Why does a Lewis structure of a covalent molecule show only the valence electrons?

Only the valence shell are shown in the Lewis electron dot structures because, the chemical reactivity of an atom of an element is determined by the valence electrons of the atom and not by all the electrons of the atom including the inner electrons.

Explanation:

Lewis electron dot structures are structures that show the bonding between atoms of a molecule, as well as the lone pairs of electrons that may exist in the molecule.

In the Lewis symbol for an atom, the chemical symbol of the element is written, and the valence electrons of the atom are represented as dots surrounding it. Only the electrons in the valence shell are shown using the Lewis electron dot structures.

Lewis structures for molecules show each atom and its position in the structure of the molecule using its chemical symbol. Dots or lines are drawn between atoms to show the electrons that are involved in the chemical bonding between the atoms in the molecule. Valence electrons in each atom that are not involved in bonding known as lone pairs, are represented as pairs of dots, and are placed next to each atom bearing the electron.

Electrons that are not in the valence level are not shown in the Lewis electron dot structures . This is because, the chemical reactivity of an atom of an element is determined by the valence electrons of the atom and not all the electrons of the atom including the inner electrons.