Write the Lewis structure of (a) \(\mathrm{SCl}_{2} ;\) (b) \(\mathrm{AsH}_{3} ;\) (c) \(\mathrm{GeCl}_{4} ;\) (d) \(\mathrm{SnCl}_{2}\).

Valence electrons are the outermost electrons of an atom and are critical in determining how an element will react chemically with others. For instance, the sulfur atom in SCl₂ possesses six valence electrons, which are available for bonding with other atoms. Similarly, chlorine has seven valence electrons readily available for making bonds. To visualize Lewis structures, it is essential to know the number of valence electrons as they govern the types of bonds that atoms form. The collective valence electrons are the ‘glue’ that holds atoms together in molecules.

For a clear understanding, counting the valence electrons is often the first step, as seen in the provided solution for SCl₂, which is done by adding the valence electrons of each atom involved. This count forms the foundation for how the electrons are distributed throughout the molecule, leading to a skeletal structure that then gets refined to satisfy the specific bonding needs of each atom.

The octet rule is a chemical rule of thumb that reflects the observation that atoms of main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electron configuration as a noble gas. Atoms will share, gain, or lose electrons to fulfill this rule. In structures like AsH₃, this rule predicts the arrangement of electrons around the arsenic atom, guiding us that arsenic can ‘expand the octet’ due to its ability to access d-orbitals. This is a point of exercise improvement - highlighting exceptions to the octet rule, like in the case of elements beyond period 3 of the periodic table.

The provided solution takes this rule into account when adding electrons to the surrounding atoms in the Lewis structures. For instance, when completing the octets for SCl₂ or GeCl₄, electrons are placed around each chlorine until they all have a complete octet, showcasing how the rule directs the electron distribution.

The arrangement of atoms within a molecule defines its molecular geometry. This arrangement is depicted through the Lewis structures and influenced by the valence shell electron pair repulsion (VSEPR) theory which states that electron pairs around a central atom will organize themselves to minimize repulsion between them. For GeCl₄, with four chlorine atoms, the molecular geometry can be predicted as a tetrahedral arrangement based on VSEPR theory, assuming that all electron pairs (bonding and lone pairs) are positioned as far apart as possible from one another.

The exercise solution could further elaborate on this by offering insights into how the number of bonding and non-bonding electron pairs affects the shape of the molecule, which ultimately has profound effects on the physical and chemical properties of the molecule. In providing an educational platform, discussing the role of molecular geometry in determining such properties can be crucial for students to understand the practical importance of Lewis structures and VSEPR theory.