Predict some possible compounds that could form between chlorine and selenium.

Valence electrons play a pivotal role in chemical bonding, as they are the electrons in an atom's outer shell that engage in the formation of chemical bonds. Understanding how many valence electrons an element has can be deduced by its position in the periodic table. For example, chlorine (Cl), located in group 17, possesses seven valence electrons. These are the electrons that chlorine can use either to gain one more, achieving the electron configuration of a noble gas (argon, in this case), or in some less common instances, lose to another element.

Selenium (Se), from group 16, similarly has six valence electrons. It is more predisposed to gaining two electrons to reach the noble gas configuration of krypton. By studying valence electrons, students can start to predict how atoms might interact when forming compounds, since the goal of most elements is to achieve a noble gas-like electron configuration, often resulting in the gain or loss of electrons to form ions.

An element's oxidation state represents its degree of oxidation or reduction; in simple terms, it reflects how many electrons an atom has gained, lost, or shared when bonding with another atom. For the elements chlorine and selenium, common oxidation states relate directly back to their valence electrons.

Chlorine most commonly has an oxidation state of -1 (Cl⁻) when it gains one electron, whereas less frequently, it can exhibit a +7 oxidation state (Cl⁺⁷) when involved in more complex molecules and reactions. Similarly, selenium typically can show a -2 oxidation state (Se²⁻) by gaining two electrons, or a +6 state (Se⁶⁺) under specific conditions. These oxidation states are essentials, as they help in predicting how atoms might combine to form compounds and provide insight into the reactivity and properties of the resulting molecules.

Compound formation is the result of chemical reactions between different atoms as they share, donate, or accept electrons to achieve stable electron configurations. For chlorine and selenium, the most feasible compound they can form is SeCl₆, where selenium is in the +6 oxidation state and chlorine is -1. In this instance, the structure arises out of one Se⁶⁺ ion and six Cl⁻ ions. Here, the chlorine atoms each donate one of their valence electrons to selenium, satisfying both elements' desire for a stable electron arrangement.

Understanding the principles of compound formation is critical for predicting the kinds of stable substances that can exist. For example, SeCl₇ is theoretically possible if chlorine were to be in the +7 oxidation state, but this is highly improbable under normal conditions due to the considerable amount of energy that would be required to remove seven electrons from chlorine atoms. Hence, SeCl₆ emerges as the more likely compound between chlorine and selenium based on principles of chemical stability and energetics.