Potassium is a highly reactive metal, whereas argon is an inert gas. Explain the difference based on their electron configurations.

Chemical reactivity refers to the tendency of a substance to engage in chemical reactions and form new substances. Elements with incomplete outer electron shells tend to be more reactive because they seek stability by achieving a full shell. In our example, potassium is a highly reactive metal due to its desire to lose an electron and reach a stable configuration, like that of noble gases. Elements like argon, with complete outer shells, show little to no reactivity as there is no need to lose or gain electrons for stability.

Reactivity also depends on other factors such as electronegativity, atomic size, and the overarching trends seen across the periodic table. The 'eager to lose' or 'gain electrons' mechanism underpins many chemical reactions, including the highly exothermic ones, which release energy, as in the case of potassium reacting with other substances.

Valence electrons play a pivotal role in determining an element's chemical properties, including its reactivity. These electrons are located in the outermost shell of an atom and are involved in forming chemical bonds. For example, the solitary valence electron in potassium's configuration, [Ar]4s1, makes it actively seek reactions that will allow it to lose this electron and achieve stability.

In contrast, argon's valence shell is already full, with an electron configuration expressed simply as [Ar], indicating that all its valence orbitals are filled. Consequently, argon does not readily form chemical bonds, hence its inert nature. Understanding the count and arrangement of valence electrons aids in predicting the types of chemical bonds an element is likely to form.

The octet rule is a chemical guideline which states that atoms tend to bond in such a way that they have eight electrons in their valence shell, giving them the same electronic configuration as a noble gas. The rule is based on the observation that elements with a full set of eight valence electrons are exceptionally stable. Potassium strives to follow the octet rule by losing its one valence electron, thereby taking on the electron configuration of the noble gas argon. Conversely, argon naturally follows the octet rule as it already possesses a complete set of valence electrons.

While the octet rule is a useful tool in predicting the behavior of elements, especially for main group elements, it has exceptions. Transition metals and some molecules do not always comply with the octet rule due to their unique electron configurations and bonding capabilities.

Attaining a noble gas configuration means achieving a full valence electron shell, which is the most stable electron arrangement for atoms. Elements in the noble gas group of the periodic table, like argon, neon, and helium, naturally have full valence shells and are thus non-reactive or inert. Other elements tend to gain, lose, or share electrons through chemical reactions to reach this highly stable state. Potassium, after losing its one valence electron, attains a noble gas configuration resembling that of argon.

This drive for stability by reaching a noble gas configuration is a critical factor shaping the chemical reactivity of elements. Understanding an element's electron arrangement provides insights into its likely reactions and the compounds it can form, which is fundamental in the study of chemistry.