Question

Bromine is a highly reactive liquid, while krypton is an inert gas. Explain this difference based on their electron configurations.

Short answer

Bromine is reactive due to its seven valence electrons, seeking one more to complete its octet. Krypton is inert because it already has a complete octet, making it stable and unreactive.

Step-by-step solution

Identifying the Electron Configurations

The first step is to determine the electron configurations for bromine (Br) and krypton (Kr). Bromine, with an atomic number of 35, has an electron configuration of [Ar] 3d^10 4s^2 4p^5. Krypton, with an atomic number of 36, has an electron configuration of [Ar] 3d^10 4s^2 4p^6.

Understanding Valence Electrons and Reactivity

Analyze the valence shells of both elements to understand their reactivity. Bromine has seven electrons in its valence shell (4p^5), which means it needs one more electron to complete its octet. Krypton has a complete octet in its valence shell (4p^6), which is a stable electron configuration.

Relating Electron Configurations to Reactivity

Bromine is highly reactive because it actively seeks an extra electron to achieve a full outer shell or octet, leading to a more stable state. In contrast, krypton already has a full outer shell with eight valence electrons, making it inert and unreactive, as it does not need to gain or lose electrons to achieve stability.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a pivotal role in chemical bonding and reactivity. Understanding valence electrons is crucial, as these electrons participate in forming chemical bonds with other atoms. In simple terms, atoms strive to reach an electron configuration similar to that of noble gases, known as the 'octet rule', where they have eight valence electrons to acquire stability.

For instance, bromine has seven valence electrons in its outer shell and is only one electron short of completing its octet, which makes it eager to react with other elements to fill this vacancy. This quest for the 'perfect eight' impacts how we predict the behavior of elements during chemical reactions. For students looking for a clearer explanation, consider valence electrons as a party waiting for the perfect number of guests—the closer to eight, the more eager the host is to invite others over to complete the party.

Chemical Reactivity

Chemical reactivity refers to the propensity of a substance to undergo a chemical change. The key to this concept lies in the elemental quest for a stable electron configuration. Atoms will gain, lose, or share electrons via bonds to achieve a complete valence shell, mirroring the stability of noble gases.

Bromine's desire to obtain one more electron to achieve a full valence shell drives its reactivity, leading it to engage readily in chemical reactions. This reactivity can be exemplified when bromine reacts with other elements to form bromides. In contrast, noble gases such as krypton have full valence shells, which render them much less chemically reactive—a property termed 'inertness'. For better understanding, think of a puzzle; an almost complete puzzle like bromine’s shell demands another piece, while a completed one, like krypton’s, has no gaps to fill.

Noble Gases

Noble gases, occupying Group 18 on the periodic table, are characterized by their full valence electron shells, leading to their well-known chemical inertness. Beginning with helium, which has a complete shell of two electrons, and going down the group to neon, argon, krypton, xenon, and radon—each has a complete octet in their outer shell (except helium).

This complete outer shell means that noble gases do not typically participate in chemical reactions under standard conditions. Krypton, for example, with its electron configuration of [Ar] 3d^10 4s^2 4p^6, is a prime example of chemical unreactiveness. Understanding noble gases sets the foundation for grasping why other elements react as they attempt to achieve a similarly stable configuration. In educational terms, noble gases could be seen as having achieved 'chemical enlightenment', and thus, rarely seeking change.