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 because it has an incomplete valence shell and tends to gain one electron to achieve a full octet, whereas krypton has a complete valence shell, making it stable and inert.

Step-by-step solution

Identify the Electron Configurations of Bromine and Krypton

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

Analyze Bromine's Electron Configuration

Bromine has seven electrons in its outermost p-orbital (4p^5). It needs one more electron to fill the p-orbital and achieve a stable octet, which is why it is very reactive, often forming compounds to satisfy the octet rule.

Analyze Krypton's Electron Configuration

In contrast, krypton has a completely filled outermost p-orbital (4p^6) along with a full s-orbital, making a full shell of 8 electrons, satisfying the octet rule. Having a complete valence shell, it has no tendency to gain or lose electrons, making it inert.

Conclude the Reactivity Difference

The difference in reactivity between bromine and krypton is due to the stability of their valence electron configurations. Bromine is reactive because it seeks to fill its valence shell, while krypton's filled valence shell makes it inert.

Key concepts

Bromine Reactivity

Bromine is an element known for its eagerness to participate in chemical reactions, a characteristic attributed to its electron configuration. With its electron arrangement as [Ar] 3d^{10} 4s^2 4p^5, bromine has seven electrons in its outermost shell, just one short of the stable octet structure. This drives bromine to readily accept an electron to fill the gap and achieve stability, explaining its high reactivity.

Commonly, bromine engages in a process called halogenation, where it adds to organic compounds, often forming bromides (compounds with Br-). Its reactive nature is also responsible for its role in many biological systems and industrial applications, such as flame retardants and pharmaceuticals due to its ability to form diverse bonds with many elements.

Krypton Inertness

In stark contrast to bromine, krypton is known for its lack of reactivity, classified as one of the noble gases. Its electron configuration, [Ar] 3d^{10} 4s^2 4p^6, is emblematic of a complete octet—a state of optimal electron stability with eight electrons in its outer shell. This full valence shell prevents krypton from easily gaining or losing electrons, rendering it largely inert.

The inertness of krypton contributes to its uses in areas where non-reactivity is a benefit, such as lighting, where it's used in fluorescent bulbs and high-intensity lighting, and in some applications as an insulating gas.

Octet Rule

Central to the understanding of chemical reactions is the octet rule, a principle stating that atoms 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 with fewer than eight electrons tend to react and form compounds to reach this stable arrangement.

This rule explains why elements like bromine are so reactive—they seek to fill their outer shell—while elements like krypton already have a full shell and are impassive. However, it's important to note that while the octet rule is a helpful guideline, it does have exceptions, especially among elements with available d or f orbitals.

Chemical Stability

Chemical stability is influenced by an element's ability to reach a state where its energy is at a minimum, usually by having a full set of valence electrons. Elements with a full valence shell, like krypton, are inherently chemically stable and, therefore, less reactive.

This stability is why inert gases are often used in environments that require non-reactive conditions. In contrast, elements with incomplete valence shells, like bromine, are less stable and more chemically active, making them react to attain stability by achieving the electron configuration of the closest noble gas.