Question

Draw Lewis structures for each of the following elements. Which two elements are most chemically stable? a. \(\mathrm{Br}\) b. \(S\) c. \(\mathrm{Kr}\) d. He

Short answer

Krypton (Kr) and helium (He) are the most chemically stable as they have complete valence electron shells.

Step-by-step solution

Draw Lewis Structure for Bromine (Br)

For bromine (Br), which is in group 17 (halogens), draw seven dots around the symbol 'Br' representing its seven valence electrons.

Draw Lewis Structure for Sulfur (S)

For sulfur (S), which is in group 16, draw six dots around the symbol 'S' to represent its six valence electrons.

Draw Lewis Structure for Krypton (Kr)

For krypton (Kr), a noble gas in group 18, draw eight dots around the symbol 'Kr' denoting its eight valence electrons, which fill its outermost shell.

Draw Lewis Structure for Helium (He)

For helium (He), a noble gas in group 18, draw two dots around the symbol 'He' to represent its two valence electrons.

Determine Chemical Stability

Since noble gases have a full valence shell, they are chemically inert or stable. Krypton (Kr) with eight valence electrons and helium (He) with two valence electrons (a full first shell) are the most chemically stable.

Key concepts

Chemical Stability

The concept of chemical stability is deeply rooted in the behavior of electrons in atoms. Generally speaking, an atom is considered chemically stable when it has little tendency to react with other substances. This stability is achieved when an atom's outermost electron shell is full, which usually means the atom has a configuration similar to that of noble gases.

To visualize this, we draw Lewis structures, which provide a vivid representation of the electron configuration. In these structures, valence electrons are shown as dots around the atomic symbols. For example, bromine (Br) and sulfur (S) depict seven and six dots, respectively, indicating they are keen to gain more electrons to become stable. However, when we look at krypton (Kr) and helium (He), their Lewis structures show full outer shells, revealing their natural stability and lack of reactivity.

By understanding chemical stability through Lewis structures, we grasp why certain elements are more reactive, while others, like noble gases, tend to maintain their inert nature.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining the chemical properties and reactivity of the element. The number of valence electrons can be easily determined from the group number of an element in the periodic table. For instance, group 17 elements, such as bromine (Br), have seven valence electrons, while group 16 elements, like sulfur (S), have six.

In drawing Lewis structures, valence electrons are depicted as dots placed around the symbol for that element. These electrons participate in the formation of chemical bonds, and an atom strives to fill its outer shell either by gaining, losing, or sharing valence electrons to achieve stability. The tendency to react is driven by an element's desire to reach an electron configuration that resembles that of the noble gases, often an octet in the outer shell, which confers chemical stability.

Noble Gases

Noble gases are elements that reside in group 18 of the periodic table and are unique in their lack of reactivity, which is a consequence of their fully occupied valence shells. This group includes helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn).

Helium, for example, has a complete first shell with two valence electrons, while the rest of the noble gases typically have eight valence electrons filling their outermost electron shell. This complete outer shell corresponds to a particularly stable electron configuration known as the 'octet rule' for most noble gases, though helium is stable with its 'duet.' Their Lewis structures are perhaps the simplest: a symbol with eight dots (except for helium with two), signifying full occupancy and the noble gases' significant chemical inertness. Thus, in our exercise, krypton (Kr) and helium (He) are identified as the most chemically stable elements due to their fulfilled valence electron shells.