Question

Show how each chemical change leads to a stable octet. (a) Sulfur forms \(\mathrm{S}^{2-}\). (b) Magnesium forms \(\mathrm{Mg}^{2+}\).

Short answer

Sulfur achieves a stable octet by gaining 2 electrons, forming \(\mathrm{S}^{2-}\), while magnesium achieves a stable octet by losing 2 electrons, forming \(\mathrm{Mg}^{2+}\).

Step-by-step solution

Determine the electron configuration of sulfur

Sulfur has an atomic number of 16, meaning it has 16 electrons in its neutral state. The electron configuration of sulfur is: 1s2, 2s2, 2p6, 3s2, 3p4. So, there are 6 electrons in the outermost energy level (3s2, 3p4).

Determine how sulfur achieves a stable octet

Since sulfur has 6 electrons in its outermost energy level, it needs 2 more electrons to achieve a stable octet. To do this, sulfur can gain 2 electrons, which would give it a charge of -2. Thus, sulfur forms \(\mathrm{S}^{2-}\). The new electron configuration with the added electrons is: 1s2, 2s2, 2p6, 3s2, and 3p6, which satisfies the stable octet rule.

Determine the electron configuration of magnesium

Magnesium has an atomic number of 12, meaning it has 12 electrons in its neutral state. The electron configuration of magnesium is: 1s2, 2s2, 2p6, 3s2. So, there are 2 electrons in the outermost energy level (3s2).

Determine how magnesium achieves a stable octet

Since magnesium has 2 electrons in its outermost energy level, it needs to lose these 2 electrons to achieve a stable octet in the 2p energy level. To do this, Magnesium can lose 2 electrons, which would give it a charge of +2. Thus, magnesium forms \(\mathrm{Mg}^{2+}\). The new electron configuration after losing the electrons is: 1s2, 2s2, and 2p6, which satisfies the stable octet rule.

Key concepts

Electron Configuration

Understanding how atoms interact to form compounds is fundamentally linked to their electron configuration. This term describes the distribution of electrons in an atom's various orbits or energy levels. For example, sulfur, with an atomic number of 16, possesses electrons that are arranged in energy levels as follows: 1s2, 2s2, 2p6, 3s2, 3p4. The 's' and 'p' denote the shape of the orbitals and the numbers tell us how many electrons are in each orbital.

Now, the key to an atom's reactivity lies in its valence electrons, which are the electrons in the outermost shell. Here sulfur has 6, requiring an additional 2 to complete its 'octet', a group of 8 valence electrons that provides stability. By adding two electrons to its valence shell, sulfur would fill its 3p orbital, resulting in a stable 3s2, 3p6 configuration. This concept is intuitive once you visualize the orbitals filling up like seats in a stadium, with the goal of completing each row to maximize stability.

Ionic Compounds

Building Bonds Between Atoms

Atoms with incomplete valence shells tend to achieve stability by forming compounds. Ionic compounds are one such type, where an atom donates or accepts electrons to fulfill the stable octet rule. When a magnesium atom loses its 2 valence electrons, it attains a noble gas electron configuration (like neon), thus becoming a positively charged ion, \(\mathrm{Mg}^{2+}\).

These 'lost' electrons can be 'gained' by another atom like sulfur, which needs 2 more electrons for stability, becoming \(\mathrm{S}^{2-}\). The resulting attraction between the positively charged magnesium cation and the negatively charged sulfur anion forms an ionic bond, creating an ionic compound, such as magnesium sulfide (\(\mathrm{MgS}\)). This transfer and sharing of electrons are central to understanding how seemingly disparate elements can combine to form compounds with entirely new properties.

Chemical Stability

A Quest for Equilibrium

In chemistry, achieving stability often drives reactions. Atoms like magnesium and sulfur undergo chemical changes to reach a state called a stable octet, which significantly lowers their energy. The stable octet is akin to a state of balance where elements are less reactive and more resistant to change.

An atom’s journey to stability isn’t a solo venture; it often involves teamwork where different atoms come together to share or transfer electrons. Magnesium gives up its 2 valence electrons freely, finding stability in a positively charged form, while sulfur accepts additional electrons to complete its set of 8, leading to chemical stability. These stable configurations are favorable and are often found in nature as part of ionic compounds which display distinct properties like high melting points, crystalline structures, and the ability to conduct electricity when dissolved or melted. Through the lens of the octet rule, students can better understand the underlying principles that govern how and why chemical compounds form.