Question

In HF, the hydrogen atom shares two electrons with the fluorine atom, but has no unshared electron pairs. Discuss why this observation is consistent with the principle behind the octet rule.

Short answer

In the HF molecule, the hydrogen atom shares its only electron with the fluorine atom, thus fulfilling its shell capacity to two electrons, and simultaneously, the fluorine atom gets an extra shared electron from hydrogen that makes a total of eight electrons in its outer shell. This way, both atoms have achieved stable electron configurations, which is the main principle of the octet rule. The fact that the hydrogen atom doesn't have unshared electron pairs doesn't violate the octet rule because it achieves a stable electron configuration by sharing its single electron.

Step-by-step solution

Hydrogen Atom's Electron Configuration

The hydrogen atom has only one electron to begin with. When it shares this electron with a fluorine atom in HF, it no longer has a need for unshared electron pairs because its outer shell is full with this shared electron.

Fluorine Atom's Electron Configuration

Now consider the fluorine atom. The atomic number of fluorine is 9, meaning it has 9 electrons in total. In its ground state, it has two electrons in the first energy level and seven in the second. When fluorine combines with hydrogen to form HF, the hydrogen shares one of its electrons with fluorine. This allows fluorine to fill its outer second energy level to hold a total of eight electrons, thereby fulfilling the octet rule.

Octet Rule Consistency

In the HF molecule, the hydrogen atom shares its only electron with fluorine, thus fulfilling its shell capacity, and the fluorine atom gets an extra shared electron from hydrogen fulfilling its outer shell capacity to eight electrons. Hence, both atoms have achieved stable electron configurations, which is the main principle of the octet rule. Therefore, the situation is consistent with the octet rule because each atom shares electrons in such a way that it achieves a stable electron configuration, even if the hydrogen atom doesn't have unshared electron pairs.

Key concepts

Electron Configuration

Understanding electron configuration is crucial to comprehend how atoms interact to form molecules. Atoms have electrons arranged in shells around their nucleus, and the number of electrons and the way they are distributed across these shells determines an element's chemical properties.
For hydrogen, there's only one electron present in its single shell, which means it's already quite simple. This single electron is not enough to follow the octet rule that other atoms use to achieve a stable electronic arrangement. However, hydrogen does follow the duet rule, which means that its single shell is full once it has two electrons. We'll see how this works in the molecule HF soon.
When we consider fluorine, it has an atomic number of 9, meaning it contains 9 electrons. These are arranged with two electrons in its first shell and seven in its second, outermost shell. Thus, fluorine needs just one more electron to complete its outer shell of eight electrons. Achieving eight electrons in the outer shell is the preferred configuration because it leads to greater stability. This desire for stability through electron configuration is the essence of the octet rule.

Covalent Bonding

Covalent bonding is the process where atoms share electrons to achieve stable electron configurations. This is how molecules form.
In a hydrogen fluoride (HF) molecule, hydrogen and fluorine share their electrons via a covalent bond. Hydrogen has one electron and fluorine needs only one more to complete its octet. By sharing their electrons, both achieve a form of stability.
Covalent bonds are particularly strong and are a primary method by which stable molecules are formed. Helium or neon already have filled electron shells, so they don't form such bonds easily. However, hydrogen and fluorine require the electrons from each other, allowing them to combine their individual atoms into a stable molecule. This bond results in both achieving a more stable electron configuration than they would separately.

Molecular Stability

Molecular stability relates to how atoms interact to maintain or reach a stable state. In chemical reactions, atoms tend to share, lose, or gain electrons to minimize energy and achieve a stable electron configuration.
For HF, hydrogen and fluorine atoms adhere to this principle by sharing electrons, thereby lowering their energy and reaching greater stability. While hydrogen achieves stability with a duet of two electrons, fluorine achieves a stable octet.
This reflects the universal tendency of atoms to find the most stable electronic arrangement—often reaching an octet for most atoms or a duet for hydrogen. Molecules like HF illustrate how fulfilling the octet or duet tendencies ensures that the atoms involved are at their most energetically favorable state. This explains why HF is a stable molecule, despite being made from two very different atoms.