How does a Lewis structure show a bond between two atoms that share four electrons?

At the heart of molecules is a type of chemical bond known as covalent bonding. This occurs when two atoms share a pair of valence electrons. The driving force behind this sharing is to achieve greater stability, usually resulting in each atom having a filled outer shell of electrons.

When atoms bond covalently, they are able to fill their outer shell without actually losing or gaining electrons outright, which would result in an ionic bond instead. A covalent bond can involve the sharing of one, two, or even three pairs of electrons, leading to single, double, or triple bonds respectively. The shared electrons are attracted to the nuclei of both atoms, which keeps the atoms together. A very common example of covalent bonding is the oxygen molecule (O₂), where two oxygen atoms share two pairs of electrons, forming a double bond for enhanced stability.

In summary, covalent bonding is a fundamental concept in chemistry that explains how molecules form and how atoms come together to achieve lower energy states and increased stability by sharing their valence electrons.

The octet rule is a principle that helps us understand the typical pattern of chemical bonding for most elements (with notable exceptions of hydrogen and helium). According to this rule, atoms are most stable when they have eight electrons in their valence shell, which is the row of electrons farthest from the nucleus.

Atoms will bond with other atoms to either gain, lose, or share electrons until they achieve an octet. In the context of covalent bonding, they share electrons to reach this state. This rule is crucial because it guides the creation of Lewis structures and helps us deduce the types of bonds (single, double, or triple) that might form between different atoms.

To illustrate, take carbon dioxide (CO₂): carbon has four valence electrons and needs four more to complete its octet. Each oxygen atom needs two electrons to complete its octet. When carbon forms double bonds with two oxygen atoms, each atom achieves a full octet, adhering to the rule.

In discussions about chemical bonding, valence electrons play a starring role. These are the electrons that reside in the outermost electron shell of an atom. Fundamentally, the chemical properties of an element are defined by the number of valence electrons it possesses. Indeed, it is these very electrons that participate in chemical bonding.

Valence electrons determine an atom's bonding capacity, or valency, and are crucial in predicting how an atom will interact with others. For instance, the noble gases, with full outer shells, are largely nonreactive due to having sufficient valence electrons to be stable without forming bonds. In contrast, alkali metals have a single valence electron that they readily lose to achieve a stable electron configuration, becoming highly reactive.

In constructing Lewis structures, the dots around the atoms symbolize these pivotal valence electrons, providing a visual representation of where bonds may form and the state of single, double, or triple bonding, as well as any non-bonded 'lone pairs' of electrons that may also impact the shape and reactivity of molecules.