How can shared electrons hold atoms together?

A water molecule remains a water molecule when it moves through the air as vapor. The two hydrogen atoms and one oxygen atom stay joined because electrons and nuclei interact in a stable arrangement. These connections are covalent bonds. Understanding them explains why boiling water and splitting water chemically are very different changes.

Each atomic nucleus is positively charged, while electrons are negatively charged. In a covalent bond, electron density is shared between atoms and attracted to more than one nucleus. At a suitable separation, these attractions help make the bonded arrangement lower in energy than the separated atoms. Pushing the nuclei much closer increases repulsion, so a bond has a characteristic length. Electrons do not sit still halfway between two hard balls, even though a simple drawing may place dots there.

The simplest example is hydrogen gas. Two hydrogen atoms form an H₂ molecule with a shared pair of electrons. A line between their symbols represents that bond. In water, oxygen forms two bonds with hydrogen. Oxygen also has electron pairs that are not shared in these bonds. Their arrangement contributes to water's bent shape. A chemical diagram records connections; understanding the three-dimensional shape requires more than counting the lines on a page.

Atoms do not share all electron pairs equally. Oxygen attracts the shared electrons in an O–H bond more strongly than hydrogen does. That unequal sharing affects how water molecules attract one another and interact with other substances. However, unequal sharing does not automatically mean that an electron has been completely transferred to form separate ions. Chemical bonding includes a range of distributions of electron density.

Breaking a bond requires energy. Forming a bond releases energy relative to the separated partners, and a chemical reaction's overall energy change depends on both the bonds broken and those formed. Boiling water mainly separates water molecules from one another; it does not normally break their O–H bonds. Steam can condense back into liquid water because those molecules survived. The important question is whether a change rearranges whole molecules or changes the connections inside them.