The particles shared between atoms in a molecule are electrons, specifically the electrons in each atom’s outermost energy level, known as valence electrons. When two or more atoms share valence electrons, they form what’s called a covalent bond, and the resulting structure is a molecule. No other subatomic particles (protons or neutrons) are shared in this process.
Why Atoms Share Electrons
Atoms are most stable when their outer energy level is full. For most elements, that means having eight electrons in the outermost shell, a pattern chemists call the octet rule. Hydrogen is an exception: it only needs two. The problem is that many atoms don’t start with a full outer shell. Oxygen, for example, has six valence electrons but needs eight. Rather than gaining or losing electrons outright, atoms can pool their resources by sharing.
Each atom contributes one electron to a shared pair, and both atoms get to count that pair toward their own outer shell. This is what makes the arrangement beneficial: both atoms effectively gain an extra electron without either one losing anything. The shared pair sits in the space between the two nuclei, and the electrical attraction between the positive nuclei and the negative electrons is what holds the molecule together.
How Many Electrons Are Shared
A single covalent bond consists of one shared pair, meaning two electrons total. But atoms can share more than one pair when they need additional electrons to fill their outer shells:
- Single bond: 2 shared electrons (one pair). Example: H₂, where each hydrogen contributes one electron.
- Double bond: 4 shared electrons (two pairs). Example: O₂, where each oxygen shares two of its electrons with the other.
- Triple bond: 6 shared electrons (three pairs). Example: N₂, where each nitrogen shares three electrons to complete its octet.
More shared electrons means a stronger, shorter bond. The distance between two bonded nitrogen atoms in N₂ (a triple bond) is about 1.1 angstroms, while the distance between two bonded fluorine atoms in F₂ (a single bond) is about 1.4 angstroms. The extra electron pairs pull the nuclei closer together.
Where the Shared Electrons Actually Are
Shared electrons don’t sit still between two atoms like marbles on a shelf. They occupy a region of space called a molecular orbital, which belongs to the molecule as a whole rather than to either individual atom. When two atoms approach each other, their individual atomic orbitals combine to create these new molecular orbitals.
In the simplest case, two hydrogen atoms each bring one electron in a 1s orbital. These orbitals combine in an additive way, producing a lower-energy bonding orbital that is concentrated in the region between the two nuclei. Because the electrons spend most of their time in this space between the nuclei, they create an attractive force that holds the atoms together. The resulting H₂ molecule has a bond length of just 0.74 angstroms.
Equal vs. Unequal Sharing
Not all electron sharing is perfectly even. When two identical atoms bond (like O₂ or H₂), both atoms pull on the shared electrons with equal strength, creating a nonpolar covalent bond. The electrons are distributed symmetrically.
When different atoms bond, one atom usually attracts the shared electrons more strongly than the other. This pulling power is called electronegativity. A small difference in electronegativity creates a polar covalent bond, where the electrons spend slightly more time near one atom than the other. Water is a classic example: oxygen pulls the shared electrons closer to itself and away from the hydrogen atoms. This gives the oxygen end a slight negative charge and each hydrogen end a slight positive charge.
A large electronegativity difference pushes things past sharing entirely. In sodium chloride (table salt), the difference is so large (about 2.1) that the electron is essentially transferred from sodium to chlorine rather than shared. This creates ions instead of a molecule, which is why sodium chloride is described as an ionic compound rather than a molecular one.
How Sharing Differs From Other Types of Bonding
Electron sharing in covalent bonds is specific: particular electrons are shared between particular pairs of atoms. Metals work differently. In a metal like copper or iron, the valence electrons aren’t attached to any one atom. Instead, they drift freely through the entire structure, forming what chemists describe as a “sea” of mobile electrons surrounding a grid of positively charged metal ions. This is why metals conduct electricity so well, and it’s why metallic bonding produces bulk materials rather than distinct molecules.
Ionic bonding, as mentioned above, involves a complete transfer of electrons from one atom to another rather than sharing. The key distinction is straightforward: molecules exist because atoms share valence electrons in covalent bonds. If electrons are transferred instead of shared, you get an ionic compound, not a molecule.