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In a hydrogen molecule (H₂), two hydrogen atoms are joined by a classic covalent bond. Each hydrogen atom contains a single proton and a single electron, and its outermost shell can accommodate two electrons. Because the atoms are identical, neither can donate its lone electron to the other; instead, they share the two electrons, creating a stable, non‑ionic bond.
Hydrogen gas consists of H₂ molecules where the two hydrogen atoms share electrons in a covalent bond. Hydrogen also forms covalent bonds in water (H₂O) and in hydrocarbons. In water, the covalently bonded hydrogens can establish weaker, intermolecular hydrogen bonds, which give water its unique physical properties.
The hydrogen atoms in H₂O each share a pair of electrons with the oxygen atom, which has six valence electrons and a shell that can hold eight. This sharing completes oxygen’s octet and creates the O–H covalent bonds that define the water molecule.
Beyond the covalent bonds, water molecules interact via dipole–dipole hydrogen bonds. The molecule’s polarity—negative on the oxygen end and positive on the hydrogen end—allows one water’s oxygen to attract the hydrogen of a neighboring molecule. These intermolecular forces, though weaker than covalent bonds, give water high surface tension and a relatively high boiling point for its molecular weight.
Carbon’s outer shell contains four valence electrons and can hold eight. In methane (CH₄), carbon shares one electron with each of four hydrogens, filling its shell and forming four stable covalent bonds. This simple arrangement exemplifies carbon’s versatility, as it can form a wide variety of organic molecules by combining with other carbons and heteroatoms.
When carbon atoms form fewer than four C–H bonds, the remaining valence electrons are shared between carbons. For instance:
Extending these chains yields longer hydrocarbons such as propane (C₃H₈), where a linear sequence of three carbons is linked by single bonds, each carbon also bonded to the appropriate number of hydrogens. This pattern underpins the vast diversity of organic chemistry.
