共有結合

Atoms form covalent bonds by sharing one or more pairs of valence electrons. The shared electron pair interacts with both atoms' nuclei, lowering the potential energy and creating a stable bond. This electron sharing allows atoms to achieve the same electron configuration as the nearest noble gas, completing their valence shells.

Nitrogen requires three more electrons to reach its nearest noble gas configuration, so it forms triple bonds with other atoms. Oxygen needs only two additional electrons, forming double bonds instead. The number of covalent bonds an atom forms depends on how many valence electrons it has and how many it needs to achieve stability.

Electronegativity is the force with which an atom attracts shared electrons in a bond. Atoms with similar electronegativities form nonpolar covalent bonds where electrons are shared equally. Atoms with different electronegativities form polar bonds, with the more electronegative atom pulling electrons toward it, creating partial negative and positive charges.

As the number of bonds between atoms increases, bond strength increases. Triple bonds are stronger than double bonds, which are stronger than single bonds. This increased strength occurs because more shared electron pairs create stronger interactions between the bonding atoms' nuclei and the electrons.

A molecule's polarity is determined by the electronegativity difference between bonded atoms. Nonpolar molecules have atoms with similar electronegativities, sharing electrons equally with no charge distribution. Polar molecules have atoms with different electronegativities, creating partial charges that make them hydrophilic and soluble in water, unlike hydrophobic nonpolar molecules.

Carbon has four valence electrons and can form four covalent bonds, as in methane (CH4). Oxygen has six valence electrons and needs two more to complete its octet, allowing it to form two single bonds or one double bond. The number of bonds each atom forms depends on its valence electron count.

Polar molecules contain partial charges that attract them to water's charged molecules, making them hydrophilic and water-soluble. Nonpolar molecules with long hydrocarbon stretches, such as fats, are hydrophobic and cannot dissolve in water. Understanding how molecules interact with water is central to energy transfer in chemical reactions and cellular transport.