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The number of covalent bonds an atom can form is dictated by its valence electrons. Oxygen has six valence electrons and needs two more to become stable, so it forms two single bonds or one double bond. Carbon has four valence electrons and can form four covalent bonds, as in methane. Each atom shares electrons to complete its outer shell and achieve stability.

Electronegativity determines how electrons are shared in a covalent bond. When atoms have similar electronegativities, they share electrons equally, forming nonpolar bonds with no charge across the molecule. When atoms have very different electronegativities, one atom pulls electrons toward it more strongly, creating a polar bond with partial positive and negative charges on opposite sides.

Water is polar because oxygen is much more electronegative than hydrogen. Oxygen pulls the shared electrons toward itself most of the time, giving it a partial negative charge while the hydrogen atoms gain partial positive charges. This unequal electron distribution makes water polar, allowing it to interact with other charged molecules and dissolve many substances.

Polar molecules are hydrophilic because their partial charges attract them to water and other charged molecules, making them soluble in water. Nonpolar molecules, such as fats containing long hydrocarbon stretches, are hydrophobic and do not dissolve in water. This difference is crucial for cells because water surrounds them and fills their cytoplasm, affecting how molecules are transported and used.

When a covalent bond forms, both atoms share a pair of electrons in a hybrid orbital that differs in shape from a normal orbital. The electrons orbit in a modified path around the nuclei of both atoms simultaneously. These covalent bonds are strong and cannot be broken by physical forces once formed, making them stable molecular connections.

Covalent bonds usually form between nonmetals such as carbon, hydrogen, nitrogen, oxygen, and chlorine. These elements have valence electrons available for sharing and are positioned on the periodic table in ways that favor electron sharing over electron transfer. Their similar electronegativities allow them to form stable covalent bonds with each other.

A single bond forms when two atoms share one pair of electrons, as when oxygen bonds with two hydrogen atoms in water. A double bond forms when two atoms share two pairs of electrons, as when two oxygen atoms bond together. The number of shared electron pairs depends on how many valence electrons each atom needs to complete its outer shell.