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Why do two nonmetal atoms usually share electrons instead of transferring them?

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Diagram showing the formation of a polar covalent bond in a water molecule, with two hydrogen atoms bonding to one oxygen atom to form H2O, illustrating shared electrons and chemical reaction. — covalent bond In polar covalent bonds, such as that between hydrogen and oxygen atoms, the electrons are not transferred from one atom to the other as they are in an ionic bond. Instead, some outer electrons merely spend more time in the vicinity of the other atom. The effect of this orbital distortion is to induce regional net charges that hold the atoms together, such as in water molecules.

When two nonmetal atoms react, they typically share electrons to form covalent bonds because they possess comparable attractive powers for electrons. Unlike metals, which have relatively low ionization energies and readily lose electrons to form cations, nonmetals have high ionization energies and high electronegativities.12

Because nonmetal atoms are reluctant to give up their own electrons, neither atom can effectively strip an electron from the other. Instead, the atoms combine by pairing their valence electrons, which allows them to achieve a more stable, lower-energy state through sharing. While these electrons are shared, they are not always shared equally; if the atoms have differing electronegativities, the electrons will spend more time near the more electronegative atom, resulting in a polar covalent bond.134

In essence, electron sharing is the mechanism that allows nonmetals to satisfy their chemical requirements when neither participant is willing or able to undergo a complete transfer of electrons.3