Water (H₂O) is a fundamental molecule. Its unique properties, including its ability to dissolve many substances and its high boiling point, are rooted in its atomic structure. Understanding the number of electrons in a water molecule is essential, as this count determines how its atoms interact and how the molecule functions.
Calculating the Total Electron Count
A neutral water molecule is composed of two hydrogen atoms and one oxygen atom. To find the total number of electrons, we sum the atomic numbers of its constituent atoms, since the atomic number equals the number of electrons in a neutral atom.
Oxygen has an atomic number of 8, meaning it contributes 8 electrons. Hydrogen has an atomic number of 1, and since there are two hydrogen atoms in water, they contribute a total of 2 electrons. Combining these electrons (8 from oxygen plus 2 from hydrogen) results in a total of 10 electrons in one water molecule.
Electrons in Action: Bonding and Lone Pairs
Although the water molecule has 10 total electrons, not all are involved in forming chemical bonds. These electrons are organized into inner-shell (core) electrons and outer-shell (valence) electrons. The oxygen atom’s 8 electrons are distributed as 2 core electrons and 6 valence electrons.
The two hydrogen atoms each contribute their single electron to the outer shell, bringing the total number of valence electrons in the water molecule to 8 (6 from oxygen and 2 from hydrogen). These 8 valence electrons form the molecule’s structure by creating two covalent bonds between the oxygen and the two hydrogen atoms. Since each covalent bond consists of a shared pair of electrons, 4 of the valence electrons are used for bonding.
The remaining 4 valence electrons on the oxygen atom are not shared with hydrogen. Instead, they exist as two distinct unshared pairs, commonly referred to as lone pairs. These two lone pairs are positioned around the oxygen atom, influencing the molecule’s final geometric arrangement. The oxygen atom thus achieves a stable octet, surrounded by four pairs of electrons: two shared pairs and two lone pairs.
The Result of Electron Distribution: Water’s Polarity
The arrangement of the 10 electrons and the nature of the covalent bonds are responsible for water’s polarity. Polarity arises from electronegativity, which is an atom’s ability to attract shared electrons in a chemical bond. Oxygen is significantly more electronegative than hydrogen, giving it a stronger pull on the shared electrons.
Because oxygen attracts the shared electrons more strongly, the electrons spend more time closer to the oxygen nucleus. This unequal sharing creates a separation of charge within the molecule. The oxygen atom acquires a partial negative charge (\(\delta^{-}\)) because the electrons, including the two lone pairs, are clustered near it.
Conversely, the hydrogen atoms are left with a lower electron density, giving each of them a partial positive charge (\(\delta^{+}\)). This permanent separation of charge, or dipole moment, makes water a polar molecule. The bent shape of the molecule, caused by the repulsion of the lone pairs, prevents the partial charges from canceling out, reinforcing the overall polarity.
This polarity allows water to dissolve many substances, earning it the nickname “universal solvent.” The partially positive hydrogen atoms on one water molecule are strongly attracted to the partially negative oxygen atoms of neighboring molecules, forming hydrogen bonds. These strong intermolecular attractions give water its high boiling point, surface tension, and other unique properties necessary for supporting biological systems.