If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Hydrogen bonds in water

The structure of water molecules and how they can interact to form hydrogen bonds.

Introduction to the properties of water

You are a tool-making, communicating, learning bag of water. Okay, that’s not completely fair, but it's close since the human body is 60 to 70% water. And it's not just humans—most animals and even tiny bacteria are made up mostly of water1. Water is key to the existence of life as we know it. That may sound dramatic, but it’s true—and dramatic things that are true are what make life interesting! Most of an organism’s cellular chemistry and metabolism occur in the water-based “goo” inside its cells, called cytosol.
Water is not only very common in the bodies of organisms, but it also has some unusual chemical properties that make it very good at supporting life. These properties are important to biology on many different levels, from cells to organisms to ecosystems. You can learn more about the life-sustaining properties of water in the following articles:
Water owes these unique properties to the polarity of its molecules and, specifically, to their ability to form hydrogen bonds with each other and with other molecules. Below, we'll look at how this hydrogen bonding works.

Polarity of water molecules

The key to understanding water’s chemical behavior is its molecular structure. A water molecule consists of two hydrogen atoms bonded to an oxygen atom, and its overall structure is bent. This is because the oxygen atom, in addition to forming bonds with the hydrogen atoms, also carries two pairs of unshared electrons. All of the electron pairs—shared and unshared—repel each other.
The most stable arrangement is the one that puts them farthest apart from each other: a tetrahedron, with the OH bonds forming two out of the four “legs”. The lone pairs are slightly more repulsive than the bond electrons, so the angle between the OH bonds is slightly less than the 109° of a perfect tetrahedron, around 104.5°.2
Because oxygen is more electronegative—electron-greedy—than hydrogen, the O atom hogs electrons and keeps them away from the H atoms. This gives the oxygen end of the water molecule a partial negative charge, while the hydrogen end has a partial positive charge. Water is classified as a polar molecule because of its polar covalent bonds and its bent shape2,3.

Hydrogen bonding of water molecules

Thanks to their polarity, water molecules happily attract each other. The plus end of one—a hydrogen atom—associates with the minus end of another—an oxygen atom.
These attractions are an example of hydrogen bonds, weak interactions that form between a hydrogen with a partial positive charge and a more electronegative atom, such as oxygen. The hydrogen atoms involved in hydrogen bonding must be attached to electronegative atoms, such as O, N, or F.
Water molecules forming hydrogen bonds with one another. The partial negative charge on the O of one molecule can form a hydrogen bond with the partial positive charge on the hydrogens of other molecules.
Water molecules are also attracted to other polar molecules and to ions. A charged or polar substance that interacts with and dissolves in water is said to be hydrophilic: hydro means "water," and philic means "loving." In contrast, nonpolar molecules like oils and fats do not interact well with water. They separate from it rather than dissolve in it and are called hydrophobic: phobic means "fearing." You may have noticed this as a not-so-handy feature of oil and vinegar salad dressings. Vinegar is basically just water with a bit of acid.

Want to join the conversation?

  • duskpin ultimate style avatar for user Andrea
    Could someone explain what a polar molecule and bond are? I'm not great at science in general, so I don't understand what a polar molecule is and why water is a polar covalent bond is.
    (15 votes)
    Default Khan Academy avatar avatar for user
    • spunky sam red style avatar for user Jamie
      a bond is when 2 or more elements combine chemically and create a molecule. Elements bond either by sharing or transferring electrons. When a bond is formed by sharing electrons, it's called a covalent bond. sometimes the electrons in a covalent bond are shared unequally which causes some parts of the molecule to be partially positive and other parts to be partially negative. When this happens, its called a polar molecule. In water, the oxygen atom gets the electrons more frequently than the two hydrogens because of unequal sharing. This causes the side of the molecule with the oxygen to be partially negative and the side with the hydrogens to be partially positive, making water a polar covalent molecule. hopefully that helps :)
      (142 votes)
  • duskpin seed style avatar for user Hi Nice to meet you
    what is used to break hydrogen bonds in water?
    (27 votes)
    Default Khan Academy avatar avatar for user
  • leaf blue style avatar for user Anastasia Stampoulis
    What is a partial positive or partial negative charge ?
    (14 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Dovid Shaw
      When one atom "shares" an electron with another atom to form a molecule, the atom with higher electronegativety (electron-greedy) will keep the shared electrons closer to itself than to the partner-atom. Since one atom gained an electron and keeps it near itself, the atom receives a "partial negative charge." It isn't fully charged because the molecule is neutral due to its balance of negative and positive regions.
      p.s. More electrons = negative charge, fewer electrons = positive charge.
      (48 votes)
  • blobby green style avatar for user yuvalboek1971
    Is every hydrophilic molecule polar?
    (8 votes)
    Default Khan Academy avatar avatar for user
    • female robot grace style avatar for user tyersome
      That is generally a safe assumption, but should not be regarded as a "law".

      An example that doesn't fit this rule perfectly is CO₂, which is non-polar, but still somewhat soluble in water — this is in part because it reacts with water, but also because of weak effects due to the presence of polar bonds within this non-polar molecule.

      Also, note that hydrophilicity and polarity are both continua, so there are many molecules that are in the middle of both of these scales.

      There are also many polyatomic ions (not formally molecules since they have charges) that are very hydrophilic, but also completely non-polar because they are symmetrical. Examples include: ammonium (NH₄⁺), sulfate (SO₄²¯), carbonate (CO₃²¯), and oxalate (C₂O₄²¯).

      It is also worth remembering that single atom ions (e.g.s Na⁺, Cl¯, Mg²⁺) are hydrophilic but not polar.
      (14 votes)
  • mr pink red style avatar for user Chadislav
    "This gives the oxygen end of the water molecule a partial negative charge, while the hydrogen end has a partial negative charge" it should be "...hydrogen end has a partial POSITIVE charge"
    (12 votes)
    Default Khan Academy avatar avatar for user
  • piceratops seed style avatar for user priscillaiscool12
    how does water come around the cycle
    (6 votes)
    Default Khan Academy avatar avatar for user
    • leafers ultimate style avatar for user Sarah Wouters
      The water cycle, in the simplest form, is evaporation, condensation, and precipitation.
      Evaporation: When the temperature raises high enough, the water particles begin to move faster and gain more kinetic energy. As they move faster, the particles spread out in all directions. This spreading converts the water into a gas, or water vapor.
      Condensation: The temperature slowly or slightly drops to where the water vapor's particles begin to slow. The decrease in speed makes the particles come closer together, making small droplets.
      Precipitation: When the droplet or cloud gets full with water, the density makes the water fall, making it rain. If the temperature is cold enough, the rain droplets will freeze as the particles slow, making snow.

      I hope that helps!
      (12 votes)
  • starky tree style avatar for user Pardhu Kaknuri
    what is the reason to bond
    (7 votes)
    Default Khan Academy avatar avatar for user
    • hopper happy style avatar for user Farhath
      Polar molecules, such as water molecules, have a weak, partial negative charge at one region of the molecule (the oxygen atom in water) and a partial positive charge elsewhere -(the hydrogen atoms in water). Thus, when water molecules are close together, their positive and negative regions are attracted to the oppositely-charged regions of nearby molecules which makes it bond! and even the answer- to become neutral is also correct! for more information visit-http://www.biology-pages.info/H/HydrogenBonds.html
      (5 votes)
  • leafers seedling style avatar for user Zesun
    Every Organism is 60 to 70 percent water . Then Why am I not fluid? Why my bones Are Solid?
    (6 votes)
    Default Khan Academy avatar avatar for user
    • starky ultimate style avatar for user rbarathshankar
      Most of your cells are filled with cytosol, which is water. Your bones are made from cells called osteoblast, they arent cells, but a type of cement made by these cells. Your epidermis (skin) holds all the water in you together. Your blood cells have water, your muscles have water, your neurons have, water. Most of you is water. I hope that makes sense and helps :)
      (7 votes)
  • blobby green style avatar for user Voldigoat27
    What is polarity in water molecules?
    (6 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Sahana Sivakumar
    Can an oxygen atom, which is partially negative attract two hydrogen atoms which are partially positive? The hydrogens that are not connected to the oxygen atom using covalent bonds.
    (5 votes)
    Default Khan Academy avatar avatar for user