why does covalent bonds determine the polarity of water?

Question

Why does hydrogen bonds determine the water's special propeties?

Answer 1

Any chemical reaction involves an energy change or transfer. Atoms combine to form molecules, because the sharing or transferring of electrons result in a lower, and more stable energy state. That is, energy input from endothermic reactions are stored in the bonds; exothemic reactions release energy as heat and radiation; oxidation-reduciton reactions involves the interconversion of chemical and electrical energy to do work, etc.

                In a covalent bond, two nonmetals share electrons equally to fullfill their valence electron configurations. Hydrogen's 1s¹ can hold one more electron, and oxygen's (1s²2s²2p⁴) 2p orbitals can hold two more electrons. Oxygen, however, is more electronegative than hydrogen; it pulls the shared electrons closer to itself, and away from the hydrogen. The result of the unequal sharing of electrons is a polar covalent bond, and a molecule that is dipolar, or a molecule that has a dipole momment. Due to the greater electron distribution around oxygen, it is the center of a partial negative charge. Hydrogen is the center of the partial positive charge.

In ionic bonding, a metal and nonmetal combine by a transfer of electrons to complete their outer shell. Generally, the more electrognegative nonmetals tend to gain electrons to form anions; whereas, metals tend to loose electrons easily, and become cations. There is no totally ionic character however, in any bond types. A transfer of electron is considered when the difference in electrogenativities between the bonded atoms are significantly large.

Furthermore, the polarity of the molecule is determined by its geometry, and the vector sum of the diploes. For example, CH₃Cl (methyl chloride) is polar, but CCl₄ is nonpolar. Thus, linear, trigonal planar, tetrahedral, etc., molecules with identical groups have dipoles which cancel, and are nonpolar. The geometry is determined by either the VSEPR model, or Molecular Orbital Model.

Answer 2

1. Why do covalent bonds determine the polarity of water?

Water (H₂O) has two hydrogen atoms covalently bonded to one oxygen atom. These covalent bonds involve sharing electrons, but oxygen is more electronegative than hydrogen, so it pulls the shared electrons closer to itself. This unequal sharing creates a partial negative charge near the oxygen and partial positive charges near the hydrogens, making the molecule polar. The shape of the molecule (bent) ensures the dipoles don’t cancel out, so overall water has a positive and a negative side.

2. Why do hydrogen bonds determine water’s special properties?

Hydrogen bonds form when the partially positive hydrogen atom of one water molecule is attracted to the partially negative oxygen atom of another. These bonds are weaker than covalent bonds but strong enough to create a network of interactions. This hydrogen bonding is responsible for many of water’s unique properties, such as:

1. High boiling and melting points (because energy is needed to break hydrogen bonds)
2. Surface tension and cohesion (water molecules stick together)
3. Excellent solvent abilities (water surrounds and interacts with many substances)
4. Ice being less dense than liquid water (hydrogen bonds create an open lattice structure when frozen)

In short, covalent bonds create the polarity of the water molecule, and hydrogen bonds between molecules give water its extraordinary behaviors.

Answer 3

Covalent bond is formed by atomic orbitals and forms the hybrid orbitals of same size and same energy. Atomic orbitals as well as hybrid orbitals show the directional property so bond formed by these orbitals also contains directional property. So Covalent bonds determine the polarity in the molecule.

Answer 4

The molecular shape of H₂O is bent, and the molecule is covalent. The bent shape is a result of the tetrahedral electron geometry (according to VSEPR theory). Since the molecule has a tetrahedral electron geometry containing 2 lone pairs of electrons, the result is a bent molecule.

The bonding is covalent; however, Oxygen has a much higher electronegativity than Hydrogen. The difference in electronegativity leads to polar covalent bonding between oxygen and hydrogen. The oxygen has a partial negative charge, and the hydrogen atoms are partially positively charged.

When you draw the molecular shape of H₂O, you will see that the partial negative charge on oxygen and the partial positive charges on the hydrogen atoms result in a net dipole. The net dipole makes water polar.

Hydrogen bonding occurs in water, and it is an intermolecular force that helps hold molecules of water together. This is not the only intermolecular force holding water molecules together. There are also London dispersion forces and dipole-dipole interactions. The dipole-dipole interactions occur because water molecules have permanent dipoles. Dipole-dipole interactions are generally stronger than London dispersion forces. However, hydrogen bonding is an even stronger intermolecular force that helps hold the molecules together even more.

Stronger intermolecular forces lead to higher boiling points and higher surface tension.

Answer 5

Water is a bent molecule. The reason this is true can be seen through application of VSEPR theory. Since the oxygen atom is "surrounded" by 4 pairs of electrons (2 shared pairs and two unshared pairs), this results in a geometry based on the four electron pairs of a slightly distorted tetrahedral (Comparable to CH₄ geometry). This places each hydrogen atom at one corner of a tetrahedron, each unshared pair oriented towards the other two corner of the tetrahedron, and the oxygen atom at the center of the tetrahedron.

Note: The hybridization for the oxygen atom is sp³, similar to that for the central atom in methane (CH₄) and ammonia (NH₃).

Since oxygen is a much more electronegative atom than hydrogen, the shared pairs of electrons between and O atom and an H atom are "pulled" towards the O atom. The bond between the oxygen atom and each hydrogen atom is polar covalent. This effect, along with the two unshared pairs of oxygen, imparts a partial negative charge to the oxygen atom and leaves each hydrogen atom with a partial positive charge. Because of the asymmetrical geometry of the water molecule (bent) and these polar covalent bonds, the water molecule is polar (it has a dipole moment).

Hydrogen bonding (a type of inter-molecular bonding) takes place between a H atom in one molecule and an oxygen, nitrogen, or fluorine atom in another atom. Additionally, the hydrogen bonding H atom must be bonded to O, N or F in its own molecule and the O, N, or F in the other molecule must be bonded to H in its molecule. (Ex: The H in NH₃ can hydrogen bond to the the F in HF or the O in H₂O or the H in another NH₃ molecule) This very strong inter-molecular attraction results in the molecules being held together tighter than might otherwise expect based on dispersion forces alone. In the case of water, the partially negative oxygen atom of one molecule has a temporary strong attraction to the partially positive hydrogen atom in another atom. In the liquid phase, these attractions make and break continually but the overall effect is to hold the molecules together relatively tightly with a resulting high surface tension, low vapor pressure, and high boiling point. In the solid, the hydrogen bonds hold the molecules together to make ice with a high melting point.

Answer 6

Brenda:
     You seem to have two questions:

1. Why is the water molecule polar?
2. How does hydrogen bonding give water special properties?

Answer to question 1: A water molecule contains 1 oxygen atom and 2 hydrogen atoms. The 3 atoms are not arranged in a straight line but rather in a triangle with the angle between the hydrogen atoms as seen from the oxygen atom being about 135°. Since the 6 outer electrons of the oxygen atom and the 2 electrons from the hydrogen atoms tend to hang around the oxygen nucleus, the oxygen corner of the triangle becomes more negatively charged and the hydrogen corners more positive. This gives the water molecule its polarity, negative toward the oxygen atom and positive opposite the oxygen atom.

Answer to question 2: A hydrogen bond is the electromagnetic attraction of a hydrogen atom with a negative pole of a neighboring atom. In water, the oxygen end of each water molecule is such a negative pole so it can form a hydrogen bond with the hydrogen atom of a neighboring water molecule. In fact, each water molecule can form a hydrogen bond with as many as 4 other water molecules. One consequence, among many, of this high number of hydrogen bonds is the high boiling point of water; it is difficult to break the hydrogen bonds so that a high temperature (lots of energy) is required for a water molecule to break away from the liquid water to become steam.

Answer 7

Brenda,

Water is said to be polar because there is a net negative charge at the Oxygen end of the molecule and a net positve charge at the hydrogens end of the molecule.  Water molecules therrefore behave like magnets in that they arrange themselves such that hydrogen bonds form between the hydrogen atoms of one molecule and the oxygen atoms of other molecules. While these are weak bonds they can hold a specific orientation and allow water to form hydrogen bonds with other species as welll.

Art

Answer 8

Hydrogen covalently bonded to a very electronegative atom (like Oxygen) is also weakly bonded/attracted to an unshared electron pair of another electronegative atom. In water, this other electronegative atom is other oxygen parts of surrounding molecules. So, not only is the hydrogen bonded directly to its oxygen but it is also attracted to the surrounding oxygen of other water molecules. Try to go with me here for the diagram, it is just a basic input for typing/symbols so the spacing is not perfect. A and B are oxygen, the little x’s are hydrogen. Each water has one oxygen with two hydrogen to make the molecule. The x’s from A are directly bonded to A, but they are also attracted to B; the x’s from B are directly boned to B but are also attracted to A. x x A B x x Polar Molecule- Molecule that has a positive and a negative end (or pole) The more electronegative atom has a slightly negative pole and the less electronegative has a slightly positive pole. In water, oxygen is the negative pole, hydrogen the positive If you needed to draw a symbol showing the polarity you would have an arrow that pointed to the hydrogen away from the oxygen and place a (+) sign on the oxygen side of the arrow O +--------> H