Polar Covalent Bonding Within the Water Molecule Explains Four Physical Properties of Water

1. Polar Covalent Bonding Within the Water Molecule Explains Four Physical Properties of Water

2. Observation 1: Water has a concave miniscus.

3. Observation 2: Water molecules stay together in a stream.

4. Observation 3: A falling water stream is attracted to an electrically charged object.

5. Observation 4: Ice floats.

6. The Lewis dot structure of water cannot explain these four observations.

7. The oxygen atom is more electronegative than the hydrogen atoms. As a result, the four pairs of shared and unshared electrons are closer to the oxygen atom than the hydrogen atoms. This unequal sharing of pairs of electrons in a covalent bond is referred to as a polar covalent bond.

8. The water molecule is bent because of the arrangement of these four pairs of electrons in three - dimensional space.

9. The four pairs of electrons repel one another. As these pairs of electrons are arranged in three-dimensional space, rather than two-dimensional space, their actual positions are not illustrated properly here.

10. This perspective diagram is a three-dimensional representation of the water molecule on two dimensions. The electron pairs are shown to be as far apart from one another as possible. The unshared electron pair in the lower left is enlarged to illustrate the idea they are coming forward out of the page. The small pair of unshared electrons is receding behind the plane of the page. The hydrogen and oxygen symbols are in the plane of the page.

11. The four electron pairs are located at the corners of a triangular pyramid called a tetrahedron. The oxygen nucleus is located in the very centre of the pyramid. The two pairs of shared electrons define two vertices of the tetrahedron while the two pairs of unshared electrons are found at the other two vertices.

12. The tetrahedral arrangement of the electron pairs in three-dimensional space causes the water molecule to be bent.

13. The bend in the molecule and the greater electronegativity of the oxygen atom causes the electron pairs to be closer to the oxygen atom than the hydrogen atom.

14. The greater electronegativity of the oxygen atom and the bend in the water molecule causes an uneven distribution of negative charge in the water molecule. The oxygen end of the molecule is slightly negative (δ-) while the hydrogen ends of the molecule are slightly positive (δ+). Water is said to have positive and negatively charged ends or poles. Thus water is said to be a polar molecule.

15. The small negative charge on the oxygen atom of one water molecule will attract the small positive charge of the hydrogen on another water molecule forming what is called a hydrogen bond. This attraction or hydrogen bonding between adjacent water molecules explains the four observations.

16. Observation 1: Water forms a concave meniscus. Water molecules are not only attracted to each other but also to the walls of the container. The attraction water molecules have for the walls of the container causes them to “climb” up the wall. This attraction of water molecules to the walls of the container is called adhesion. The attraction water molecules have for each other is termed cohesion. Together, adhesion and cohesion are responsible for the formation of the meniscus.

17. Observation 2: Water molecules stay together in a stream. In flowing water the hydrogen bonds between adjacent water molecules are transient; forming, breaking and reforming as water molecules slide past one another in the liquid state. This attraction or cohesion between water molecules is responsible for holding water molecules together in a drop and a stream.

18. Observation 3: A falling water stream is attracted to an electrically charged object. As water molecules stream past a positively charged object, water molecules rotate because the negative oxygen end of the water molecule is attracted to the positive charge. The entire stream is pulled towards the charged object because of the transient hydrogen bonds between adjacent water molecules

19. Observation 4: Ice floats. Water molecules are fixed in position in the ice crystal by hydrogen bonds between water molecules. The hexagonal arrangement of six water molecules in ice takes up more space than the same six molecules in the liquid state. Hence ice is less dense than water and floats. Water is rare in that the solid state is less dense than its liquid state. For most substances the solid state is more dense than the liquid state.