As we delve into chemistry, we found that certain concepts in physics are ever present. Chemistry is not just about mixing compound A and B to produce C, but how and why these thing happen.
In this article we explain how electrical charges (a familiar concept in physics) actually makes chemistry work. |
"Just as individual bonds are often polar, molecules as a whole are often polar also. As a practical matter, strongly polar substances are often soluble in polar solvents like water, whereas less polar substances are insoluble in water"
John McMurry, Organic Chemistry 8th Edition, 2012
We have seen that atoms differ in their capacity to "hold onto" their electrons; some gain electrons, some lose electrons. Certain atoms, oxygen and nitrogen, for example, do not have sufficient electron-attractive power to become fully charged negative ions.
However, the attractions of electrons are sufficiently great so that, when covalently bonded to hydrogen, the electrons are not equally shared between the two nuclei. The electrons tend to spend more time around the oxygen nucleus and consequently less time around the hydrogen nucleus. This means that one portion of a molecule is slightly positive or slightly negative in relation to another portion of the same molecule. When such an uneven distribution of charge occurs, the molecule is said to exhibit polarity. The molecule has a positive and a negative end, separated from each other like the poles of a bar magnet. Because this is not a full -1 or +1 charge but a smaller charge, it is represented as delta positive or delta negative.
Textbook Case: Water
Water is a good example to illustrate this point. Although the water molecule is electrically neutral, it does have a positive and a negative end. The geometric configuration of the molecule places both hydrogen atoms at one end. The nucleus of the oxygen atom attracts electrons more than the nuclei of the hydrogen atoms. This results in two slightly positively charged regions on one end of the molecule and a single slightly negatively charged region on the other. The molecule thus has a positive and a negative end, or two poles.
The water molecule is a polar molecule, meaning that there is an uneven distribution of electron density. Water has a partial negative charge near the oxygen atom due the unshared pairs of electrons, and partial positive charges near the hydrogen atoms.
An electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond as shown in the illustration below, and how this can be affected by static electrical charge.
However, the attractions of electrons are sufficiently great so that, when covalently bonded to hydrogen, the electrons are not equally shared between the two nuclei. The electrons tend to spend more time around the oxygen nucleus and consequently less time around the hydrogen nucleus. This means that one portion of a molecule is slightly positive or slightly negative in relation to another portion of the same molecule. When such an uneven distribution of charge occurs, the molecule is said to exhibit polarity. The molecule has a positive and a negative end, separated from each other like the poles of a bar magnet. Because this is not a full -1 or +1 charge but a smaller charge, it is represented as delta positive or delta negative.
Textbook Case: Water
Water is a good example to illustrate this point. Although the water molecule is electrically neutral, it does have a positive and a negative end. The geometric configuration of the molecule places both hydrogen atoms at one end. The nucleus of the oxygen atom attracts electrons more than the nuclei of the hydrogen atoms. This results in two slightly positively charged regions on one end of the molecule and a single slightly negatively charged region on the other. The molecule thus has a positive and a negative end, or two poles.
The water molecule is a polar molecule, meaning that there is an uneven distribution of electron density. Water has a partial negative charge near the oxygen atom due the unshared pairs of electrons, and partial positive charges near the hydrogen atoms.
An electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond as shown in the illustration below, and how this can be affected by static electrical charge.
Many other unique properties of water are due to the hydrogen bonds. For example, ice floats because hydrogen bonds hold water molecules further apart in a solid than in a liquid, where there is one less hydrogen bond per molecule. The unique physical properties, including a high heat of vaporization, strong surface tension, high specific heat, and nearly universal solvent properties of water are also due to hydrogen bonding. The hydrophobic effect, or the exclusion of compounds containing carbon and hydrogen (nonpolar compounds) is another unique property of water caused by the hydrogen bonds. The hydrophobic effect is particularly important in the formation of cell membranes. The best description is to say that water "squeezes" nonpolar molecules together.
Polarity in Biochemistry and Health
The significance of molecular polarity to the biological sciences come from two main areas: First, polar molecules tend to become oriented with respect to other molecules. Because of this, polar molecules are important in helping to establish the three-dimensional structure or orientation of other larger molecules. For example, molecules of fatty acids, found in all living matter are composed of a nonpolar carbon chain with a polar carbon-oxygen group (COOH) at one end. When placed in water, the polar ends of the fatty acid molecules are attracted to water molecules, which are also polar. The nonpolar carbon chains are at the same time repelled by the water. As a result, fatty acid molecules are oriented on the water's surface. Of particular importance to living things is the orientation of phospholipid molecules, which are a combination of a fat molecule with a phosphate group. Phospholipids are among the most important parts of cell membranes. They tend to become oriented on surface or boundary regions in a manner similar to the fatty acids in water (left). It is partly in this way that cell membranes are given a distinct structure. |
Second, polarity is important in understanding both the geometry and the chemical characteristics of large molecules, such as proteins. Proteins are so large that they may possess a number of polar groups on one molecule. Polar groups, like radicals, are simply groups of atoms which bear as a unit a partial positive or a partial negative charge. The specific geometry of proteins exists in part because polar groups on one part of the molecule attract polar groups on another part of the same molecule. This stabilizes the specific twisting and folding of the molecule which is all-important to the chemical characteristics it displays.
Without its correct three-dimensional structure a protein does not work. And in certain cases, if proteins do not fold into their native shape, they are usually toxic. Several diseases are believed to result from misfolded proteins resulting in a class of diseases termed ‘proteopathy’ which includes Alzheimer’s disease, Huntington’s disease, and even sickle-cell anemia.
Ponder this
The structure of an ozone (O3) molecule approximates that of water, and have the same bipolarity even though it consists of three atoms of the same element, all containing the same number of valence electrons. How do we explain this?
In the video, it is explained that dissolving compounds (such as salt, NaCl) using a polar solvent (such as water) will result in a solution of ions suspended in that solvent. How do we explain how certain compounds dissolve in only a particular solvent, but not another?
Discuss
How does the biomolecular elements in our bodies react to electrical charge? Considering that things like cell membranes depend on the polarity of its constituent molecules to hold its structure? Provide evidence and justifications to your hypotheses.
Further readings
Chemical polarity, at Wikipedia
Molecule polarity simulator, a great clasroom tool to explain the concept