Electrons are found in defined regions in a atom called orbitals, with each orbital containing up to two electrons. Some orbitals, for example, are spherical, while others are shaped more or less like barbells. The orbital does not define the exact position of the electrons, but only the probability of finding it in a region.
Atoms and molecules are most stable if each orbital contains two paired electrons. (The two have opposite spins.) But it is possible for an atom or molecule to have an orbital with only one unpaired electron, in which case the atom or molecule is called a free radical. Free radicals tend to react readily with other atoms or molecules and in a way that ends the condition of the unpaired electron. More on this when we discuss the immune system.
SEE IF YOU REMEMBER: Substance A gives an electron to substance B. Which one of the following is correct?
BY ANY CHANCE, can you name a molecule inside cells that is widely used to reduce other molecules?
Another important consideration is that the orbitals are arranged in shells about the nucleus of the atom. The first shell closest to the nucleus has only one orbital, which can have up to two electrons. The next shell has four orbitals, which together can contain up to eight electrons. The third likewise can contain up to eight electrons.
Moving from the inside out, electrons in each shell possess more energy than those in the shell to the inside. Thus, the lowest energy configuration is for the innermost shells to be filled first.
But also, and this fact unlies much of chemistry, atoms tend to be most stable if their outmost shell is completely filled with electrons.
The way in which the outmost shell tends to become full depends on the specific atom involved. Elemental sodium, for example, has only one electron in its outmost shell and in water immediately gives this up, forming the positively charged sodium ion in the process. On the other hand, the element chlorine avidly seeks one more electron to fill its outmost shell and thus readily becomes the negatively charged chloride ion.
But the most common atoms we encounter in biology are carbon, hydrogen, oxygen and nitrogen, and all these do not tend to completely give up or grab electrons to fill their outmost shell. Rather, they fill their outmost shell by sharing electrons with other atoms. In the process, the atoms involved are held together with covalent bonds.
Now here is one of the main reasons for this review of electrons. Covalent bonds are not all alike because some types of atoms attract electrons more than others. For example, in covalent bonds between carbon and hydrogen, the electrons are equitably shared. Such covalent bonds are called nonpolar covalent bonds. In molecules with only nonpolar covalent bonds, the negative charges of the electrons tend to be evenly distributed about the atoms in molecule.
By contrast, oxygen and nitrogen attract electrons more strongly due to the configuration of the electrons about the atoms. If either oxygen or nitrogen form covalent bonds with carbon or hydrogen, the result is a polar covalent bond. With a polar covalent bond, there is a net separation of charge because the electrons spend more time near the oxygen or nitrogen than near the carbon or hydrogen. The result is a polar molecule. Such a molecule, with its net separation of charge, tends to have distinctly different physical properties than a nonpolar molecule. This is an issue you will encounter frequently in physiology.
Water is the most important polar molecule, of course, since it is the medium in which biological reactions take place. Check out the structure of water to see how the two polar covalent bonds give the molecule a negative "pole" and a positive "pole". But realize that the separated charges shown are much less that the full charge of one electron. (Thus the Greek delta is used to mean "partial".)
One consequence of the partial separation of charge in water is that the opposite charges attract, forming what are called hydrogen bonds. In liquid water, these are forming and breaking rapidly. Nonetheless, they make water molecules cohesive, giving water an unusually high surface tension. Similarly, the mutual attaction gives water a high heat capacity and heat of vaporization.
However, the most important issue is the effect the polar covalent bonds have on water's solvent properties........
QUESTION: How and why do you suppose the polar nature of water affects its solvent properties?
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Some larger molecules can have both a polar region and a nonpolar region coexisting in the same molecule. Such molecules are termed amphipathic . Naturally the two ends of such a molecule tend to have the opposite solvent properties. As you will find, a number of important molecules have this property, which, indeed, is the basis of their physiological roles.
Incidentally, consider what would happen if we remove an electron from an orbital in one atom and transfer it to another atom in which the electron is in an orbital where it has less energy. Energy would be liberated by this process, of course. And this, in fact, is the fundamental means by which our metabolic reactions generate most of the energy that powers all our various energy requiring processes.
The electron might begin, for example, in a bond between carbon and hydrogen in a molecule we eat, such as a carbohydrate or fat. It would end up typically in a covalent bond between oxygen and hydrogen in water. Here it would have far less energy. This is why we breathe oxygen. It serves as a repository for all those electrons that have lost energy in the process of generating ATP.
QUESTION: What term refers to a substance in which there is an orbital with an upaired electron?
QUESTION: Name three biological, nonpolar molecules.
QUESTION: What type of chemical bond involves shared electrons?
QUESTION: In common biological molecules, which types of atoms form polar covalent bonds (with carbon or hydrogen)?