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|Chapter Two: The Chemistry of Biology|
Through chemical reactions, atoms can combine and form complex structures called molecules. The atoms in molecules stay together due to forces called bonds between the atoms. Bonds come in two major types: ionic bonds and covalent bonds. A full understanding of these bonds is not necessary for understanding the biology principles that rest on them, but knowing the distinction between them is still important.
One important thing to know about bonds is that they have nothing to do with gravity. Gravity is a force of attraction between all atoms. Objects which are very massive (the Earth, for example) exert a very large gravitational force. That's why we stick to the ground. Objects with very little mass, like the protons, neutrons, and electrons in an atom, produce an incredibly tiny gravitational force. This miniscule force does not play any role in the formation of bonds between atoms.
Sometimes, atoms will become more stable if they either obtain or lose electrons. For example, the element chlorine becomes more stable when it gains an electron, and sodium becomes more stable when it loses an electron. If an atom of chlorine and an atom of sodium come near enough to one another, an electron from the sodium can actually jump to the chlorine atom. However, the sodium atom now has more protons than electrons, so it has a positive charge. The chlorine atom now has an additional electron and so has a negative charge. These charged atoms are called ions. Ions of opposite charges, like the sodium and chlorine ions, are attracted to one another in what is called an ionic bond.
In other cases, two atoms will share the electrons so that each becomes more stable. For example, oxygen, which normally has eight electrons, happens to become more stable when it has two additional electrons. If two oxygen atoms come together, they can each contribute a pair of electrons to be shared by both atoms. This type of sharing is called a covalent bond. In the case of the oxygen atoms, the electrons are shared exactly equally. However, in most cases, one atom pulls on the electrons more than the other. In these bonds, one side of the molecule is slightly more negative since the negatively charged electrons are closer to that side. These molecules are said to be polar (water is an example), whereas molecules which share the electrons equally are nonpolar. Polar and nonpolar molecules are very significant in how many aspects of unicellular organisms function.