CHAPTER 2 Chemical Elements of Microorganisms^31
becomes positively charged. The atom is oxidized.An atom that is in-
volved in this exchange is called an ion.The atom that gives up an electron
is called a cation.A cation is positively charged. The atom that receives an
electron is called an anion,which is negatively charged. The reaction that
creates table salt from sodium and chlorine causes an ionic bond between
these atoms (Fig. 2-5).- Covalent bond.Atoms share electrons in their valence shell (Fig. 2-5).
The shared electron orbits the nucleus of both atoms. A covalent bond is the
strongest bond and the most commonly found in organisms. There are three
kinds of covalent bonds: single, double,and triple.These names reflect
the number of electrons that are shared between the two atoms that form
the bond. Atoms that share electrons equallyform nonpolar covalent
bond. Atoms that share electrons unequallyform polar covalent bond. - Coordinate covalent bond.A bond is formed when electrons of the shared
pair come from the same atom. - Hydrogen bond.Ahydrogen bond forms a weak (5% the strength of a
covalent bond), temporary bond that serves as a bridge between either dif-
ferent molecules or portions of the same molecule. For example, two water
molecules are physically combined using a hydrogen bond.
Decoding Chemical Shorthand
Over the years chemists have developed a way of describing atoms, chemical
elements, and reactions so they can convey ideas to each other. Table 2-2 shows
commonly used chemical notations that you’ll need to know when learning
about microbiology.
I Just Want to See Your Reaction
The process of bonding together atoms and separating atoms that are already
bonded together is called a chemical reaction.Achemical reaction causes a
change in the properties of atoms or to a collection of atoms, but the atoms
remain unchanged because of a change in the electron configuration. For example,