Chemical Bonding – Covalent Bonding (Sharing Electrons)
In Plate 9 you learned that many atoms gain or lose electrons to become ions and that these ions wander around
separately in water or collect into crystals in the absence of water. The electrical attraction holding two ions together in a
crystal is often called an ionic bond. Many elements, however, have atoms that are unable to form ions that will last for
more than the tiniest fraction of a second. One such element is carbon.
Color the titles in the upper third of the plate. Choose contrasting colors for C and H. Color the protons, the
neutrons, the electrons, and all the space between the electron shells and the nucleus as labeled. Color the
symbols for the elements (H and C).
A neutral carbon atom has six protons and six electrons. It also has six, seven, or eight neutrons. If it were to gain four
electrons to satisfy its "desire" to fill its outer shell to its capacity of eight, the atom would be in the rather unbalanced
condition of having six positive charges and ten negative charges. Since it is only the attraction of opposite electrical
charges that keeps the electrons around a nucleus, the outer electrons would not be held very strongly and would easily
be removed by other atoms. If that same carbon atom were to lose four electrons to get rid of its second shell entirely so
that the already filled first shell becomes the outer one, it would have six positive charges balanced by only two negative
charges; the overabundance of positive charges would have a strong attraction for additional electrons, and the atom
would attract electrons away from other atoms to become a neutral atom again. Instead, carbon almost always fills its
outer shell by sharing electrons. One good example of this is the compound methane (CH4), which is the principal component of the natural gas that is widely used for heating and cooking.
Color the heading Methane Electron Diagram. Use the Ce- and He- colors for the title Shared Electrons. Color the
title Empirical Formula and the structures of the diagram as you did at the top of the plate. To emphasize that the
shared electrons now travel around both the hydrogen and the carbon, one half of each shared electron should
receive the hydrogen electron color and the other half the carbon electron color as labeled. Note that in methane
all of the electrons are shared except those of the inner shell of carbon. Color the symbol CH 4 as well.
While carbon needs four additional electrons to fill its outermost shell, hydrogen needs only one additional electron to fill
its outermost shell, which has a capacity of only two electrons. So a carbon atom "makes a deal" with four hydrogen
atoms. "You let me use one of your electrons, and I'll let you use one of mine." Thus one electron from carbon spends part
of its time going around the hydrogen atom at the top of our diagram while the electron of that hydrogen atom spends part
of its time going around the carbon atom. One other electron from the carbon atom spends part of its time going around
the hydrogen atom on the left of our diagram while the electron of that hydrogen atom spends part of its time going around
the carbon atom. The same occurs with the other two hydrogen atoms in the diagram. This satisfies carbon, since it now
has eight electrons going around it in the outer shell, and it satisfies the hydrogen atoms, which each have two electrons
going around them. Where atoms are concerned, it apparently doesn't matter whether shared electrons are present fulltime or part-time. This sharing of electrons holds the atoms together to form a molecule, and each pair of shared electrons
is called a covalent bond.
Color the remaining headings, title S, and all related structures.
The projection formula provides a simple way to represent the arrangement of the atoms in a molecule that is formed by
covalent bonds. Each atom is represented by the letter of its name, and the bond, or shared electron pair, is represented
by a single straight line. In an actual methane molecule the hydrogen atoms are not radiating out at 90 degrees from one
another in a flat plane. They are actually separated by equal angles in three-dimensional space, as is shown in the balland-stick model, which shows the angles correctly but does not give an idea of the space occupied by the electrons. That
space is more accurately represented by the space-filling model, which represents the space within which the electrons
spend 90 percent of their time. The angles at which the various atoms project out from a molecule and the spaces they
occupy have great importance to their functioning in a living thing, so biological scientists are generally most interested in
the space-filling model of a molecule.