Atoms are the building blocks of all substances. But what is it that keeps atoms connected together? They
are held together by CHEMICAL BONDS, strong attractive forces between atoms. Without these ties that
bind, the universe would be nothing more than a mass chaos of individual atoms.
So what constitutes a chemical bond? A bond is formed when electrons from two atoms interact with each
other and their atoms become joined. The electrons that interact with each other are VALENCE
ELECTRONS, the ones that reside in the outermost electron shell of an atom.
As you sit reading this sentence, billions of atoms surround you. When any two of them approach each
other, their outermost electrons come into close contact. Recall that electrons are negatively charged
particles, so a chance meeting between atoms usually results in the two repelling each other.
Occasionally, however, atoms will stick together by forming a chemical bond.
There are two main types of bonding discussed here. A COVALENT BOND results when two atoms "share"
valence electrons between them. An IONIC BOND occurs when one atom gains a valence electron from a
different atom, forming a negative ion (ANION) and a positive ion (CATION), respectively. These oppositely
charged ions are attracted to each other, forming an ionic bond.
Why are chemical bonds important? The type of chemical bond that occurs in a molecule
or substance in part defines its properties. For example, consider sodium chloride (NaCl)
and hydrogen chloride(HCl). Both substances contain chlorine, but NaCl is the white solid
crystalline substance sprinkled on French fries, and HCl is a foul smelling gas.(note: when
this gas is dissolved in water, it forms a solution known as hydrochloric acid. This is the
acid that your stomach uses to digest food.)
Table salt (NaCl)
How can this be if both materials have chlorine in them? The chemical bonding that takes
place in NaCl is different than that in HCl. This gives NaCl and HCl very different structures, appearances,
and properties.
What other differences are noticeable among molecules that result from different types of chemical
bonding? Think about what happens when a bunch of sodium and chlorine ions join together to form rock
salt. If we hit this with a hammer, it shatters into tiny pieces. It does this because the bonds between the
atoms in rock salt are ionic. The particles are arranged in such a way that they line up along rows of
positive and negative charge. Under enough stress, the salt crystals break along those lines into much
smaller pieces.
The bonds that hold the carbon and hydrogen atoms in rubber together, on the other hand, are not ionic
but covalent. Each carbon atom shares four of its outermost electrons with its immediate neighbors.
Under stress, the bonds stretch, then snap back as each atom pulls on the shared electrons. And that's
the way the ball bounces.
These silver atoms are joined by metallic bonds.
There is a third type of bonding, called METALLIC BONDING. As the name implies, metallic bonding usually
occurs in metals, such as copper. A piece of copper metal has a certain arrangement of copper atoms.
The valence electrons of these atoms are free to move about the piece of metal and are attracted to the
positive cores of copper, thus holding the atoms together.
Essential to understanding all types of chemical bonding is realizing that all bonds use electron "glue."
Every substance is made up of atoms, and all atoms are surrounded by the charged particles called
electrons. Electrons in the outermost shell can and do bond with those of other atoms to create everything
from quartz and quarters to plastic and pennies. In large part, the difference between materials as diverse
as diamonds and pencils is how they're glued together.
3a. Atoms That Share: Covalent Bonds
Don't look now, but you're surrounded by covalent bonds. They're
in the air you breathe and in the water you drink. You even make
them yourself when you exhale. What are these bonds? What do
they want?
Take a look at water (H2O). H2O is a MOLECULE, a discrete unit of
atoms that are bonded together. A molecule of water consists of
an oxygen atom that is bonded to two hydrogen atoms. A
hydrogen atom and the oxygen atom each donate one valence
electron to form a chemical bond. These two valence electrons forming the bond are shared by both
atoms, resulting in a SINGLE COVALENT BOND. Think of this in terms of two pieces of wood that are nailed
together. The pieces of wood are the atoms, and the nails holding them together are the electrons that
form the covalent bond. Each piece of wood shares a portion of the nails.
A water molecule is an example of the single
covalent bond between oxygen and hydrogen.
Air, too, contains oxygen. Oxygen does not exist as a single oxygen atom, but as a
molecule of two oxygen atoms. These two oxygen atoms share two pairs of
valence electrons (four valence electrons total) between them, forming a DOUBLE
COVALENT BOND. This is true of any double covalent bond; four valence electrons
are shared between two atoms. Another component of air is nitrogen. Like oxygen,
nitrogen does not exist as a single nitrogen atom, but as a molecule made up of
two nitrogen atoms. The two nitrogen atoms in a molecule of nitrogen share three
pairs of valence electrons (six valence electrons total) to form a TRIPLE COVALENT
BOND.
Oxygen molecules shares
two electrons to make a
double covalent bond.
Is it possible to predict whether bonds are covalent or not? A good rule of thumb is that bonds between
nonmetals (remember that hydrogen is considered a nonmetal) are usually covalent bonds. For example,
the carbon dioxide (CO2) molecules you exhale are bonded together covalently.
So you see, you are surrounded, but it's okay. Relax. Take a deep breath. Get a drink of water, maybe.
Then read on to learn about some other ways molecules stick together.
3b. Fatal Attraction: Ionic Bonds
Now that you've gotten over being surrounded by covalent bonds, it's time you knew something else. You
ate ionic bonds. Lots. Today. You can find some in your saltshaker. In salt (sodium chloride), atoms do
not share electrons. Rather, one type of atom strips at least one valence electron from another type of
atom, creating ions of opposite charges. These two oppositely charged atoms are held together by an
ELECTROSTATIC INTERACTION, an attraction between oppositely charged particles.
A sodium atom transfers an electron to a chlorine atom to create table salt.
An IONIC BOND is an electrostatic interaction that holds together a positively charged ion (cation) and a
negatively charged ion (anion). In an ionic bond, one atom loses an electron to another atom, forming a
cation and anion, respectively. And, as everyone knows, opposites attract.
A neutral atom becomes an ion either by losing an electron (cation) or by
gaining an electron (anion).
In table salt, for example, a valence electron from a sodium atom is transferred to a chlorine atom,
forming Na+ and Cl-. Because the ions have opposite charges, they are attracted to each other. The loss
of a valence electron and the attraction to the atom that took it happen simultaneously.
It is possible for more than one valence electron to be drawn away from another atom, as in barium
chloride (BaCl2, a substance used in medicinal preparations). In barium chloride, two chlorine atoms each
take one valence electron away from barium, leaving the barium ion Ba2+.
Unlike water or oxygen, which have covalent bonds, substances that have ionic bonds do not exist
naturally as discrete molecules. Rather, they form IONIC SOLIDS, three-dimensional networks in which
each cation is surrounded by anions and each anion is surrounded by cations.
Look again in the saltshaker. Each sodium cation is surrounded or ionically
bonded to six chloride anions, and each chloride anion is ionically bonded
to six sodium cations. The formula NaCl for sodium chloride shows that for
every sodium atom present in a piece of salt, there is one chlorine atom
present. What, don't see any ionic bonds? All right, it looks like this:
Where are ionic bonds found? These types of bonds usually form when
metal atoms bond with nonmetal atoms. In salt, the metal sodium bonds
with the nonmetal chlorine. Besides salt, some other examples are lithium
fluoride (LiF), strontium oxide (SrO) and calcium chloride (CaCl2).
Sodium chloride results from ionic
bonding.
3c. Rules of the Atomic Playground: Polar Covalent Bonds
Atoms, like preschoolers, don't always play fair. This means that sometimes in a covalent bond the
electrons are not shared equally between the two atoms. On average, one of the atoms partially "pulls"
the bonding electrons toward itself, creating an unequal sharing of those bonding electrons. This is called
a POLAR COVALENT BOND. In order to determine whether a covalent bond is polar or not, it's necessary to
understand electronegativity.
Recall that electronegativity is a measure of an atom's ability to draw its bonding electrons to itself. Each
element has a numeric value corresponding to its electronegativity. The values used here were devised
by Linus Pauling, though there are a few other scales of electronegativity values.
Fluorine was determined to be the most electronegative element and has an electronegativity value of
4.0. Francium, the least electronegative element, has a value of 0.7. It is important to remember a general
trend in the periodic table: electronegativity increases from left to right going across a period, and it
increases from the bottom to the top of a group. For example, in period 2, nitrogen (group 5A) has an
electronegativity value of 3.0, compared to 2.5 for carbon (group 4A). In group 5A, phosphorous (period 3)
has an electronegativity value of 2.1, compared to 3.0 for nitrogen (period 2).
In an action that resembles toddlers tugging on a toy, a polar covalent bond occurs when one atom with a
higher electronegativity draws the bonding electrons toward itself, pulling those electrons away from the
atom with the lower electronegativity value. This creates an unequal sharing of electrons known as
UNEQUAL CHARGE DISTRIBUTION, or charge separation. The charge separation makes the bond polar
because the more electronegative atom becomes partially negatively charged and the atom with the lower
electronegativity becomes partially positively charged.
Consider H2 and HBr. In H2, the atoms have an equal "pull" on the bonding electrons, making the bond
NONPOLAR.
In HBr, however, Br has an electronegativity of 2.8, compared to 2.1 for hydrogen. The Br atom pulls the
bonding electrons toward itself, creating a partial negative charge on itself and a partial positive charge on
the hydrogen atom.
The larger the absolute difference in electronegativity between two atoms, the more polar that bond is.
For example, the electronegativity difference of a carbon-oxygen bond is -1.0, the result of 2.5 (the
electronegativity value for carbon) minus 3.5 (the value for oxygen). The absolute value for the difference
in electronegativity is the value without the minus sign (1.0 for a carbon-oxygen bond). For a carbonchlorine bond, the difference in electronegativity is 0.5 (2.5 - 3.0 = -0.5). Thus, a carbon-oxygen bond is
more polar (1.0) than a carbon-chlorine bond (0.5).
Question: Which of the following bonds is most polar? 1. N-H 2. O-H 3. C-Cl 4. P-Br