Three Different Ways Atoms Bond Mean Three Different Kinds of Compounds
All atoms have electrical charges. What does this mean? At the macroscopic level, it
means you can see that matter attracts and repel. What does it mean at the
microscopic level? It means plus and minus charges make up all matter. But you
have seen that different kinds of matter have different electrical properties. Some
things conduct electricity when dissolved in water and some do not. A material's
electrical properties also affect what it will dissolve in, how hard it will be, and how
high the melting and boiling points will be (or whether they will boil or melt at all).
A microscopic view of matter helps explain these differences.
Polar Covalent Bonds
Do you remember what it means for a molecule to be polar? How did a stream of
water interact with a positively charged object? A negatively charged object? Water
bent toward both. That means water demonstrated characteristics of both plus and
minus charges.
Many other molecules have some parts that are more positive and other parts that
are more negative. These molecules are called polar covalent (often shortened to
just polar). Remember that polar refers to having a part of the molecule that is
somewhat positive and a part that is somewhat negative. Covalent is the type of
bond that holds atoms together in molecules (we will see how these bonds form in
the next activity).
Polar molecules do not conduct electricity even when they are liquids or dissolved
in liquids. This is because the charges, which make one end of the molecule partly
positive end and the other end partly negative, can’t leave the molecule, even though
the molecule itself can move around. When charges are not moving freely, no
electricity flows.
A simplified sketch of a polar molecule.
Individual atoms are not shown.
An atom level sketch of a water molecule.
You can see that the side with hydrogen
atoms is more positive than the other
side.
Substances that are made of molecules with polar bonds easily dissolve in liquids
that are also made of polar molecules. Why do think this is? What would make polar
molecules mix well with other polar molecules?
Three Different Ways Atoms Bond Mean Three Different Kinds of Compounds
All atoms have electrical charges. What does this mean? At the macroscopic level, it
means you can see that matter attracts and repel. What does it mean at the
microscopic level? It means plus and minus charges make up all matter. But you
have seen that different kinds of matter have different electrical properties. Some
things conduct electricity when dissolved in water and some do not. A material's
electrical properties also affect what it will dissolve in, how hard it will be, and how
high the melting and boiling points will be (or whether they will boil or melt at all).
A microscopic view of matter helps explain these differences.
Ionic Bonds
Figure 3.9 shows charges in solid
sodium chloride (table salt). These
charged atoms are called ions. Solids
made of ions are known as ionic
compounds.
Notice how the ions in figure 3.9 are
not mobile. They cannot move about
freely like dust particles in air. Instead,
plus and minus attractions lock them in
place. The name for the orderly
microscopic structure of solids is the
crystal lattice. This is an important
feature of ionic compounds: ions form
crystals where each ion is either
positively or negatively charged, and
the attraction between the charges hold
the crystal together. These ions do not
form individual molecules. There is no
set number of atoms in an ionic
compound- the crystals grow as long
as their are more positive and negative
ions around. In figure 3.9, the ions in
the crystal lattice are not free to move
from one wire of the conductivity
meter to the other. When charges are
not moving, no electricity flows.
Ionic Compounds Dissolve in Polar Liquids
Solid ionic compounds such as sodium chloride
dissolve in water. Imagine an ionic crystal in
water. Figure 3.10 shows what that might look
like. Find the plus and minus charges on the
water molecules. The charged ends of water
molecules attract sodium chloride ions. The
plus end of water attracts negative chloride
ions. Similarly, the negative end of water
attracts positive sodium ions. Water molecules
pull ions free from their crystal lattice. Soon
sodium and chloride ions are pulled into the
liquid and become part of the solution. Once in
solution, water molecules surround each ion.
The result is a solution of dissolved sodium
chloride.
Now these ions are free to move about in the
solution along with the water molecules. As a
result, the solution conducts electricity. Why
does the solution conduct electricity? The
answer is because now there are charged
particles, positively charged sodium ions and
negatively charged chloride ions, free to move
in the solution. When charges can move,
electricity can flow through a material.
Interestingly, there is a limit to how much of an
ionic compound can dissolve in water. Can you
think of a reason why you can’t dissolve an
unlimited amount of ionic crystals in water?
Figure 3.10
Three Different Ways Atoms Bond Mean Three Different Kinds of Compounds
All atoms have electrical charges. What does this mean? At the macroscopic level, it
means you can see that matter attracts and repel. What does it mean at the
microscopic level? It means plus and minus charges make up all matter. But you
have seen that different kinds of matter have different electrical properties. Some
things conduct electricity when dissolved in water and some do not. A material's
electrical properties also affect what it will dissolve in, how hard it will be, and how
high the melting and boiling points will be (or whether they will boil or melt at all).
A microscopic view of matter helps explain these differences.
Nonpolar Covalent Bonds
Molecules in the same category as baby oil and paraffin wax are not like water. Baby
oil and paraffin wax are made of molecules with different electrical properties from
water. What does that mean? It means molecules of baby oil and paraffin wax do not
have plus and minus sections like water molecules do, nor are they made of ions like
sodium chloride. Instead, we can think of their positive and negative charges as
being balanced and spread evenly throughout the molecule. Molecules such as baby
oil and paraffin are typically called nonpolar covalent molecules (often shortened to
nonpolar).
Nonpolar molecules do not dissolve in water. Why not? Since nonpolar molecules
are neutral, the positive and negative sides of water molecules do not have the same
attraction or repulsion to nonpolar molecules compared to molecules that do have
negative and positive sections. Nonpolar molecules do mix well with other nonpolar
molecules. As a general rule substances with similar electrical properties will mix
with each other. The rule is sometimes stated “like dissolves like.
Figure 3.11 A simplified
sketch of a nonpolar molecule.
Individual atoms are not shown
Figure 3.12 An atom level sketch of a nonpolar molecule.
Black are carbon atoms and white are hydrogen atoms. Since
hydrogen and carbon atoms have similar electrical properties no
part of the molecule is significantly more positive or negative.
Nonpolar molecules do not conduct electricity even when they are liquids or
dissolved in other liquids. This is because the microscopic charges they have are
balanced and spread evenly throughout the molecule, making the molecule neutral.
These microscopic charges stay in the molecule and can’t move freely through the
liquid. When charges are not moving freely, no electricity flows.
Three Different Ways Atoms Bond Mean Three Different Kinds of Compounds
All atoms have electrical charges. What does this mean? At the macroscopic level, it
means you can see that matter attracts and repel. What does it mean at the
microscopic level? It means plus and minus charges make up all matter. But you
have seen that different kinds of matter have different electrical properties. Some
things conduct electricity when dissolved in water and some do not. A material's
electrical properties also affect what it will dissolve in, how hard it will be, and how
high the melting and boiling points will be (or whether they will boil or melt at all).
A microscopic view of matter helps explain these differences.
Not all Bonds Make Compounds
In the previous two activities, we have only considered compounds: chemical bonds
between different kinds of atoms, like oxygen and hydrogen bonding to make water
molecules or sodium and chlorine bonding to make salt crystals. While some bonds
only produce compounds we know that not all bonds are between different kinds of
atoms.
Oxygen gas (O2) is an element, not a compound, because it is a molecule with a bond
between two atoms of the same element. O2 has a nonpolar covalent bond because
the atoms are identical and have identical electrical properties. One side does not
become more negative than the other. Do you think an ionic bond could happen
between two atoms of the same element? A polar bond? Why or why not?
Figure 3.13 An atom level sketch of an oxygen molecule
Metallic Bonds: a special case
Metal atoms can also bond with other metal atoms.
When metal atoms bond with each other, the atoms
themselves become more positive by freely sharing
part of their negative charge with their neighboring
metal atoms. The attraction between the positive
metal atoms and freely flowing negative charges
help hold all the metal atoms together. These
negative charges can move all over the metal object
because they are not fixed on any one atom. This is
how we can get a chunk of gold atoms to stay
together, or an iron nail or a copper wire. Can you
think of any properties of a metal that these freely
moving negative charges can help explain?
Figure 3.14 Negative charges
move freely around partly
positive metal atoms