compound basics
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Compound BasicsLet’s start with molecules. Molecule is the general term used
to describe any atoms that are connected by chemical bonds.
Every combination of atoms is a molecule. A compound is amolecule made of atoms from different elements. All
compounds are molecules, but not all molecules arecompounds. Hydrogen gas (H2) is a molecule, but not a
compound because it is made of only one element. Water (H2O)
can be called a molecule or a compound because it is made of
hydrogen (H) and oxygen (O) atoms.
There are two main types of chemical bonds that hold atoms
together: covalent and ionic/electrovalent bonds. Atoms thatshare electrons in a chemical bond have covalent bonds. An
oxygen molecule (O2) is a good example of a molecule with a
covalent bond. Ionic bonds occur when electrons are donated
from one atom to another. Table salt (NaCl) is a common
example of a compound with an ionic bond.
You may also learn about a third type of bond. Metallic bonds
occur between metal atoms. We’re going to focus on ionic and
covalent bonds.
Physical and ChemicalTraits of CompoundsWhen we discuss phase changes
in matter, we are looking
at physical changes. Physical
forces alone (unless you'reinside of the Sun or something
extreme) rarely break downcompounds completely. You can
apply heat to melt an ice cube,
but there will be no change in
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the water molecules. You can also place a cup of water in a
container and decrease the pressure. The water will eventually
boil, but the molecules will not change.
Chemical changes in compounds happen when chemical bondsare created or destroyed. Forces act on the bonds betweenatoms, changing the molecular structure of a substance. You
can pour liquid acid on a solid and watch the solid dissolve.
That process is a chemical change because molecular bonds are
being created and destroyed. Geologists pour acids on rocks to
test for certain compounds.
There are millions of different compounds around you. Probably
everything you can see is one type of compound or another.When elements join and become compounds, they lose many
of their individual traits. Sodium (Na) alone is very reactive.
But when sodium and chlorine (Cl) combine, they form a non-reactive substance called sodium chloride (table salt, NaCl).
New compounds have few or none of the physical or chemical
traits of the original elements. They have a new life of their
own.
Different Bonds AboundIf you look at sodium chloride, it is held together by one
ionic/electrovalent bond. What about magnesium chloride
(MgCl2)? It contains one magnesium(Mg) and two chlorine (Cl)atoms. There are two ionic bonds. Methane (CH4) is made up of
one carbon (C) and four hydrogen (H) atoms. There are four
bonds and they are all covalent.
Those examples have very simple chemical bonds. However,
most compounds have combinations of ionic and covalent
bonds. Let's look at sodium hydroxide (Na-OH)...
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You can see the sodium (Na) part on the left and
the hydroxide (-OH) part on the right. The bond that binds
the hydrogen (H) to the oxygen (O) is covalent. The sodium isbonded to the hydroxide part of the compound with an ionic
bond. This is a good example of how there can be different
types of bonds within one compound.