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.