Environmental Engineering Reference
In-Depth Information
Atoms, Ions, and Compounds
Atoms, ions, and compounds are the building blocks
of matter.
If you had a supermicroscope capable of looking at in-
dividual elements and compounds, you could see they
are made up of three types of building blocks. The first
type is an
atom:
the smallest unit of matter that has the
characteristics of a particular element.
The second type is an
ion:
an electrically charged
atom or combination of atoms. Examples encountered
in this topic include
positive
hydrogen ions (H
+
),
sodium ions (Na
+
), calcium ions (Ca
2+
), and ammonium
ions (NH
4
+
) and
negative
chloride ions (Cl
-
), nitrate ions
(NO
2
-
), sulfate ions (SO
4
2-
), and phosphate ions (PO
4
3-
).
A third building block is a
compound:
a substance
containing atoms or ions of more than one element
that are held together by chemical bonds. Chemists
use a
chemical formula
to show the number of atoms
(or ions) of each type in a compound. This shorthand
contains the symbol for each element present and uses
subscripts to represent the number of atoms or ions of
each element in the compound's basic structural unit.
Examples of compounds and their formulas encoun-
tered in this topic are water (H
2
O, read as “H-two-O”),
oxygen (O
2
), ozone (O
3
), nitrogen (N
2
), nitrous oxide
(N
2
O), nitric oxide (NO), hydrogen sulfide (H
2
S), car-
bon monoxide (CO), carbon dioxide (CO
2
), nitrogen
dioxide (NO
2
), sulfur dioxide (SO
2
), ammonia (NH
3
),
sulfuric acid (H
2
SO
4
), nitric acid (HNO
3
), methane
(CH
4
), and glucose (C
6
H
12
O
6
).
Table sugar, vitamins, plastics, aspirin, penicillin,
and most of the chemicals in your body are
organic
compounds
that contain at least two carbon atoms
combined with each other and with atoms of one or
more other elements, such as hydrogen, oxygen, nitro-
gen, sulfur, phosphorus, chlorine, and fluorine. One
exception, methane (CH
4
), has only one carbon atom.
All other compounds are called
inorganic compounds.
The millions of known organic (carbon-based)
compounds include the following:
There are three major types of organic polymers:
com-
plex carbohydrates,
consisting of two or more monomers
of simple sugars (such as glucose) linked together;
pro-
teins,
formed by linking together monomers of amino
acids; and
nucleic acids
(such as DNA and RNA), made
by linking sequences of monomers called nucleotides.
Genes
consist of specific sequences of nucleotides
found within a DNA molecule. These coded units of
information about specific traits are passed from par-
ents to offspring during reproduction.
Chromosomes
are combinations of genes that make up a single DNA
molecule, together with a number of proteins. Each
chromosome typically contains thousands of genes.
Genetic information coded in your chromosomal DNA
is what makes you different from an oak leaf, an alliga-
tor, or a flea, and from your parents. The relationships
of genetic material to cells are depicted in Figure 2-4.
What Are Atoms Made Of?
Each atom has a tiny nucleus containing protons, and
in most cases neutrons, and one or more electrons
whizzing around somewhere outside the nucleus.
If you increased the magnification of your supermicro-
scope, you would find that each different type of atom
contains a certain number of
subatomic particles.
There
are three types of these atomic building blocks: posi-
tively charged
protons (p),
uncharged
neutrons (n),
and negatively charged
electrons (e).
Each atom consists of an extremely small center, or
nucleus.
It contains one or more protons, and in most
cases neutrons, and one or more electrons in rapid mo-
tion somewhere outside the nucleus. Atoms are in-
credibly small. In fact, more than 3 million hydrogen
atoms could sit side by side on the period at the end of
this sentence.
Each atom has equal numbers of positively
charged protons inside its nucleus and negatively
charged electrons outside its nucleus. Because these
electrical charges cancel one another,
the atom as a whole
has no net electrical charge.
Each element has a unique
atomic number,
equal
to the number of protons in the nucleus of each of its
atoms. The simplest element, hydrogen (H), has only
1proton in its nucleus, so its atomic number is 1. Car-
bon (C), with 6 protons, has an atomic number of 6,
whereas uranium (U), a much larger atom, has 92 pro-
tons and an atomic number of 92.
Because electrons have so little mass compared
with the masses of protons or neutrons,
most of an
atom's mass is concentrated in its nucleus.
The mass of an
atom is described in terms of its
mass number:
the total
number of neutrons and protons in its nucleus. For ex-
ample, a hydrogen atom with 1 proton and no neutrons
in its nucleus has a mass number of 1, and a uranium
atom with 92 protons and 143 neutrons in its nucleus
has a mass number of 235 (92
■
Hydrocarbons:
compounds of carbon and hydrogen
atoms. An example is methane (CH
4
), the main com-
ponent of natural gas, and the simplest organic
compound.
■
Chlorinated hydrocarbons:
compounds of carbon, hy-
drogen, and chlorine atoms. An example is the insecti-
cide DDT (C
14
H
9
Cl
5
).
■
Simple carbohydrates
(simple sugars): certain types of
compounds of carbon, hydrogen, and oxygen atoms.
An example is glucose (C
6
H
12
O
6
), which most plants
and animals break down in their cells to obtain energy.
Larger and more complex organic compounds,
called
polymers,
consist of a number of basic structural
or molecular units (
monomers
) linked by chemical
bonds, somewhat like cars linked in a freight train.
143
235).
