Chemistry Reference
In-Depth Information
CHEMISTRY
S
C
E
YMBOLS
OF
OMMON
LEMENTS
O
Oxygen
A gaseous element essential to human life; comprises
about 20% of the air
H
Hydrogen
The most abundant element in the universe
N
Nitrogen
Necessary element in protein; air is 80% N
C
Carbon
All living matter contains carbon compounds
Cl
Chlorine
Abundant element, always combined in nature with
other elements
S
Sulfur
Occurs free in nature
Na
Sodium
Combined with other elements in nature
P
Phosphorous
Combined with other elements in nature; another
element essential to life
Chemists know how much by weight of an element or a compound will react
exactly with another element or compound because the atoms of each ele-
ment have an assigned weight called the atomic weight. By scientific agree-
ment, each element's atomic weight is proportional to that of carbon with an
assigned weight of 12. In all chemical weight calculations, hydrogen has an
atomic weight of 1 (the lightest element), while nitrogen is 14 and oxygen
is 16. The molecular weight of a compound is the sum of the weights of all
of the atoms in the compound as shown by its chemical formula; for example,
the molecular weight of water with the formula H
O is 2
×
1 (2 hydrogens)
2
+
16 (1 oxygen), which totals 18.
It is desirable for chemical calculations to deal with weighable amounts
of various substances that we know contain equal numbers of molecules.
For example, the molecular weight of hydrogen is 2 and that of oxygen is 32.
This means that 2 g of hydrogen contains the same number of molecules as
32 g of oxygen. These actual weights are called
1
×
gram-molecular weights
or
moles
, for short. Thus, a gram-molecular weight (or mole) of any chemical
is the quantity of that chemical whose weight is numerically equal to its
molecular weight. It has been determined that one gram-molecular weight
of a chemical contains 6.023
23
×
10
molecules (this is called Avogadro's
number).
