Gases
The States of
Matter
The term state can be defined as a set of
conditions that describe a person or thing at a given time.
It is in this sense of the word that scientists divide matter
into the three states shown in the figure
below.
 |
The three states of matter have characteristic
properties. Solids have a distinct shape. When they
melt, the resulting liquid conforms to the shape of
its container. Gases expand to fill their containers.
|
There are two reasons for studying gases
before liquids or solids. First, the behavior of gases is
easier to describe because most of the properties of gases do
not depend on the identity of the gas. We can therefore
develop a model for a gas without worrying about whether the
gas is O2, N2, H2, or a
mixture of these gases. Second, a relatively simple, yet
powerful, model known as the kinetic molecular theory
is available, which explains most of the behavior of gases.

Elements or
compounds that are Gases at Room Temperature
Before examining the chemical and physical properties of
gases, it might be useful to ask: What kinds of elements or
compounds are gases at room temperature? To help answer this
question, a list of some common compounds that are gases at
room temperature is given in the table below.
Common Gases at Room Temperature
Element or Compound |
|
Atomic or Molecular Weight |
|
H2 (hydrogen) |
|
2.02 |
|
He (helium) |
|
4.00 |
|
CH4 (methane) |
|
16.04 |
|
NH3 (ammonia) |
|
17.03 |
|
Ne (neon) |
|
20.18 |
|
HCN (hydrogen cyanide) |
|
27.03 |
|
CO (carbon monoxide) |
|
28.01 |
|
N2 (nitrogen) |
|
28.01 |
|
NO (nitrogen oxide) |
|
30.01 |
|
C2H6 (ethane) |
|
30.07 |
|
O2 (oxygen) |
|
32.00 |
|
PH3 (phosphine) |
|
34.00 |
|
H2S (hydrogen sulfide) |
|
34.08 |
|
HCl (hydrogen chloride) |
|
36.46 |
|
F2 (fluorine) |
|
38.00 |
|
Ar (argon) |
|
39.95 |
|
CO2 (carbon dioxide) |
|
44.01 |
|
N2O (dinitrogen oxide) |
|
44.01 |
|
C3H8 (propane) |
|
44.10 |
|
NO2 (nitrogen dioxide) |
|
46.01 |
|
O3 (ozone) |
|
48.00 |
|
C4H10 (butane) |
|
58.12 |
|
SO2 (sulfur dioxide) |
|
64.06 |
|
BF3 (boron trifluoride) |
|
67.80 |
|
Cl2 (chlorine) |
|
70.91 |
|
Kr (krypton) |
|
83.80 |
|
CF2Cl2
(dichlorodifluoromethane) |
|
120.91 |
|
SF6 (sulfur hexafluoride) |
|
146.05 |
|
Xe (xenon) |
|
131.30 |
|
There are several patterns in the table
above.
- Common gases at room temperature include both
elements (such as H2 and O2)
and compounds (such as CO2 and NH3).
- Elements that are gases at room temperature are all nonmetals
(such as He, Ar, N2, O2, and so
on).
- Compounds that are gases at room temperature are all covalent
compounds (such as CO2, SO2,
and NH3) that contain two or more
nonmetals.
- With only rare exception, these gases have relatively
small molecular weights.
As a general rule, compounds that consist of relatively
light, covalent molecules are most likely to be gases at room
temperature.

