Air refers to the mixture of gases and water vapour which make up the atmosphere of the Earth. Note that this particular balance is specific to Earth - other planets have other atmospheres, and therefore different air. For example, the atmosphere of Mars is 95% carbon dioxide, plus small amounts of oxygen, water, methane, nitrogen, and argon, but Jupiter's atmosphere is almost entirely hydrogen, smaller amounts of helium, and trace amounts of neon, oxygen, carbon, and nitrogen. The study of the chemical makeup of the air is known as atmospheric chemistry.
Here on Earth, however, the exact proportions are known to be as follows:
WATER - amount varies.
Water vapour is an important component of the atmosphere. However, unlike all of the gases below, water is a liquid at room temperature - which means that it is never a true part of the gaseous atmosphere. Instead, the proportion of water vapour in the air varies widely according to location (distance from major water sources, and altitude), and the weather (cloudy and rainy conditions obviously have much higher water content in the air). On average, water makes up about 1% of the air we breathe in.
NITROGEN - 78%
The largest component of the Earth's atmosphere is nitrogen. In a way this is surprising, since we do not rely on nitrogen directly in the way we do oxygen. Still, all living things do contain some nitrogen, and it is an important component of plant fertilizer.
The fact that nearly-inert nitrogen forms such a large percentage of our atmosphere can actually be of some help to us when, for example, a fire occurs. If the Earth's atmosphere were instead made up mostly of oxygen, fires would spread extraordinarily rapidly, almost instantly becoming dangerously large. NASA's spacecraft once had artificial atmospheres without nitrogen; when a fire broke out in Apollo 1 during training, all of the astronauts inside were killed in about thirty seconds.
OXYGEN - 20%
Oxygen is the essential component to all animal life. It is so important that people who travel between sea-level and mountainous regions can experience noticeable side effects of the difference in oxygen levels. (In this case, the proportion of oxygen has not changed, but the atmosphere has grown thinner at high altitudes, so that overall one breath draws in less oxygen).
Fortunately for animals like us, Earth also has a renewable source of oxygen in the form of plants, which take in carbon dioxide molecules, divide them into atoms, and then release oxygen into the atmosphere. (In contrast, animals and humans do the reverse: we breathe in oxygen, and breathe out carbon dioxide). It is speculated by scientists that early Earth, prior to plants, had much lower oxygen content. The creation of an oxygen-rich atmosphere by the first generation of plants, therefore, was arguably the first and most important incident of climate change on early Earth.
ARGON - 1%
Argon is a noble gas, which means that it is even less reactive than nitrogen; for this reason, its existence escaped the notice of chemists until the late 1800s. As a noble gas, it does not play any especially important role in life or in chemical processes on the Earth. However, some isotopes are slightly radioactive, so that scientists are able to perform certain geological and paleontological rock dating by means of an analysis of the argon content of the rocks.
CARBON DIOXIDE - 0.04%
Carbon dioxide is a molecule which consists of a single carbon atom bonded to two oxygen ions. This is the principal compound of concern among those who argue that human pollution is causing dangerously rapid climate change, because carbon dioxide is known to absorb more solar radiation than the other gases which make up large proportions of the atmosphere. This means that, over time, the Earth will gradually take in more and more heat from the Sun, and therefore grow hotter and hotter. (The scientific argument against climate change, therefore, is that carbon dioxide is not increasing to levels at which this increased absorption of sunlight is dangerous.)
All living things produce small amounts of carbon dioxide as they breathe, taking in oxygen, mixing it with carbon within the body, and then exhaling it as carbon dioxide. However, natural processes on the Earth, and industrial processes in the human economy, release much more carbon dioxide than we do simply by breathing.