Atmospheric Composition, Atomic Structure, and the Physical Properties of Air

Composition and Characteristics of the Atmosphere The atmosphere is defined as a gaseous envelope that surrounds the Earth. Its density is not uniform; it becomes increasingly less dense as one rises in altitude. Consequently, as the height increases, the atmospheric pressure decreases. The atmosphere serves a critical protective function by shielding the Earth from solar radiations, a task performed by the couche d’ozone (the ozone layer). The specific layer of air in which humans and other organisms live is known as the troposphere. This layer is predominantly composed of diazote (nitrogen), which makes up $78\%$ of its volume, followed by the gas essential for life, dioxygène (oxygen), at $21\%$. The remaining $1\%$ of the air's composition includes other gases, such as the dioxyde de carbone (carbon dioxide) that our organisms produce. While carbon dioxide represents only a very small percentage ($0.03\%$), it is vital because it enables photosynthesis in plants and helps regulate the Earth’s temperature through the greenhouse effect (effet de serre). Air is classified as a homogeneous mixture of several gases. For general calculations, its composition is often approximated as $80\%$ (or $\frac{4}{5}$) diazote and $20\%$ (or $\frac{1}{5}$) dioxygène. # Environmental Effects and Air Pollution The principle of the greenhouse effect (effet de serre) is essential for life, but modern production levels have led to negative impacts. The excessive production of dioxyde de carbone and other gases like méthane (methane)—which is linked to transport, heating, and livestock farming—increases the greenhouse effect, which in turn causes an abnormal warming of the Earth. Furthermore, other gases such as sulfur oxides (oxydes de soufre), nitrogen oxides (oxydes d’azote), or ground-level ozone (ozone au niveau du sol) can cause significant health and environmental issues. These pollutants, resulting from transport, industries, and habitations, can cause respiratory ailments (affections respiratoires) and the formation of acid rain (pluies acides). # Historical Perspectives and Atomic Theory The understanding of matter has transitioned through various theories over time. For approximately $2000$ years, a false theory dominated chemistry. Aristote believed that matter was composed of 4 elements: terre, eau, air, and feu (earth, water, air, and fire). However, Lavoisier eventually contradicted this theory. Lavoisier proved the earlier assertion by Démocrite that matter is made of "grains de matière," known today as molecules and atoms. Lavoisier and Dalton worked to understand how atoms assemble to form molecules. A primary example provided is the water molecule, which is the smallest particle of water and is composed of one atom of oxygen (un atome d’oxygène) and two atoms of hydrogen (deux atomes d’hydrogène). # Classification and Representation of Atoms There are a multitude of different atoms, all categorized within a table called the classification périodique (periodic table). Atoms can be represented in two ways. First, they have a chemical symbol (symbole chimique), which consists of either a single uppercase letter or two letters (where the first is uppercase and the second is lowercase). Second, they are represented by molecular models (modèle moléculaire), which are spheres of different sizes and colors. Examples of atoms include carbone (Carbon, symbol $C$), oxygène (Oxygen, symbol $O$), hydrogène (Hydrogen, symbol $H$), and azote (Nitrogen, symbol $N$). Other notable examples provided are chlore (Chlorine, symbol $Cl$) and calcium (Calcium, symbol $Ca$). # Structure and Representation of Molecules A molecule is defined as an assembly of atoms (un assemblage d’atomes entre eux). Every molecule is represented by a chemical formula (formule chimique), which specifies the name and the exact quantity of atoms it contains, and a molecular model (modèle moléculaire), which shows the physical assembly of the corresponding atomic spheres. Examples and their formulas include: dihydrogène ($H_2$), dioxyde d’azote ($NO_2$), trioxyde de soufre ($SO_3$), dioxygène ($O_2$), diazote ($N_2$), dioxyde de carbone ($CO_2$), monoxyde de carbone ($CO$), eau ($H_2O$), protoxyde d’azote ($N_2O$), méthane ($CH_4$), and butane ($C_4H_{10}$). # Characteristics and Molecular Models of the States of Matter Matter exists in three states: solide, liquide, and gazeux. In the solid state, molecules are very close together and have limited movement, maintaining a compact and ordered arrangement. Solids have a "forme propre" (proper shape), meaning they keep their form regardless of their container, and they can be grasped between fingers. In the liquid state, molecules are close but agitated, arranged in a compact and disordered way. Liquids possess a "volume propre" (proper volume) but no fixed shape, as they flow and take the form of their container. In the gaseous state, molecules are very far apart and highly agitated, resulting in a dispersed and disordered arrangement. Gases have no proper volume because they occupy all available space and will escape an open container. It is important to note that certain visual phenomena are not gases. Fog (le brouillard) seen above a boiling pot consists of small liquid water droplets in suspension. Similarly, smoke (la fumée) from a fire consists of solid microparticles in suspension. Neither phenomenon is considered a gas. # Air Volume, Pressure, and Compressibility Air does not have a property volume. If a certain quantity of air is trapped in a syringe, the piston can be pushed or pulled to change the volume. Decreasing the volume is called compressing the air (it is compressible), while increasing the volume is called expanding the air (it is expansible). Atmospheric pressure is measured with a baromètre (barometer) in units of pascal ($Pa$). The pressure of a gas inside a container is measured with a manomètre (manometer). On a molecular level, pressure is generated by molecules striking the walls of the container. When volume is decreased, molecules strike the walls more frequently, causing pressure to increase. When volume is increased, molecules strike the walls less often, causing pressure to decrease. Unlike air, water is incompressible. When a syringe is filled with water, the volume cannot be decreased because the molecules cannot be brought any closer together than they already are.