A gas is one of the three primary states of matter, alongside solid and liquid. It is made up of tiny particles, such as atoms or molecules, that are far apart and move rapidly in all directions. Gases expand to fill the shape and volume of their container. Common examples include hydrogen, oxygen, and helium. ^[1-4]

Properties of Gases ^[1-4]

• Shape and Volume: Gases have neither a fixed shape nor a definite volume. They expand to fill the entire space available to them.
• Particle Movement: The particles move quickly and randomly in all directions. This rapid movement gives gases high kinetic energy.
• Intermolecular Forces: The forces of attraction between gas particles are extremely weak or negligible. Some gases, like helium, exhibit weak Van der Waals forces.
• Density: Gases have low density compared to solids and liquids, as their particles are spread far apart.
• Compressibility: Because gas particles are widely spaced, gases are highly compressible under pressure.
• Pressure: The particles exert pressure by colliding with each other and with the walls of their container.

Types of Gases

Gases can be classified based on their chemical composition into elemental gases and compound gases. ^[1-4]

1. Elemental Gases

These gases are made of only one type of element. Depending on the number of atoms bonded together, they may be monoatomic, diatomic, or triatomic.

i. Monoatomic Gases

Consist of single atoms not bonded to each other:

• Helium (He) – Used in balloons and cooling systems
• Neon (Ne) – Used in neon signs and lighting
• Argon (Ar) – Used in welding and light bulbs

ii. Diatomic Gases

Consist of two atoms of the same element bonded together:

• Oxygen (O₂) – Vital for respiration and used in medical applications
• Nitrogen (N₂) – Makes up most of Earth’s atmosphere; used in food preservation
• Hydrogen (H₂) – Used as a fuel and in chemical manufacturing
• Chlorine (Cl₂) – Used in water treatment and bleaching

iii. Triatomic Gases

Contain three atoms of the same element:

• Ozone (O₃) – Found in the upper atmosphere and protects against ultraviolet radiation

2. Compound Gases

These gases are composed of molecules that contain two or more different elements chemically bonded.

• Carbon Dioxide (CO₂) – Produced by respiration and used in carbonated drinks
• Methane (CH₄) – A main component of natural gas used for cooking and heating
• Ammonia (NH₃) – Used in fertilizers and household cleaners

Understanding Gas Behavior ^[1-4]

The behavior of gases is explained by a set of scientific principles known as the gas laws. These laws describe how gases respond to changes in temperature, pressure, volume, and the number of particles. Key laws include:

• Boyle’s Law – Describes the inverse relationship between pressure and volume
• Charles’ Law – Shows how gas volume increases with temperature
• Gay-Lussac’s Law – Relates pressure and temperature
• Avogadro’s Law – Explains the relationship between gas volume and the number of moles

These laws are combined in the Ideal Gas Law, written as: 

PV = nRT

This equation allows scientists to predict how a gas will behave under various conditions.