The density of a substance is defined as its mass per unit volume. In general, the denser a substance is, the more tightly packed its particles are. Solids have the highest density because their particles are packed very tightly together. Liquids are less dense than solids because their particles are not as tightly packed. Gases are the least dense because their particles are very far apart from each other.

There are a few reasons why gases are less dense than solids and liquids. First, the particles in a gas move around more freely than the particles in a solid or liquid. This means that they can spread out more and take up more space. Second, the particles in a gas are less likely to stick together than the particles in a solid or liquid. This also means that they can spread out more and take up more space.

The amount of space that the particles in a gas take up is also affected by the temperature of the gas. The higher the temperature, the more energy the particles have and the more they move around. This means that the particles will spread out more and the gas will become less dense.

The density of a gas can also be affected by the pressure of the gas. The higher the pressure, the more the particles are squeezed together and the denser the gas becomes.

In summary, gases are less dense than solids and liquids because their particles are farther apart from each other and because they move around more freely. The density of a gas can also be affected by the temperature and pressure of the gas.