Our concept of solids and liquids depends largely on our ability to see and touch them.
If we have two lumps of solid with roughly the same volume. If one lump is light while the other is heavy, we say the heavy lump has a higher density (mass per unit volume (lb/in ^{2}, g/cc, kg/m^{3}, etc.)).
Gases present a challenge to our ability to see and/touch, and new terms have been introduced to describe the “gaseous state.”

**Note:** The gas laws used to derive the values quoted below are correct only for **ideal** gases.
However, in room temperature chambers as pressure decreased, all gases approach ideal behavior.
For vacuum applications, the appropriately scaled value — to allow for pressure change — will be sufficiently accurate for precise calculations.

### Number Density

Avogradro determined that equal volumes of gas at the same temperature and pressure contained equal numbers of molecules.
It does not matter if the gas is pure N^{2}, CO^{2}, Ar, H^{2}, or a mixture of all four.
Later, Loschmidt determined that 22.4 liters of gas at 760 Torr and 0° contain 6.022 × 10 ^{23} molecules (the present day value, often called **Avogadro's number**).

Since gas fills any volume that contains it, its “density” (in g/cc units) depends on that volume, the gas composition, and molecular weights of the components.
If instead of density (mass per unit volume) we use number density (number of molecules in 1 cc) we can describe a “quantity” of gas without knowing anything about composition or molecular weights.
For Avogadro's number (which refers to 22.4 liters) we know the number density (which refers to 1 cc) of any gas at 760 Torr and 0° C is 2.69 × 10^{19} cm^{-3}.

### Mean Free Path

The huge number density at atmospheric pressure and the high velocities of the gas molecules mean that in each cc there are many, many gas phase collisions every second.
Expressed another way, even though a molecule travels at high speed, on average it travels a very short distance before hitting another gas phase molecule.
This average distance is called the **mean free path (mfp)**. For air at 760 Torr the mfp is 6.5 × 10 ^{-6} cm.

### Particle Flux

In addition to colliding with each other in the gas phase, gas molecules hit the containing vessel walls and every other surface inside the enclosure.

The rate at which they hit these surfaces, called **particle flux**, depends on the gas's number density.

The flux of air at 760 Torr and 0°C is 2.9 × 10^{23} cm^{-2} s^{-1}.

### Flow Regimes

The mean free path (described above) and the chamber/component dimensions determine the gas's flow conditions. If mfp is:

- Very short compared with the chamber's
**“characteristic dimension's”**the gas is in**continuum flow.** - Shorter than the chamber's characteristic dimension's but approaches them, the gas is in
**transitional flow**. - Equal to or longer than the chamber's characteristic dimension's then the gas is
**molecular flow**.

The flow regime is used to identify the appropriate equations needed to calculate conductances, pump down times, and other characteristics.

### Vacuum Doesn't Suck!

There is a common misunderstanding that vacuum pumps suck. **There is no such force as suction.**
If the gas molecules in one “section” of a vacuum volume could be instantaneously removed, molecules from the remaining section in their normal high-speed flight would randomly collide and bounce off walls until they filled the whole volume at a lower pressure.

For vacuum pumping this means, until a gas molecule in its random flight enters the pumping mechanism, that molecule cannot be removed from the volume.
In effect the pump acts like a one-way valve: gas molecules may enter but not return. But for that to happen, molecules must first arrive at the pump; it cannot reach out and grab them.
Understanding that **vacuum doesn't suck** makes the basic aspects of vacuum technology much easier to grasp.