States of Matter

• Gas pressure comes from molecules colliding with the container walls.
• An ideal gas has zero particle volume and no intermolecular forces of attraction.
• pV = nRT in SI units: p in Pa, V in m³, T in K, R = 8.31 J K⁻¹ mol⁻¹. It can be used to find M_r.
• Four lattice types: giant ionic (NaCl, MgO), simple molecular (I₂, C₆₀, ice), giant molecular (SiO₂, graphite, diamond), giant metallic (Cu).
• Structure and bonding explain melting and boiling points, electrical conductivity and solubility.

Collisions with the walls

Gas molecules move randomly at high speed. When they collide with the walls of the container they change momentum, so they exert a force on the walls. Pressure is the total force per unit area from these collisions. More collisions per second (more molecules, or faster ones) means higher pressure.

Two assumptions

An ideal gas is one where:

• the molecules have zero particle volume (negligible compared with the container), and
• there are no intermolecular forces of attraction between them.

Real gases behave most like ideal gases at low pressure (molecules far apart, their own volume negligible) and high temperature (molecules move fast enough that attractions are insignificant). They deviate most at high pressure and low temperature, and for molecules with strong intermolecular forces (e.g. NH₃ more than He).

What the particles are and what holds them

Details worth knowing

• Iodine
• Ice
• Diamond