How does matter change states (i.e from solid to gas)? What happens to the particles? How do you calculate the relative atomic mass of an element?...

The Basic Structure of Matter

Matter is composed of particles which are held together by intermolecular forces. Particles are composed of one or more atoms. Atoms are composed of subatomic particles called, protons, neutrons, and electrons.

States of Matter

• 
  Solids: Solid particles are held tightly together by strong intermolecular forces. There is very little movement of the particles. This means that the particles have low kinetic energy.

• 
  Liquids: Liquid particles are held together more loosely than solid particles. They able to slide past one another and they possess more kinetic energy.

• 
  Gases: The intermolecular forces between gas particles are negligible. The particles move around rapidly and have high kinetic energy.

When heat energy is absorbed by a solid, the intermolecular forces between the particles are reduced. The particles are then able separate and move around (i.e. the substance becomes a liquid).

When heat energy is absorbed by a liquid, it eventually overcomes the intermolecular forces holding the liquid particles together. The particles are then free to move around (i.e. the substance becomes a gas).

Chemical and Physical Change

• 
  Physical Change: In a physical change, the particles of a substance are rearranged, but do not change into different particles. Examples of physical changes include: state changes such as melting and evaporation, shredding paper, mixing sand and water.

• 
  Chemical Change: In a chemical change, the particles of a substance do change into different particles. Examples of chemical changes include: burning, digesting, reacting.

Relative Atomic Mass

The relative atomic mass of an element is a "weighted average" of the masses of all of the isotopes of the element. Isotopes are different forms of the same element with different numbers of neutrons.

A weighted average is one that takes into account the percentages of each substance. The relative mass of an element is calculated by multiplying the percentage of each isotope times its mass, and adding the resulting answers together. 

Example: The relative mass of chlorine can be calculated by determining the weighted average mass of chlorine's isotopes using the information shown below.

    Isotope            Percentage          Mass (amu)

    Chlorine-35          75%                    35

    Chlorine-37          25%                    37

Relative mass = (.75)(35) + (.25)(37) = 35.5 amu. 

The relative mass is the number that is used as the atomic mass in the periodic table.