valence electron

• Electrons orbit the nucleus at certain distances from the nucleus. Each orbit corresponds to a certain energy level. 
• The electrons in the outermost orbit are called valence electrons. They have higher energy and are less tightly bound to the atom than those closer to the nucleus.
• If a valence electron acquires a sufficient amount of energy, it can escape from the outer orbit.
•  The escaped valence electron is called a free electron. It can migrate easily from one atom to another.

structure of atom

• An atom is the smallest particle of an element possessing the unique characteristics of that element.
•  Atoms have a planetary type of structure.
• They consist of a central nucleus surrounded by orbiting electrons.
• The nucleus consists of positively charged protons and uncharged particles called neutrons. An atom has the same number of electrons and protons. Thus the atom is electrically neutral. 

activation energy

what is activation energy

the energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction. The activation energy of a reaction is usually denoted by E_a and given in units of kilojoules per mole.

         sources: 
http://www.youtube.com/watch?v=VbIaK6PLrRM