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.