The role of electrons
Electrons are live in shells or energy levels which are numbered to 4 and have certain letters that are K, L, M and N. They spin around the nucleus in these shells.
Atoms are electrically neutral because the number of proton's and neutron's are equal. If this is changed then an atom becomes charged and is called an ion. If an atom loses a electron it becomes a positive ion & if it gains electrons it becomes a negative ion.
Periodic table (groups, periods)
If you carefully inspect an electron configurations of the first 18 elements shows that, the number of occupied shells is in an atom is the same number as the periodic table. Elements in the same group have the same number of outer shell electrons and will have similar chemical properties. As you move down a group, more shells are used. The atoms get bigger, causing their properties to change slightly.
Th valence electrons are how many electrons are in the outside shell.