long form of periodic table | What Are Periods & Groups In The Periodic Table? | Properties of Matter | Chemistry

 As you move from the light elements to the heavy elements you keep periodically coming across the same properties which is why it's called the periodic table the recurring properties are organized so that you can easily see similarities between elements the periodic table is arranged in periods and groups going from the light elements at the top to the heavy elements at the bottom the rows going across from left to right are

  Periods elements in the same period all share something in common they have the same number of energy shells each new period row represents a new shell elements in the first period have one shell and as we go down the shells increase hydrogen is in the first period it has one shell potassium is in the fourth period and has four energy shells as do all the other elements in this period the columns going down from top to bottom are the groups elements in the Same group also have something in common elements in the same group have the same number of electrons in their outermost shell the electrons in the outer shell are called valence electrons this just means that these are the electrons available for reactions and bond formation the number of electrons in the outer shell governs elements reactivity which is why elements in the same group have similar properties the group number can tell you how many electrons are in This shell for example let's look at Group seven fluorine chlorine iodine bromine and astatine they all have seven electrons in their outermost shell and all exhibit similar chemical characteristics the properties show a gradual change going down the group as we go from period to period so if we look at group seven again we can see that they are each in a different period in the periodic table telling us that each Element in this group has its outer electrons on a different shell so chlorine is in group 7 period 3 therefore it has 3 energy levels with 7 electrons in its outermost shell