All elements in the same horizontal row create a period, have the same amount of electron shells. Since the number of electron shells doesn't affect the properties of the atom to any bigger extent, the elements of the same period don't share many characteristics. Sodium, sulfur, and chlorine are examples of elements from period 3, meaning they have 3 electron shells containing electrons. That the number of the period is identical to the number of electron shells is good to remember. In the picture below, period 3 is marked in purple.

As you may have noticed, period 6 and 7 look a bit different. In these periods, there are too many elements to fit, so some were moved below the rest of the table. The first of these rows is the lanthanoids (marked in orange), and the second row is the actinoids (marked in red).

For each step to the right in a period, the atomic number is increased by one. Fluorine, which is one step right of oxygen (atom number 8), thus has atom number 9.

The next article is about trends in the periodic table.

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