Mr

Notice how the elements are arranged on the periodic table in increasing order of atomic number. There is, first of all, hydrogen--then to its right is helium. Below hydrogen is lithium--then to its right is beryllium, boron, and so forth on across with increasing atomic number.

The elements on the periodic table are arranged so that their atomic numbers increase in order from left to right. There is a special name given to the horizontal rows on the periodic table. They are called periods. The first row, the one containing hydrogen and helium, is called the first period. The second row is called the second period. And so on.

You may wonder why helium, with atomic number 2, is put way over on the right side instead of next to hydrogen. The reason is that the chemical properties of helium are similar to those of the other elements in the far right column. They are called the inert gases because they are all very unreactive gases. Remember that the periodic table is arranged so that elements with similar chemical properties are lined up in vertical columns. It would not be correct to place the inert gas, He, in the second column, the one just to the right of H, because He's properties aren't anything like those of the other elements in the second column (Be, Mg, etc.) They are all reactive metals, not inert gases. In many ways, H does not belong in the first column either, but it is usually put there anyway for reasons that we will get into later.