Dmitri Mendeleev developed a chart like arrangement of the elements called the __(1)Periodic table__. He stated that if the elements were listed in order of increasing __(2)Period__, their properties repeated in a regular manner. He called this the __(3)__ of the elements. The arrangement used today, devised by __(4)__, differs from that of Mendeleev in that the elements are arranged in order of increasing __(5)__. Each horizontal row of elements is called a(n) __(6)__. Each vertical column is called a(n) __(7)__, or, because of the resemblance between elements in the same column, a(n) __(8)__. In rows 4 through 7, there is a wide central section containing elements, each of which is called a(n) __(9)__. Rows 6 and 7 also contain two other sets of elements that are listed below the main chart. These are called the __(10)__ and the __(11)__, respectively. Each of these elements, as well as those in the first two columns at the left end of the chart, is classified as a(n) __(12)__. Each of the elements at the right side of the chart is classified as a(n) __(13)__. Each of the elements between these two main types of elements, having some properties in common with each, is called a(n) __(14)__. Each of the elements in the column labeled 1 is called a(n) __(15)__. Each of the elements in the column labeled 2 is called a(n) __(16)__. Each of the elements in column 17 is called a(n) __(17)__. Each of the elements in column 18 is called a(n) __(18)__.