Yet amid all this bewildering variety there is a pattern, although it’s quite hard to find. Scientists call this pattern the periodic law. Whenever it’s mentioned, the name of one man springs instantly to mind, a man born in 1834 amid the forests of Siberia; Dmitri Ivanovich Mendeleev. At the March meeting of the Russian Chemical Society 1869, I communicated a paper on the correlation of the properties and atomic weights of the elements. The substance of this paper is embraced in the following conclusions: 1) The elements if arranged according to their atomic weights exhibit an evident periodicity of properties. Today, we use atomic number where Mendeleev used atomic weight, but what did Mendeleev mean by the periodicity of properties? Let’s try to find out by laying out the elements as he laid them out in this book.
We’ll ignore hydrogen and start with element number two, helium. The eighth element in this row is fluorine, next comes neon. Now, neon resembles helium in two very obvious ways. First, it’s a gas, and second, it doesn’t react with anything. We’ll mark this similarity by putting neon underneath helium. Off we go again. Another row of eight and we get a sign that we’re on the track of something very fundamental. The next element is argon and again it’s a gas which doesn’t react with anything. So argon goes beneath neon and off we go again. Let’s stop there at scandium. The vertical columns of this arrangement are called groups and the horizontal rows are called periods. What we have here are the beginnings of a periodic table. The groups are numbered from zero up to seven. The most important thing to remember about the periodic table is that the groups contain elements which resemble each other.
This video introduces what's known as 'periodic law'. This is the way that certain properties of elements repeat periodically when arranged by atomic number. The Periodic Table arranges elements into vertical columns (groups) and horizontal rows (periods) to display these trends.