Although Dmitri Mendeleev is most often cited as the inventor of the modern periodic table, his table was just the first to gain scientific credibility. It wasn't the first table that organized the elements according to periodic properties.
There are about 94 elements on the periodic table that occur in nature. All of the other elements are strictly human-made. Some sources state more elements occur naturally because heavy elements may transition between elements as they undergo radioactive decay.
Technetium was the first element to be made artificially. It is the lightest element that has only radioactive isotopes (none are stable).
The International Union of Pure Applied Chemistry, IUPAC, revises the periodic table as new data becomes available. At the time of this writing, the most recent version of the periodic table was approved in December 2018.
The rows of the periodic table are called periods. An element's period number is the highest unexcited energy level for an electron of that element.
Columns of elements help to distinguish groups in the periodic table. Elements within a group share several common properties and often have the same outer electron arrangement.
Most of the elements on the periodic table are metals. The alkali metals, alkaline earths, basic metals, transition metals, lanthanides, and actinides all are groups of metals.
The present periodic table has room for 118 elements. Elements aren't discovered or created in order of atomic number. Scientists are working on creating and verifying elements 119 and 120, which will change the appearance of the table, though they were working on element 120 before element 119. Most likely, element 119 will be positioned directly below francium and element 120 directly below radium. Chemists may create much heavier elements that may be more stable because of the special properties of certain combinations of proton and neutron numbers.
Although you might expect atoms of an element to get larger as their atomic number increases, this does not always occur because the size of an atom is determined by the diameter of its electron shell. In fact, element atoms usually decrease in size as you move from left to right across a row.
The main difference between the modern periodic table and Mendeleev's periodic table is that Mendeleev's table arranged the elements in order of increasing atomic weight, while the modern table orders the elements by increasing atomic number. For the most part, the order of the elements is the same between both tables, though there are exceptions.
