Everything in the universe is made of elements. An element is a substance that cannot be broken down or made into anything simpler by chemical reactions. Several elements, like gold, silver and lead, have been known for thousands of years. Darmstadtium, on the other hand, was created in a lab in the 1990's.
Siberian chemist Dimitri Mendeleev arranged the elements into the periodic table in 1869. He organized them in groups (columns) and periods (rows) leaving gaps for elements that were still undiscovered. Today, those gaps have been filled to a total of 118 known elements but certainly there may be others that have yet to be discovered.
The vertical groups of the table make up "families" that are closely related and liking the same types of chemical characteristics. Mendeleev organized the elements so that each had a higher atomic weight than the one on its left and by similar chemical properties to other elements in the same column.
In 1913 Henry Moseley discovered it was the atomic number (charge) and not the atomic weight that is most fundamental to the chemical properties of any element. The element's chemistry is determined by the way its electrons are arranged.
There is a progression from metals to non-metals across each period. The block of elements in groups 3-12 contain the transition metals. The rare Earth elements are groups 58-71 (lanthanides–chemically similar to each other) and 90-103 (actinides-less chemically similar to each other). Naturally occurring rare Earth elements are found only in very small amounts. The actinides include most well known elements that take part in or produce nuclear reactions. No element with an atomic number higher than 92 occurs naturally and are produced in nuclear reactors or particle accelerators.
In researching the periodic table, I found conflicting information regarding how many elements were on the periodic table and even how some of them are grouped. But for the most part, it looks like there are 118.
Now you’re a little smarter, Girlfriend — And so am I.