The periodic table has undergone some changes since it was first introduced. List three ways the modern periodic table differs from the one first published by Mendeleev in 1869.

Although some elements, such as gold and iron, have been known to humans since prehistoric times, it wasn't until the 17th century that the first scientific discovery of an element (phosphorus) was made. Only 12 elements were known prior to 1700, but as more and more elements were discovered-by 1900 there were more than 80-scientists tried to find a way to organize them systematically, according to their physical and chemical properties.

Today, the periodic table organizes the elements in horizontal rows, or periods, by order of increasing atomic number, which equals the number of protons in the atomic nucleus of each element. The elements are also organized in vertical columns, or groups, based on similar physical characteristics and chemical behavior. This arrangement developed side by side with atomic theory over about 200 years, and it continues to evolve as new elements are discovered.