The elements in the periodic table are organized by their atomic numbers, their electron configurations and the recurring properties found in them. Elements are arranged in blocks, with elements found in that block all containing consistent properties. For instance, all alkali metals are highly reactive, and all noble gases are inert, meaning they cannot react with other elements under normal conditions.
The lower the atomic number of an element, the higher and further to the left the element is found on the periodic table. Two groups, known as the lanthanide and actinide groups, contain elements that do not fit in the commonly understood structure of the periodic table. Elements at the atomic number of neodymium and lower are all primordial elements, which are elements that can be found naturally in the universe outside of the process of decay or artificial means.
Diatomic nonmetals are the elements that can be found the most frequently in the universe and share roughly the same place as polyatomic elements, which together are the elements that are most vital to the existence of life. These groups carry a strange structure on the table, with hydrogen, a diatomic nonmetal, being found at the very upper left of the table, and all the others found in a roughly triangle-shaped grouping on the right side of the table.