Final answer:
Mendeleev first ordered the periodic table by increasing atomic mass, but later it was discovered that atomic number provides a clearer periodic pattern. Atomic weight is important for calculations but not used for ordering the periodic table.
Step-by-step explanation:
Mendeleev first ordered the periodic table of elements by increasing atomic mass. He arranged the elements in such a way that elements with similar properties appeared in the same vertical column or group. He left gaps in the table to account for elements that were not yet discovered.
Later, it was discovered that elements can be better ordered by their atomic number, which represents the number of protons in an atom's nucleus. This arrangement resulted in a clearer periodic pattern and allowed for a more accurate prediction of an element's properties.
Atomic weight, also known as relative atomic mass, represents the average mass of an element's atoms, taking into account the abundance of different isotopes. It is an important value for calculating stoichiometry and other chemical reactions, but it is not used to order the periodic table.
Learn more about Ordering of the periodic table of elements