Mendeleev developed an early version of the periodic table. Describe how Mendeleev arranged the elements in his periodic table and explain two ways in which his arrangement differed from the modern periodic table.

Mendeleev arranged the elements in order of increasing atomic mass, placing them in rows (periods) and columns (groups) so that elements with similar chemical properties appeared in the same column.

He deliberately left blank spaces in the table where no element was known, anticipating that future discoveries would fill those gaps.

Two ways his arrangement differed from the modern table:
1. The modern periodic table is ordered by increasing atomic number (number of protons) rather than atomic mass.
2. Mendeleev’s ordering sometimes placed elements out of sequence because the atomic masses of some elements (especially those with multiple isotopes) did not match the expected trend, whereas the modern table uses atomic number to avoid this problem.