The period in Mendeleev's atomic table is defined by the horizontal rows. In the rows, the atomic mass and the atomic number of elements increase gradually compared to the increase in a group (vertical column). The increase in atomic mass across a period is gradual as the outermost shell of the atoms are getting filled with each horizontal traversal in the row. Example, the third period of the atomic table that starts with the element sodium (Na) has an atomic mass of 23. This atomic mass becomes 24 for magnesium which is the next element after sodium, then comes aluminium with an atomic of 27. Hence the answer is atomic mass.
