The modern periodic table is organized by atomic number.

The modern periodic table is organized by the atoms atomic number as opposed to Mendeleev's periodic table which was organised by their atomic weight. The atomic number of an atom is the number of protons found in the nucleus of a certain atom.

Order generally coincides with increasing atomic mass. The rows are called periods

Distance=slopeslope is -2/4which equals-1/2the distance between these two points is -1/2