As the atomic masses of elements increase down a group on the periodic table, the number of atoms in 1 mol of the element decreases. This is because the atomic mass is the mass of one atom of an element expressed in atomic mass units (amu). The atomic mass is the sum of the protons and neutrons in the nucleus of an atom, so the heavier the element, the more protons and neutrons it has. As a result, 1 mol of a heavier element contains fewer atoms than 1 mol of a lighter element.

For example, 1 mol of hydrogen contains 6.022 x 10^23 atoms, while 1 mol of lead contains only 2.072 x 10^23 atoms. This is because lead has a much higher atomic mass than hydrogen (207.2 amu vs. 1.008 amu), so there are fewer lead atoms in 1 mol of lead than there are hydrogen atoms in 1 mol of hydrogen.

The relationship between atomic mass and the number of atoms in 1 mol of an element is an important concept in chemistry, as it allows chemists to calculate the mass of a given number of atoms or the number of atoms in a given mass of an element.