Atomic Mass
The average mass of one atom, in atomic mass units — driven almost entirely by proton and neutron count.
See atomic mass colored on the interactive table →Across a period (left → right)
Rises steadily left to right (each element has one more proton, and generally more neutrons).
Down a group (top → bottom)
Rises steadily down a group.
Why it happens
This is the most straightforward trend on this page: atomic mass is overwhelmingly determined by the number of protons and neutrons in the nucleus, and both of those simply increase as atomic number increases — there's no competing effect the way there is with radius or ionization energy. It climbs almost monotonically from hydrogen through oganesson.