exam 1\nwhich characteristic is not a periodic trend?\no atomic radius\no electron affinity\no ionization…

Brief Explanations:

Atomic radius, electron affinity, and ionization energies show trends across periods and groups in the periodic table. However, the number of elements in a group is a fixed - characteristic for each group and not a periodic trend that varies in a regular way across the table.

Answer:

number of elements in a group