Find the chemical symbol of the first atom on the list of elements sorted by name. Some elements are easy: C for carbon, H for hydrogen. Some are not so obvious: Fe is the symbol for iron, Na represents sodium. This is because some elements have classical names that come from other languages, such as Latin.
Find the chemical symbol for the second atom as you did for the first in step 1.
Locate the two symbols on the periodic table. Note the column and row where they’re located.
Determine the relative sizes of the atoms. It’s easy if they happen to be in the same row or column. The smallest atoms are at the top of every column in the periodic table. They get bigger as you go down the column. Also, atoms get smaller going left to right for every row.
If they’re not in the same row or column, and they’re close neighbors in the periodic table, their sizes may be similar. In this case, use a periodic table that includes atomic radius. Since the radius is the atom’s size, that number determines which is larger. A bigger atom has a larger radius.