The mass of an element listed in the Periodic Table is the weighted average of all its naturally occurring isotopes.
Naturally occurring carbon is about
99 % carbon-12 (12.000 u) + 1 % carbon-13 (13.003 u).
That extra carbon-13 makes the <em>average atomic mass</em> greater than 12.000 u.
Oxygen and selenium both have 6 valence electrons. They're in the same group hope this helps
Tomic radius INCREASES as you go DOWN a Group because each successive Period (row) has an additional occupied energy level. you can think of it this way: each time you drop down a row, you add a "ring."
