PLZ HELP ME ASAP NEED THIS FOR CLASS IN 5 MIN

the standard on which all relative masses are based is the 12C isotope, which is given a mass of 12 exactly. Explain why then, in the table of relevant atomic masses in appendix 4, the relative atomic mass of carbon is listed as 12.011

carbon atoms are about 99% C-12 and 1% C-13, so the weighted average will be a little over 12.00.
(0.99x12 + 0.01x13 = 12.01)

Carbon 12 is just one isotope of Carbon. There is also Carbon =14. The rel at mass is an average of all isotopes as they occur in nature