Each element found in nature occurs as a mixture of isotopes

Isotopes of an element have the same number of protons $($same Atomic Number$),$ but differ in the number of neutrons $-$ so$,$ they have different masses.

Table $1$: Natural isotopic abundance on Earth for some elements.

$\backslash$table$[[$Isotope$,$Abundance on Earth $(\%),$Isotopic Mass $($amu$)],{[}^{12}C($carbon$-12),98\mathrm{.}89,12\mathrm{.}00],{[}^{13}C($carbon$-13),1\mathrm{.}11,13\mathrm{.}00],{[}^{16}O($oxygen$-16),99\mathrm{.}76,16\mathrm{.}00],{[}^{17}$ O $($oxygen$-17),0\mathrm{.}04,17\mathrm{.}00],{[}^{18}O($oxygen$-18),0\mathrm{.}20,18\mathrm{.}00]]$

How many isotopes of oxygen are listed in the table?

What do isotopes of oxygen have in common? How are they different?

If you selected one carbon atom at random, the mass of that atom is most likely to be amu.

What is the mass $($in amu$)$ of $100$ carbon$-12$ atoms?

If you select $100$ carbon atoms at random, the total mass will likely be:

a$.1200\mathrm{.}0a$

b$.$ slightly more than $1200a$

c$.$ slightly less than $1200a$

d$.1300\mathrm{.}0a$

The average mass of a marble

In a collection of $100$ marbles, $25$ of the marbles have a mass of $5\mathrm{.}00g$ and $75$ have a mass of $7\mathrm{.}00g.$

The average mass of a marble is $6\mathrm{.}50g.$

The average mass can be determined by dividing the total mass of the marbles by the total number of marbles:

$(255\mathrm{.}00)+(757\mathrm{.}00)100=6\mathrm{.}50g$

$=6\mathrm{.}50g$

The average mass of a marble can also be determined by first multiplying the fraction $($or decimal percent$)$ of marbles of a particular type by the mass of a marble of that type and then taking a sum over all types of marbles:

$=6\mathrm{.}50g$

$=6\mathrm{.}50g$

$(2)$

$(2)$