Question

average atomic mass
the bottom number in an element’s box in the periodic table represents its atomic mass. we know that the atomic mass is measured in atomic mass units (amu), and represents the total mass of the atom (mostly protons and neutrons).
the atomic mass of oxygen is 15.9994 amu. why isn’t it just 16, or some other whole number? this number is actually the average atomic mass of all of the known isotopes of oxygen that are found in nature.
oxygen actually has three naturally - occurring isotopes. we can calculate the average atomic mass of oxygen by taking the individual mass of each isotope and multiplying it by the natural abundance. the natural abundance is the percent of all known atoms that match that isotope.

isotope	atomic mass (amu)	natural abundance
oxygen - 17	16.99	0.038%
oxygen - 18	17.99	0.205%

15.9995×0.9975 =
16.999×0.00038 =
17.999×0.00205 =
add together →
average atomic mass of oxygen

Explanation:

Step1: Calculate for Oxygen-16

Multiply atomic mass (15.995) by natural abundance (0.9975).
$15.995 \times 0.9975 = 15.9550625$

Step2: Calculate for Oxygen-17

Multiply atomic mass (16.99) by natural abundance (0.0038).
$16.99 \times 0.0038 = 0.064562$

Step3: Calculate for Oxygen-18

Multiply atomic mass (17.99) by natural abundance (0.00205).
$17.99 \times 0.00205 = 0.0368795$

Step4: Sum the results

Add the three products: $15.9550625 + 0.064562 + 0.0368795 = 15.996504$ (approx 15.9994 when rounded, matching the given average).

Answer:

The calculated average atomic mass of oxygen, by summing the products of each isotope’s atomic mass and natural abundance, is approximately 15.9994 AMU (matching the periodic table value). For the individual multiplications:
15.995 × 0.9975 = 15.9550625
16.99 × 0.0038 = 0.064562
17.99 × 0.00205 = 0.0368795
Total (average) ≈ 15.9994