Atomic Mass Of Oxygen 16

Why is the Relative Diminutive Mass of Oxygen Less than 16?

Oxygen has three naturally occurring isotopes (see table below). The relative atomic mass of oxygen is xv.9994. Why is the relative atomic mass of oxygen less than 16?

Isotope	% Natural Abundance
oxygen-sixteen	99.762%
oxygen-17	0.038%
oxygen-18	0.200%

The atomic mass of an element is defined every bit the average of the isotopic masses, weighted according to the naturally occurring abundances of the isotopes of the chemical element. Although oxygen has nine isotopes with mass numbers ranging from 13 to 21, oxygen-13, oxygen-14 and oxygen-15 are not naturally occurring isotopes. So, we do not demand to include them when we calculate the relative atomic mass of oxygen. Why is the relative atomic mass of oxygen less than 16?

Actually, we cannot determine the mass of an atom simply past adding up the masses of its protons and neutrons. Thus, the post-obit adding is wrong:

Relative atomic mass = 16 x 99.762% + 17 10 0.038% + 18 x 0.200% = 16.008

The actual masses of individual oxygen atoms are not whole numbers even though their mass numbers are whole numbers. When protons and neutrons combine to form the oxygen nucleus, a very small amount of the total mass is converted to nuclear binding energy. Withal, we cannot predict the corporeality of nuclear bounden energy just by counting the numbers of protons and neutrons. It must be experimentally determined using a mass spectrometer. For instance, since the mass of 1 atom of oxygen-16 is i.33291 times the mass of one carbon-12 atom, the mass of one oxygen-16 atom should exist equal to one.33291 x 12 = 15.99492 atomic mass units.

It is important to notation that only the mass of 1 carbon-12 atom is a whole number because, by international agreement, i cantlet of the carbon-12 isotope has been assigned a mass of 12 atomic mass units.

The concept of isotopic mass is non clearly described in many textbooks. Writers commonly emphasize that the relative mass of an electron is close to zip and so the relative mass of an isotope is approximately equal to its mass number. This kind of caption may result in pupil misconceptions of diminutive mass.

David Dice has developed an interesting experiment with atomic mass (http://www.carlton.paschools.pa.sk.ca/chemical/Molemass/moles3a.htm). Y'all may suit information technology to meet the needs of your chemistry students.