4.3. Isotopes and Atomic Mass http://www.ck12.org
Atomic Mass and the Mass Spectrometer
Beginning in the early part of the twentieth century, scientists were approaching a new understanding of the com-
position of the atom. Several major discoveries demonstrated that the atom contained a nucleus, where protons
and neutrons are situated. It was discovered that electrons surrounded the nucleus and it was later determined that
electrons primarily determine the chemical properties of elements. Several devices were created during this time
which demystified the inner workings of the atom and the composition of elements. One such device was themass
spectrometer, which was developed in 1918 by Arthur Jeffrey Dempster. The mass spectrometer is an instrument
for determining the masses of atoms, molecules, and molecular fragments. TheFigure4.13 illustrates a modern
mass spectrometer.
FIGURE 4.13
An electron is removed from an atom to
yield a positive ion (such as H+, O+, or
N+). The ions are then accelerated and
deflected by a magnetic field. The degree
of deflection directly relates to the mass
of the ion: the lighter the ion, the greater
the deflection and the heavier the ion, the
lesser the deflection. The beam of ions is
then detected and the relative abundance
of each isotope of an element can then be
determined.If we were to place a sample of carbon into a mass spectrometer and analyze its mass, we would find that some of the
carbon atoms have a relative mass of 12, while other atoms have a relative mass of 13, and still others have a relative
mass of 14. The mass spectrometer measures thepercent abundanceof these carbon isotopes. Percent abundance
is the percentage of atoms in a naturally occurring sample of the pure element that are a particular isotope. We can
represent the percent abundance of carbon with what is known as a mass spectrogram, shown in theFigure4.14.
The spectrogram reveals the percent abundances of the variants of carbon atoms consists of 98.9%^12 C, 1.1%^13 C,
and «0.1%^14 C. Because we generally deal with very large amounts of atoms, it is more practically useful to know
the average mass of each atom in a large sample as determined by the percent abundance of each isotope. The
atomic massof an element is the weighted average of the atomic masses of the naturally occurring isotopes of that