Which of the following has the
smallest mass? (A) A proton, (B) an electron, (C)
a neutron, (D) an atom, or (E) a nucleus.
The choices in this question are
the constituent parts of an atom. Protons are the positively charged
particles found in the nucleus of the atom. Electrons are the negatively
charged particles found in the electron cloud that surrounds the nucleus. And neutrons are the neutrally
charged particles also found in the nucleus of the atom. The nucleus is the region at the
center of the atom that contains the protons and neutrons, whereas the atom is the
entire structure of protons, neutrons, and electrons arranged in a nucleus and
electron cloud. These different particles, or
groups of particles, have different masses, so we want to find the smallest one.
Since atoms are made up of protons,
electrons, and neutrons and nuclei are made up of protons and neutrons, we can
eliminate them from consideration. Even the lightest atom of hydrogen
has a proton and an electron. So there’s no way an atom could be
lighter than a proton, electron, or neutron. In addition, since nuclei are made
up of protons and neutrons, there’s no way that a nucleus could be lighter than a
proton or a neutron, just as there is no way for a car to be lighter than one of its
tires. The constituent parts must be
lighter than the whole.
So, among these three remaining
subatomic particles, proton, electron, or neutron, which one has the smallest
mass? If we reference the known masses of
these three particles, we will find that protons and neutrons have the same mass,
one unified atomic mass unit. Electrons, on the other hand, have
a much smaller mass of one over 1840 unified atomic mass units. Since these particles are so small,
it’s impractical to use the more common mass unit of gram when working with their
masses. One unified atomic mass unit is
very roughly one trillionth of one trillionth of a gram, quite conveniently,
protons, each way one unified atomic mass unit, and neutrons, each way one unified
atomic mass unit.
This allows us to calculate the
mass number of an atom quite easily by simply adding together the number of neutrons
and protons. The mass number is how we
differentiate different isotopes of elements. For example, helium-4 is the
isotope of helium with a mass number of four. Since the mass of an electron is so
small in comparison to the mass of a proton and neutron, we ignore this number when
making this calculation. The majority of the mass of the
atom is found in the nucleus, which is made up of protons and neutrons. The part of the atom with the
smallest mass is the electron.