Video Transcript
Where is most of the mass of an
atom located? (A) In the nucleus. (B) In the electron shells. (C) In the space between the
nucleus and the electron shells.
This question is testing our
understanding of the structure of an atom and how mass is distributed within the
atom. The structure of an atom contains
two main regions: the nucleus and the electron cloud. The nucleus is the dense core in
the center of the atom. Inside the nucleus, which is not
shown to scale here, the subatomic particles, called protons and neutrons, are found
closely packed together. The electrons are found outside of
the nucleus but still within the atom in the electron cloud. Within the electron cloud,
electrons are organized into electron shells based on their energy.
In this video, we won’t be talking
about how the electrons are organized into the electron shells. Instead, let’s discuss the
approximate masses of each of the subatomic particles. A proton has essentially the same
mass as a neutron, roughly one unified atomic mass unit, or one u each. In contrast, an electron is
substantially less massive than a proton or neutron. Its mass is about 0.00055u. In fact, about 1840 electrons would
have the same mass as one proton or neutron. This means that most of an atom’s
mass is located in the nucleus due to the presence of the heavier protons and
neutrons found there.
To get some perspective, let’s take
a quick look at our diagram of an atom. The nucleus contains five protons
and six neutrons. Each proton and neutron has a mass
of one u. So, five protons contribute five u
and six neutrons contribute six u to the mass of the nucleus. After adding, the total mass of the
nucleus is 11u. The electron shells outside of the
nucleus contain five electrons. Altogether, these five electrons
have a total mass of about 0.00275u.
It’s clear to see from this example
that most of the mass of an atom is located in the nucleus. Therefore, the correct answer is
answer choice (A).
