Video Transcript
The atomic mass unit, u, is defined
as one twelfth of the mass of an atom of carbon-12. What is the mass of a silicon-30
atom in atomic mass units?
Sometimes, you may see unified
dropped from unified atomic mass unit. That’s fine as long as you’re using
the symbol u. The symbol amu is sometimes used
interchangeably, but they’re actually defined slightly differently. But it’s perfectly okay because we
actually have the definition in the question, one twelfth of the mass of an atom of
carbon-12.
Carbon is an element we can find on
the periodic table. The atomic number for carbon is
six. The atomic number of an element is
the number of protons we find in atoms or ions of that element. So, in an atom of carbon-12, we
know that there are six protons in the nucleus. In the naming of isotopes like
carbon-12, we put the mass number at the end. Atomic number tells us the number
of protons in the nucleus. And mass number tells us the number
of protons plus the number of neutrons.
What this tells us is that in our
nucleus of carbon-12, we have six protons and enough neutrons that when you sum them
together, you get 12. 12 minus six is six. So, we have six neutrons. And since we’re dealing with atoms,
which must be neutral, we must have six electrons to balance out the positive charge
of the protons.
The next thing we have to do is
work out the mass of a silicon-30 atom in atomic mass units. Silicon is the element just below
carbon in the periodic table with an atomic number of 14. So, we know in our atom of
silicon-30, there must be 14 protons in the nucleus. But of course, silicon-30 is an
isotope of silicon where the nuclei contain a strict number of neutrons and the
number of protons plus the number of neutrons is 30.
So, our atom of silicon-30 also
contains 16 neutrons. Just like with the atom of carbon,
we’re dealing with a neutral combination of protons and electrons. So, we need 14 electrons to balance
out the charge of the 14 protons. The first thing we need to
understand is that an atom of carbon-12 has a mass of exactly 12 unified atomic mass
units. We know that because from the
definition, we know one unified atomic mass unit is the mass of a carbon-12 atom
divided by 12.
The next thing we need to know is
that the mass of a proton and the mass of a neutron are about the same. And electrons have much less mass
than protons or neutrons. So, for the purposes of this
question, we can ignore them. Protons and neutrons are known
collectively as nucleons. So, the question is, if 12 nucleons
together have a mass of 12 unified atomic mass units, what will be the mass of 30
nucleons? And the answer is 30 unified atomic
mass units.
Now, bear in mind this is only
approximate because when neutrons and protons bond together, there is a slight
change in their mass and things are a bit more complicated. You may have noticed that I steered
clear of the average atomic mass for silicon, which is 32.06. This represents the average mass
for a silicon atom on Earth based on the abundance of different silicon
isotopes.
Our estimate of 30 unified atomic
mass units for an atom of silicon 30 is the best we can do with the figures we have
available.
