Video Transcript
The unified atomic mass unit u is
defined as one twelfth of the mass of an atom of carbon-12. What is the mass of a fluorine-19
atom in unified atomic mass units?
There are two approaches in
determining the mass of a fluorine-19 atom. The first approach focuses on
utilizing all the information provided in the question. Let’s begin with the information
provided about the element fluorine-19. Fluorine is an element found on the
periodic table that has an atomic number of nine, where the atomic number is equal
to the number of protons found in the nucleus, and an average atomic mass of
18.998u, where the symbol u stands for unified atomic mass units.
Before we continue discussing what
we know about the fluorine-19 atom, one might wonder why we simply cannot use the
average atomic mass of 18.998u listed under the chemical symbol for fluorine as the
answer to this question as the mass of a fluorine-19 atom. The reason is because, by
definition, the average atomic mass calculates the average mass of an element based
on the abundance of all different isotopes of that element. This question does not ask for the
average atomic mass of fluorine atoms, but rather for the mass of a single
fluorine-19 atom. Therefore, we cannot use the
average atomic mass of fluorine as the answer for this question. And we will continue mapping out
the information we have been provided.
The number 19 in fluorine-19 is the
mass number, which is the sum of the protons and neutrons in the nucleus of an atom
of an element. Since the question is focused on an
atom of fluorine-19, it would be beneficial to draw a fluorine-19 atom and label its
components. The nucleus of a fluorine-19 atom
contains nine protons because it has an atomic number of nine. As far as determining the number of
neutrons for fluorine-19, we can use the mass number equation by substituting the
number 19 for the mass number and the number nine for the number of protons. Upon subtracting nine protons from
the mass number 19, we’ve determined there are 10 neutrons in the nucleus of a
fluorine-19 atom.
Since the question states we are
dealing with an atom of fluorine-19, we must have nine negatively charged electrons
in the electron cloud, which is represented by the blue circle in this drawing, to
match the nine positively charged protons in the nucleus in order to generate a
neutral entity.
Another piece of information that
we’ve been given in the question to assist in determining the mass of a fluorine-19
atom is the definition of the unified atomic mass unit. One unified atomic mass unit is
defined as one twelfth of the mass of one atom of carbon-12, which means there are
six protons in the nucleus. And the number 12 in the isotope
notation carbon-12 is the mass number of this isotope of carbon. Since the definition indicates
we’re dealing with an atom of carbon-12, there must be six negatively charged
electrons in its electron cloud to match the six positively charged protons in the
nucleus.
As we did in determining the number
of neutrons for the fluorine-19 atom, for the carbon-12 atom, we can subtract six
protons from its mass number of 12 to give this carbon isotope six neutrons in the
nucleus. Another way to write the definition
of unified atomic mass units is to show that one u is equal to the mass of a
carbon-12 atom divided by 12. Based on this definition, an atom
of carbon-12 has a mass of exactly 12 unified atomic mass units, or 12 u.
We can confirm this by taking into
account that the mass of a proton and the mass of a neutron are about the same. And electrons have a negligible
mass in comparison to protons and neutrons, which means if the 12 nucleons or the
sum of the six protons and six neutrons in an atom of carbon-12 are equal to 12
unified atomic mass units or 12 u, then the 19 nucleons or the sum of the nine
protons and 10 neutrons in fluorine-19 equates to a mass of 19 unified atomic mass
units or 19 u.
The second approach to solving this
question focuses solely on one proton and one neutron each being approximately equal
to one unified atomic mass unit, or one u. Again, not including the mass of
the electrons because of their negligible mass in comparison to the proton and
neutron, the nine protons approximately equal to a mass of nine u added to 10 u for
the approximate mass of 10 neutrons in the fluorine-19 atom equates to a mass of 19
unified atomic mass units, or 19 u, which is the same answer we obtained by using
the relationship of unified atomic mass units to the mass of a carbon-12 atom.
Therefore, the mass of a
fluorine-19 atom in unified atomic mass units is 19 u.
