This video
will cover the strategies needed to
solve the following types of isotope
problems.
One, determine the symbol of a specific
isotope if given its name.
Two, determine the name of a specific
isotope if given its symbol.
Three, evaluate the numbers of protons,
neutrons, and electrons in a neutral
isotope.
And four, evaluate the numbers of protons,
neutrons, and electrons in an ion.
To go from name to symbol notation, let's
start by interpreting the two pieces of
information
provided in the name.
The number 57 refers to the mass
(A) of this specific isotope.
The element iron has the symbol
Fe. Recall
that the symbol notation for an isotope
is set up like this,
A-Z-X, where the mass number (A)
is at the top and the atomic number (Z) is
at the bottom.
The atomic number is never explicitly
part of the name,
but we can always find it on the
periodic table.
Viewing the element block for iron, we
see that the atomic number
Z equals 26.
To write the isotope notation, we first
write the symbol
Fe. Then we write the mass number from
the name,
57, at the top left and the atomic number
we looked up,
26, at the bottom.
Continuing with this problem we can now
evaluate the numbers of protons, neutrons,
and electrons
in the iron-57 isotope.
The number of protons equals the atomic
number Z,
so protons equals 26.
Because protons plus neutrons equals
mass number
(or red plus blue equals purple) we can
rearrange this to solve for neutrons
as A minus Z equals n.
Here's a useful trick: always set up this
neutron math
directly on the isotope symbol. 57
minus 26 equals 31,
so neutrons equals 31.
If the number of neutrons turns out to
be negative...which
isn't possible according to the laws of
physics, then it probably means you have
the A and Z reversed.
Because this is a neutral atom, the
number of electrons
must balance the number of protons. So
the number of electrons
is also equal Z or 26.
Let's analyze the information we're
given in the symbol,
to determine which parts make it into
the name.
The top number, 84, is the mass
(A) and is the number that is used in the
name.
The bottom number, 36, is the atomic
number (Z)
and is not used in the name. The symbol
Kr is for the element krypton.
When we write the name it takes the
format
element name hyphen mass,
so the name of this isotope is krypton-
84.
To figure out the protons, neutrons, and
electrons,
we'll first look up the atomic number
for Be,
which is beryllium, and we find that Z
equals four.
We write this value of Z at the bottom
of the notation.
The number of protons equals the atomic
number Z,
which is 4.
Remember that the top number, 10, is the
mass number,
and for neutrons we'll do the
subtraction of
A minus Z within the isotope symbol.
Ten minus four equals six, so the number
of neutrons
equals six. And finally,
the number of electrons for a neutral
atom is equal to the number of protons,
so electrons equals Z equals four.
Here, our isotope is an ion, which we can
recognize from the charge
shown as a superscript on the right.
Determining the protons and neutrons
follows the same process for an ion
as it does for a neutral atom. It's only
the number of electrons that will change
and we'll talk about those details in a
little bit.
The mass number (A) equals 80.
The atomic number is 34,
so the protons equals 34.
And the neutrons are calculated as
80 minus 34
equals 46.
For the electrons, be sure to follow the
steps in
order. First, start with the number of
electrons in the neutral isotope,
which is Z equals 34.
Then let's look at the two parts of the
charge of the ion.
The value, which is two in this case,
is how many electrons are involved in
forming the ion.
The sign tells us what to do.
Because this is a negative ion, we will
add
2 electrons to Z, to get the total.
Electrons equals 34
plus two equals thirty six.
The mass number equals 65.
The atomic number Z from the periodic
table
is 29.
So the number of protons equals 29.
Calculate the neutrons as 65
minus 29 equals 36.
For the electrons start with
Z equals 29 the electrons in the neutral
atom.
Then look at the value and the sign of
the charge.
The value is one, and the sign
is positive, which means one electron
will be removed.
So 29 minus 1 equals 28.
 
