Chemistry Reference
In-Depth Information
the atomic number you could simply look it up on the periodic table. The
number in the lower-right corner of the hydrogen example indicates the
number of atoms in the sample. The plus in the upper-right corner of the
hydrogen example represents the net charge on each of the atoms, so we
are dealing with positive ions in this example.
We can use the information in the elemental notation in order to de-
termine the number of each of the subatomic particles in the atom. For
example, look at the following elemental notation:
We know that the number in the lower left-hand corner is the atomic
number, which gives us the number of protons, so this atom of iron (Fe)
has 26 protons. We know that the number in the upper left-hand corner is
the mass number, which is the total number of protons and neutrons. If we
subtract the atomic number from the mass number, we find the number of
neutrons to be 30 (56 - 26 = 30). This particular atom of iron has a +2
charge, which means that it has 2 more protons than electrons. By sub-
tracting the charge number from the number of protons, we find that there
are 24 electrons in the atom (26 - (+2) = 24).
Let's try a few example problems.
56
26
2+
Fe
Example 1
Use the following elemental notations to fill in the missing
information in the table below.
20
10
0
40
18
+
40
2+
32
16
2-
Ne
Ag
Ca
S
Number of protons
Number of electrons
Number of neutrons
The first thing asked for is the number of protons in each element,
which is very easy to determine. The atomic number, which is identical to
the number of protons, is located in the lower left-hand corner of each of
the elemental notations. But wait! Where is the atomic number for cal-
cium? It appears to be missing, but does that mean that we can't solve it?
Not at all. As I mentioned earlier, the atomic number is redundant; if you
know the identity of the element, you can look up the atomic number on
the periodic table. Find calcium on the periodic table, and you will see that
it has an atomic number of 20. Now you can fill in the number of protons
for each of the elements.
