Download Extension 18.2: Isotopes

Isotopes
Recall that we have defined the nucleus of an atom by specifying the number of protons,
Z (the atomic number) and the number of neutrons, N. These added together give the total
number of nucleons in the nucleus, A (the mass number). So, as we have said,
A = Z + N.
Why do we need to specify N or A? After all, the atomic number Z defines the sort of
nucleus it is—if Z = 1, it is hydrogen, if Z = 2, it is helium, and so on. The number of
neutrons is just a hanger-on, isn’t it?
The answer is—no!
The number of protons does determine which element it is, but the mass number
essentially determines its mass (the nucleus’s mass is approximately equal to the mass
number times the proton’s mass):
M nucleus ≈ A x mproton.
So different forms of an element may have different mass.
To distinguish the different nuclei having the same atomic number but different masses,
we say that they are different isotopes. The word isotope is stolen from Greek and
transmogrified into English. It roughly means “same number of protons.”
Take, for example, hydrogen. It may have a single proton as its nucleus, or a proton and a
neutron, or a proton and two neutrons. There are therefore three isotopes of hydrogen:
they are named hydrogen, deuterium, and tritium. We denote them:
Energy, Ch. 18, extension 2 Isotopes
hydrogen
deuterium
tritium
2
1
H,
1
2
H,
1
3
H.
1
1
Naturally-occurring hydrogen, 99.985% of it, is 1 H, and the rest, 0.015%, is deuterium,
2
H.
1
Tritium is not stable.
Normally, isotopes of elements do not have separate names the way hydrogen does. For
example, we might speak about different isotopes of carbon. We could speak of three of
the isotopes of carbon, the elements with Z = 6 and 6, 7, or 8 neutrons in the nucleus, as
carbon-12
( C), carbon-13 ( C), and carbon-14 ( C).
12
6
13
6
14
6
You may have heard people using this way of designating nuclei. Two isotopes of
( U) is the most common isotope, and
uranium-235 is the less common isotope ( U).
uranium are found, for example. Uranium-238
238
92
235
92
The isotopes have virtually no effect on the chemical properties of the atoms. (The
number of neutrons has a very slight effect on the configuration of the electrons. This
difference is of importance for laser enrichment of uranium, discussed in Extension
E19.1, Enrichment.)