Download All About Isotopes

All About Isotopes
The atomic number of any atom (element) is a whole number and represents the number of
protons in the atom, but that’s not true of atomic mass which is not a whole number. Since
atomic mass is the number of the protons plus neutrons in the nucleus does that mean the nucleus
of atoms have fractions of protons or neutrons? No. This is due to the varying amounts of
different versions, called isotopes, for each element.
Isotopes are versions of atoms with different numbers of neutrons. For example hydrogen (H) is
mostly found as 1 proton, no neutrons, and 1 electron. But there is a version with 1 proton, 1
neutron, and 1 electron and another version with 1 proton, 2 neutrons, and 1 electron. Notice that
in each version the number of protons is always 1 since the number of protons defines the atom: 1
proton always means hydrogen. Almost all hydrogen has 1 proton and no neutron, but about 1 in
3000 H atoms has 1 proton and 1 neutron and even fewer hydrogen atoms have 1 proton and 2
neutrons. So 99.9% of all hydrogen has no neutrons but there is a little with 1 and 2 neutrons
respectively. So when we average the amounts of each isotope then atomic mass ends up as a
non whole number! Isotopes are very important in scientific research for a number of reasons.
1. Below draw the three isotopes of hydrogen. Use different colored circles to represent protons
(label with a plus (+) sign) and neutrons (label with a zero (0) sign) in the center (the nucleus) and
a lower case e with a minus sign (-) for electrons circling the nucleus. Draw a dotted line in a
circle to represent the electron flying around the nucleus.
Hydrogen 1 (1 proton, no neutron, 1 electron)
Deuterium (1 proton, 1 neutron, 1 electron)
Tritium (1 proton, 2 neutrons, 1 electron)
2. Carbon has 3 isotopes: Carbon 12, Carbon 13, and Carbon 14. Carbon always has 6 protons
(remember the number of protons, the atomic number, defines the atom). Draw the three isotopes
of carbon below. Draw the electrons for each isotope on the back of this page with 2 electrons in
an inner dotted circle line and 4 electrons in an outer doted circle line and space the electrons
equally apart from each other (they repel each other because they have the same negative
charge!).
3. Protons have the same charge and yet they are packed tight next to each other in the nucleus
with neutrons. Why aren’t they flying apart since like charges repel? If you add up the mass of
the number of protons and neutrons in an atom and then actually find the mass of the nucleus, it
weighs less than it should! Where did that missing mass go?