Download Atomic number, atomic mass and isotopes

13.2 THE NUCLEUS 1 - ATOMIC NUMBER, ATOMIC MASS AND ISOTOPES
Atomic number. The atomic number (Z) of an element is the number of protons in the nucleus of
one atom of that element. This is also the number of electrons orbiting the nucleus. Every
element has its own atomic number. Hydrogen is the lightest element and has an atomic number
of one. Hydrogen atoms have one proton in the nucleus and one electron in orbit. The second
lightest element is the inert gas helium which has an atomic number of two (two protons and two
electrons). For the element carbon, which is the basis of all living things, Z = 6 (six protons and
six electrons) and for the heaviest naturally occurring element uranium, Z = 92.
neutron
proton
electron
Hydrogen (Z=1, A=1)
Helium (Z=2, A=4)
Lithium (Z=3, A=7)
Carbon (Z=6, A=12)
Simple models of four atoms (with their atomic numbers)
Atomic mass. The atomic mass (A) of an element used to be defined as the mass of one atom
of that element compared to the mass of one atom of hydrogen. On that standard, the atomic
mass of hydrogen was exactly one, and the atoms of all other elements were compared to that.
Now the standard has been adjusted
slightly so that an atom of carbon
Other terms and standards. Atomic mass, relative
with 6 protons, 6 neutrons and 6
atomic mass and atomic weight are alternative terms that
electrons, has an atomic mass of
have been used in various circumstances and times. On
exactly 12. For most purposes this
the hydrogen standard, the atomic mass of hydrogen was
makes very little difference (see the
exactly 1. On the present carbon standard, the atomic
text box on the right) so we will use
mass of hydrogen is 1.008 so the difference is very small.
the old standard in this book. Look at
the simple models of the atoms shown above. Remember that protons and neutrons are equally
heavy, and that the mass of the electrons is so tiny that we can ignore it. You can see that
hydrogen has an atomic mass (A) of 1 (the nucleus contains one proton only), helium has an
atomic mass of 4 (2 protons and 2 neutrons), lithium has an atomic mass of 7 (3 protons and 4
neutrons), and carbon has an atomic mass of 12 (6 protons and 6 neutrons). Now study
Appendix B and check out the atomic numbers and atomic masses of your favourite elements.
Isotopes. Isotopes are atoms of the same element that have different atomic masses; they have
the same number of protons, but different numbers of neutrons. Look at the element chlorine in
Appendix B. Its atomic number Z is 17 so all atoms of chlorine have 17 protons. About 75% of
chlorine atoms have 18 neutrons (so the atomic mass A of these atoms is 35), while the rest have
20 neutrons (so A = 37). We say that chlorine has two isotopes, one with an atomic mass of 35,
and the other with an atomic mass of 37. The atomic mass given in Appendix B is 35.5 which is
the average mass of chlorine atoms as compared to a standard hydrogen atom.
99.99%
0.01%
a trace
Z=1, A=1
Z=1, A=2
Z=1, A=3
The three isotopes of hydrogen

1. Define (i) atomic number, (ii) atomic mass, (iii)
isotopes.

2. What element has fewest neutrons in its atoms?

3. What elements have Z numbers: 1, 6, 8, 14, 79?
Many elements have two or more isotopes,
but one is usually much more common than
the rest. The diagram on the left shows the
three isotopes of hydrogen. 99.99% of all
hydrogen atoms are the first isotope with one
proton only in the nucleus (A = 1); 0.01% are
the second isotope which has a neutron as
well as a proton (A = 2); only a very few are
the third isotope with two neutrons (A = 3).
13 - 2

4. What do you get if you subtract Z from A?

5. The element zinc has an atomic number of 30 and
an atomic mass of 65.4. Explain all that you can work
out about zinc atoms from this information.