Geology Reference
In-Depth Information
6
WHAT WE CAN LEARN FROM
THE PERIODIC TABLE
Ionization energy
A chemical element is identified by its atomic number
Z
, which defines both the number of protons in the
nucleus (and hence the nuclear charge) and the number
of electrons in the neutral atom (Box 6.1.). In the present
chapter we consider how the atomic number, in con-
junction with the electron energy structure developed
in Chapter 5 (Figure 5.7), determines the chemical
properties of the element concerned. The structure of
the energy-level diagram leads to a
periodic repetition
of
chemical properties, which is conveniently summa-
rized by tabulating the elements on a grid known as the
Periodic Table (see inside rear cover).
Although the architecture of the Periodic Table can
be thought of as an outcome of wave-mechanical the-
ory, it was originally worked out from chemical obser-
vation. It was first published in its modern form by the
Russian chemist Dimitri Mendeleev in 1869, almost 60
years before Schrödinger published his paper on wave
mechanics.
The bonds formed by an atom involve the transfer or
sharing of electrons. It therefore makes sense to illus-
trate the periodicity of chemical properties by looking at
a parameter that expresses how easy or difficult it is to
remove an electron from an atom. The
ionization energy
of an element is the energy input (expressed in J mo1
−1
)
required to detach the loosest electron from atoms of
that element (in its ground state). It is the energy differ-
ence between the 'free electron at rest' state (the zero on
the scale of electron energy levels) and the highest occu-
pied energy level in the atom concerned. What this
means in the simplest case, the hydrogen atom, is shown
in Figure 5.6. A low ionization energy denotes an easily
removed electron, a high value a strongly held one.
We can picture how ionization energy will vary
with atomic number by considering the highest occ-
upied energy level in each type of atom (Figure 5.7). In
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