Geology Reference
In-Depth Information
Box 5.4 The absolute essentials
Of all the chapters so far in this topic, understanding
Chapter 5 requires the most physical insight. The reader
is strongly encouraged to persevere until the end of the
chapter because - armed with the wave-mechanical view
of the atom - you will be best equipped to cope with suc-
ceeding chapters of the topic. Nevertheless, for the
reader to whom the concept of electron 'waveforms' is
unacceptably remote from everyday experience, the main
conclusions of Chapter 5 are presented below in non-wave
terms.
The essential properties of any atom are determined by the number of protons (positively
charged nuclear particles) in the nucleus. This is known as the atomic number
Z
of the atom.
The value of
Z
identifies the chemical element to which the atom belongs. The other particles
shown in the figure are neutrons (uncharged nuclear particles), the number of which deter-
mines the isotope of the element.
Z
= 3, lithium
3-
+
+
The positive charge
(Z+)
is neutralized by a cloud of
Z
electrons (negatively charged particles)
surrounding the nucleus, held in position by its electrostatic attraction.
+
The atom forms chemical bonds with other atoms by 'trading' electrons with them. The
Z
electrons in an atom may be divided into:
(a) valence electrons (light shading), constituing the 'liquid assets' with which the atom
can trade to form chemical bonds with other atoms, and
(b) core electrons (darker shading), the 'capital reserve' of electrons too tightly bound to
the nucleus to be involved in bonding.
The number of valence electrons, among other factors, determines the number of bonds that
an atom can form (its 'valency').
+
+
+
Each electron in an atom resides in a specific region of space close to the nucleus called an
orbital, each orbital accommodating up to two electrons. The various orbitals differ in their
symmetry around the nucleus, and this determines the geometry of directional bonds such
as those responsible for the structure of diamond.
The size and shape of each orbital is indicated by the values of quantum numbers
n
and
l
(Table 5.3).
The distribution of electrons between orbitals, and the division into 'core' and 'valence' cat-
egories, is determined by the
energy-level diagram
shown in Figure 5.7. Each box represents
an orbital accepting up to two electrons. To predict the chemical properties of an atom,
Z
electrons are 'fed in' from the bottom (lowest energy, most stable) upward. Valence elec-
trons are those residing in the highest occupied energy levels (least strongly bound electrons,
most easily removed).
3p
3s
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