Page 73 - Science at the nanoscale

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June 12, 2009
4.1. Atoms and the Periodic Table of Elements
For example, the set (n = 2, l = 1 and m = 0, ±1) corresponds
l
to the 2p x , 2p y and 2p z orbitals (Fig. 3.12). According to Pauli
2
exclusion principle, there can only be six 2p electrons pair-wise
(m s = ±1) in each of these orbitals. Correspondingly, we see that
completely filled d orbitals (l = 2 and m = 0, ±1, ±2) have 10
l
10
electrons (d ) and that of f orbitals (l = 3 and m = 0, ±1, ±2, ±3)
l
14
have 14 electrons (f ). For any n value, there will be an element
which has all its corresponding AOs occupied fully with electrons.
We called such electron configuration a closed shell structure. The
elements situated at the right-most column in the Periodic Table
constitute the group of noble gases, i.e. He, Ne, Ar, Kr, Xe and
2
6
Rn. Due to the outermost ns np closed shell structure, they exist
mainly as monoatomic inert gases in the atmosphere.
In contrast, if we now move to the second column from the
right in the Periodic Table, we find the group of halogens: F, Cl,
2
5
Br, I and At, with electron configuration ns np . This group of
elements is known to have a strong tendency to accept one elec-
tron to form anions such as F , Cl , etc. Conversely, elements in
−
−
the left-most column of the Periodic Table have the tendency to
donate one electron to form alkali metal cations such as Li , Na ,
+
+
etc. The element gold (Au, Z = 79) has the electron configuration:
10
14
1
[Xe]4f 5d 6s . Silver (Ag, Z = 47), which is in the same group
10
1
as gold, has a similar electronic structure: [Kr]4d 5s . Both these
elements form metallic solids that are available in many common
forms such as wires, foils, and bars. These materials are good
conductors of heat and electricity, and are known for their gen-
eral inertness to chemicals (although Ag readily reacts with sul-
fur). We shall see later that these properties are modified when
the materials exist in nanometre sizes.
The Periodic Table contains all known 117 elements that have 63 ch04
been discovered to date. We have thus seen how different per-
mutations of fundamental entities such as protons and electrons
can form vastly different elements that make up all matter sur-
rounding us — e.g. complex life-forming structures such as DNA
and RNA are simple combinations of C, N, O and H; modern
day IT devices are built mainly using Si. While the atom is the
2 Pauli exclusion principle states that no two fermions (particles with 1 2 integral
spin, e.g. electrons) can occupy the same state. Hence in a single atom, if two
electrons have the same (n, l, and m l ) value, then m s must be different, i.e. the
electrons in the same orbital must have opposite spins.