Page 57 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

OVERVIEW 39

Table 3.2 The first 10 elements in the periodic table
z Element Notation n / m m s
1 Hydrogen Is 1 1 0 0 + | or -
2 Helium Is 2 1 0 0 +4 or
2
3 Lithium Is 2s 1 2 0 0 + \ or -5
2
4 Beryllium Is 2s 2 2 0 0 + or -|
2
2
5 Boron Is 2s 2p 1 2 1 -1, or 0, or 1 + 5 or 2 1
2
2
6 Carbon Is 2s 2p 2 2 1 — 1 , or 0, or 1 +lor 1
2
2
7 Nitrogen Is 2s 2p 3 2 1 -1, orO, or 1 + | or 2
2
2
8 Oxygen Is 2s 2p 4 2 ] - 1 , or 0, or 1 + or
~2
2
2
9 Fluorine Is 2s 2p 5 2 1 -1, or 0, or 1 + or 1
2
2
2
10 Neon Is 2s 2p 6 2 1 -1, or 0, or 1 + or I
and the superscript 1 stands for the number of electrons occupying this state. Note that
the electron can take on spin quantum number of either +| or — ^, as the two spin states
have the same energy in the absence of magnetic field and are equally accessible to the
electron. The second element helium has two electrons that occupy the two lowest energy
states. These states are (n, l, m, m s) equal to (1, 0, 0, +-1) and (1, 0, 0, — |), that is, the
2
ground state of the helium atom can be labeled as 1s . Note that for helium the lowest
shell (n = 1) is full. The chemical activity of an element is determined primarily by the
valence electrons that are electrons in the outermost unfilled shell. Because the valence
energy shell of helium is full, helium does not react with other elements and is an inert
element.
The third element lithium has three electrons, two of which will occupy the states
(n, l, m, m s) equal to (1, 0, 0, +5), (1, 0, 0, —|), and the third will occupy one of the
other eight states in the next shell with n = 2. From among these eight states, the third
electron in the lithium atom occupies one of the two states (2, 0, 0, +^) or (2, 0, 0, — |).
2
1
Therefore, the ground state configuration of lithium is written as 1s 2s .
In theory, we may continue to build up the periodic table following the method illus-
trated in the preceding discussion, as shown in Table 3.2. However, in practice, electrons
will start to interact with each other as the atomic number of the element increases. This
electron-electron interaction is not taken into account in the model that was presented
earlier for the atomic structure, and the buildup of the periodic table will, therefore, some-
what deviate from the predicted one. This deviation is shown in Table 3.3 and Figure 3.2,
which gives the energy scheme for the first six different shells and orbitals.
Proceeding in the same manner as described earlier, we can arrange the 105 or so
elements that are known to us presently to complete the periodic table, as shown in
Figure 3.3. In the periodic table, elements are arranged in horizontal rows in order of
atomic number. A new row is begun after each noble gas (e.g. He, Ne, Ar, Kr, Xe,
and Rn) is encountered. Notice that the elements in each vertical column have similar
properties. Also notice that the elements on the left-hand side of the diagonal dividing
band are metals, whereas those on the right-hand side are nonmetals. The elements within
the bands are semimetals, which are more commonly known as semiconductors.