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Periodic Table (Chemistry) 681

TABLE II Electronic Configurations of the Elements in Periods 2 and 3
Period 2 Period 3
Atomic Electronic Atomic Electronic
Name Symbol number configuration Name Symbol number configuration
2
6
2
2
Lithium Li 3 1s 2s Sodium Na 11 1s 2s 2p 3s
2
2
2
6
Beryllium Be 4 1s 2s 2 Magnesium Mg 12 1s 2s 2p 3s 2
2
2
6
2
2
2
Boron B 5 1s 2s 2p Aluminum Al 13 1s 2s 2p 3s 3p
2
2
2
6
2
2
Carbon C 6 1s 2s 2p 2 Silicon Si 14 1s 2s 2p 3s 3p 2
2
2
2
2
2
6
Nitrogen N 7 1s 2s 2p 3 Phosphorus P 15 1s 2s 2p 3s 3p 3
6
2
2
2
2
2
Oxygen O 8 1s 2s 2p 4 Sulfur S 16 1s 2s 2p 3s 3p 4
2
6
2
2
2
2
Fluorine F 9 1s 2s 2p 5 Chlorine Cl 17 1s 2s 2p 3s 3p 5
2
6
2
2
2
2
Neon Ne 10 1s 2s 2p 6 Argon Ar 18 1s 2s 2p 3s 3p 6
begins to fill with boron until all possible quantum num- IV. TABULAR TRENDS
ber combinations are exhausted for n = 2 and l = 1bythe
tenth atom, neon. Hence, the second row accounts for all A. Atomic Radius
possible quantum number combinations for n = 2. Simi-
Chemical and physical properties of the elements and their
larly, the third row proceeds to fill the shells for all possible
compounds constituted the evidence for the periodic sys-
quantum number combinations for n = 3,l = 0or1.The
tem in the last half of the 19th century. Modern under-
similarities of the electron arrangements for members of
standing of the arrangement of electrons within the atoms
the same family are apparent. Of particular importance is
of the various elements has provided an explanation of
the configuration of the valence electrons. The conven-
these properties and their periodic nature. One property
tional view is that atoms with the same number of outer
that is quite obviously associated with atomic structure is
electrons (isoelectronic atoms) exhibit similar chemical
atomic size. For metallic elements, atomic radii can be de-
properties. Later on, we will see that a different scenario
termined using diffraction. Calculations are made from the
is supported by many pieces of data.
angles at which the X-rays bounce off the atoms. Another
Starting with the first element of the fourth row, potas-
experimentally based technique involves computing the
sium,weseethatinorderforitsvalenceconfigurationtobe
distance between chemically bonded atoms from spectro-
similar to that of the other alkali metals, it must begin fill-
scopic measurements. Quantum mechanical calculations
ing the 4s shell, even though all possible n = 3 shells have
also yield atomic radii, including those plotted in Fig. 4,
not been filled [the 3d (n = 3,l = 2) shell is still empty].
which is a graph of atomic radius against atomic number.
This assignment is correct because it has been shown, ex-
A periodic pattern is visible: the maxima are occupied by
perimentally and computationally, that the 4s energy level
the alkali metals and the minima by the rare gases. Within
for potassium is lower than the 3d. The 3d does not begin
any column or family, the radius increases with increasing
to fill until scandium, the first of the transition elements.
atomic number. This is to be expected, since the atomic
This brings up the point that the assignment of quan-
number equals the number of electrons in the atom. Each
tum numbers indicates that orbital energy levels do not
new shell increases the size of the atom. However, within
always follow the sequence of increasing n and l values.
any row or period, the atomic radius decreases with in-
The valence electron configuration of the alkali metals and
2
the alkaline earths are always ns and ns . Therefore, the creasing atomic number. Here, the added electrons make
the atom smaller. This latter trend is attributable to the fact
elements of columns 1 and 2 are known as members of
that the positive nuclear charge (the number of protons)
the s block. Groups 3 through 12 constitute the d block,
also increases along a row. The added outer electrons do
because in these transition elements electrons are intro-
not effectively screen the attractive force due to the pro-
duced into the d orbitals. The valence electronic config-
tons, and hence, these forces increase with atomic number.
urations of this group follow a regular trend belonging to
0
v
1
any one of three types: (n − 1)d ns , (n − 1)d v−1 ns ,or This means the electrons are pulled closer to the nucleus
2
(n − 1)d v−2 ns , where v is the number of valence elec- and the atomic radius decreases.
trons. Interelectronic repulsion is a crucial determinant of
B. Metallic Character and Ionization Energy
the configuration type. Finally, the p block contains the
elements of Groups 13 through 18, and the lanthanides Onereadilyobservableperiodicpropertyisthevariationin
and actinides comprise the f block. the metallic character of the elements. The members of the