The properties of elements - Geochemistry: An Introduction - page 6

n

6

Cs Ba

In Sn Sb Te I Xe

Y r b Mo Tc Ru Rh Pd Ag Cd

5

Rb Sr

Ga Ge AsSeBr Kr

Sc Ti V Cr Mn Fe Co Ni Cu Zn

4

K Ca

Al Si PS Cl Ar

d

3

Na Mg

BCNOF e

Li Be

2

p

HHe

1

s

l = 0 1 2

Figure 1.4 The filling order of the lowest orbitals of the elements in the periodic table. The vertical scale

shows the energy levels.

Outer
electron

-

+

Nucleus

Inner electron
cloud

Figure 1.5 Shielding of the nuclear charge by the cloud of electrons orbiting between the outer electrons

and the nucleus.

orbitals of a neon atom; and their divalent ions, such as Fe 2 + and Cu 2 + , have a configura-
tion [Ne]3s 2 3p 6 3d i . These transition elements differ only by the number i of electrons in
orbital 3d but have an identical outer electron shell 4s, which explains why their chemical
properties are so similar. This phenomenon is further amplified in the rare-earths (or lan-
thanides), such as La and Ce (shell 4f ), and the actinides (5f ), such as U and Th, where
the s and p orbitals of the external shells are identical.
Simple rules hold for the prediction of atomic radii. First, the potential energy and atomic
radius increase with n and therefore down each column of the periodic table. Second,
the atomic radius decreases across each row. This is due to the reduction of electrostatic
attraction of the outer electrons by the cloud of the inner electrons ( Fig. 1.5 ) , a phenomenon
known as shielding. For the lanthanides and the actinides, the f electrons on their multi-
lobate orbitals leave some parts of the nucleus exposed ( Fig. 1.3 ) and therefore do not
screen the increasing charge of the nucleus as efficiently as the more smoothly shaped
lower-order orbitals. As a result, their atomic radii decrease smoothly with their increasing
atomic number, a phenomenon known as lanthanide (and actinide) contraction.