Elements having electronic configurations different from that of the noble
gases either lose or gain electrons to achieve the structure ns^2 np^6 of the
nearest noble gas atom. The electrons in these shells are called the valence
electrons and are primarily responsible for the chemical bond formation.
Electrons in different shells are held by binding energyin different shells
of the atom. The binding energy of an electron is defined as the energy that
is required to be supplied to remove it completely from a shell. The binding
energy of the electron is the greatest in the Kshell and decreases with
higher shells such as L,M, and so on. The binding energy also increases with
increasing atomic number of the elements. Thus, the K-shell binding energy
(21.05 keV) of technetium, with atomic number 43, is higher than the K-
shell binding energy (1.08 keV) of sodium, with atomic number 11. The K-
shell binding energy of electrons in several elements are: carbon, 0.28 keV,
gallium, 10.37 keV, technetium, 21.05 keV; indium, 27.93 keV; iodine,
33.16 keV; lead, 88.00 keV.
When an electron is removed completely from an atom, the process is
called ionization. The atom is said to be ionized and becomes an ion. On
The Atom 5Table1.3. Electron configurations in different energy shells.
Principal quantum number Orbital No. of electrons =2(2l+1)
Principal shell (n)(l) in each orbital 2 n^2
K 1 s(0) 2 2
L 2 s(0) 2
p(1) 6 8
M 3 s(0) 2
p(1) 6
d(2) 10 18
N 4 s(0) 2
p(1) 6
d(2) 10
f(3) 14 32
O 5 s(0) 2
p(1) 6
d(2) 10
f(3) 14
g(4) 18 50
Fig. 1.1. The electronic configuration of 28 Ni. The
Kshell has 2 electrons, the Lshell has 8 electrons,
and the Mshell has 18 electrons.