The Foundations of Chemistry

a noble gas, has a filled first main shell (two electrons). The atom is so stable that no
chemical reactions of helium are known.

Orbital Notation

1 s Simplified Notation

1 H __

h 1 s 1

2 He __

hg 1 s 2

Row 2.Elements of atomic numbers 3 through 10 occupy the second period, or hori-
zontal row, in the periodic table. In neon atoms the second main shell is filled completely.
Neon, a noble gas, is extremely stable. No reactions of it are known.

Orbital Notation

1 s 2 s 2 p Simplified Notation

3 Li __

hg h 1 s (^22) s (^1) or [He] 2s 1
4 Be
hg hg 1 s (^22) s (^2) [He] 2s 2
5 B
hg hg h 1 s (^22) s (^22) p (^1) [He] 2s (^22) p 1
6 C
hg hg hh 1 s (^22) s (^22) p (^2) [He] 2s (^22) p 2
7 N
hg hg hhh 1 s (^22) s (^22) p (^3) [He] 2s (^22) p 3
8 O
hg hg hghh 1 s (^22) s (^22) p (^4) [He] 2s (^22) p 4
9 F
hg hg hghg h 1 s (^22) s (^22) p (^5) [He] 2s (^22) p 5
10 Ne
hg hg hghg __hg 1 s (^22) s (^22) p (^6) [He] 2s (^22) p 6
We see that some atoms have unpaired electrons in the same set of energetically equiv-
alent, or degenerate,orbitals. We have already seen that two electrons can occupy a given
atomic orbital (with the same values of n,
, and m
) onlyif their spins are paired (have
opposite values of ms). Even with pairing of spins, however, two electrons that are in the
same orbital repel each other more strongly than do two electrons in different (but equal-
energy) orbitals. Thus, both theory and experimental observations (see the following
Enrichment section) lead to Hund’s Rule:
Electrons occupy all the orbitals of a given subshell singly before pairing begins.
These unpaired electrons have parallel spins.
Thus, carbon has two unpaired electrons in its 2porbitals, and nitrogen has three.
Helium’s electrons can be displaced
only by very strong forces, as in
excitation by high-voltage discharge.
In the simplified notation, we indicate
with superscripts the number of
electrons in each subshell.
5-17 Electron Configurations 217
As with helium, neon’s electrons can be displaced by
high-voltage electric discharge, as is observed in
neon signs.
In writing electron configurations of
atoms, we frequently simplify the
notations. The abbreviation [He]
indicates that the 1sorbital is
completely filled, 1s^2 , as in helium.