A Guidebook to Mechanism in Organic Chemistry

Structure, Reactivity and Mechanism

two electrons, these electrons being distinguished from each other by

having opposed (' paired') spins.

It can be shown from wave-mechanical calculations ttaat the Is

orbital (corresponding to the classical K shell) is spherically symme­

trical about the nucleus and that the 2s orbital is similarly spherically

symmetrical but at a greater distance from the nucleus; there is a

region between the two latter orbitals where the probability of finding

an electron approaches zero (a spherical nodal surface):

spherical nodal surface

3p qfv^

/M shell ggl;

2s If. }L shell

U K shell

1*

As yet, this marks no radical departure from the classical picture

of orbits, but with the 2p level (the continuation* of the L shell) a dif­

ference becomes apparent. Theory mow requires the existence of tfttg^

2/7 orbitals, all of the same energy and shape, arranged mutually at

right-angles along notional x, y and z ajies and, therefore, designated

as 2px, 2py and 2p„ respectively. Further, these three 2p orbitals are

found to be not spherically symmetrical, like the Is and 2s, but

* dumb-bell' shaped with a plane, in which there is zero probability

of finding an electron (nodal plane), passing through the nucleus

(at right-angles to the x, y and z axes, respectively) and so separating

the two halves of each dumb-bell: ^

plane '.

2px 2py (^2) Pi 2px, 2py and Ip,
combined
We can thus designate the distribution of the six electrons of the
carbon atom, in orbitals, as Is^2 !?^2 2p\2py; orbitals of equal energy
(e.g., 2p„ 2py, 2p,) accommodating a single electron, in turn, before
any takes up a second one—the 2pz orbital thus remains unoccupied.