The Science Book

256 LINUS PAULING

I

n the late 1920s and early

1930s, in a series of landmark

papers, American chemist

Linus Pauling figured out a

quantum-mechanical explanation

of the nature of chemical bonds.

Pauling had studied quantum

mechanics in Europe with the

German physicist Arnold

Sommerfeld in Munich, with Niels

Bohr in Copenhagen, and with

Erwin Schrödinger in Zurich.

He had already decided that he

wanted to research the bonding

within molecules, and realized that

quantum mechanics gave him the

right tools to do so.

Hybridization of orbitals

When he returned to the US,

Pauling published about 50 papers,

and, in 1929, he laid down a set

of five rules for interpreting the

X-ray diffraction patterns of

complicated crystals, now known

as Pauling’s rules. At the same

time, he was turning his attention

to the bonding between atoms in

covalent molecules (molecules in

which atoms are bonded by sharing

two electrons with each other),

especially of organic compounds—

those based on carbon.

A carbon atom has six electrons

in total. The European pioneers of

quantum mechanics designated

the first two as “1s-electrons”:

these have a spherical orbital or

shell around the carbon nucleus—

like a balloon inflated around a

golf ball in the center. Outside the

1s shell is another shell containing

two “2s-electrons.” The 2s shell

is like another, bigger balloon

outside the first. Lastly, there are

“p-orbitals,” which have big lobes

sticking out either side of the

nucleus. The px orbital lies on the

x-axis, the py on the y axis, and

the pz orbital on the z-axis. The last

two electrons of the carbon atom

occupy two of these orbitals—

perhaps one in px and one in py.

The new quantum-mechanical

picture of electrons treated their

orbits as “clouds” of probability

densities. It was no longer quite

right to think of the electrons as

points moving around their orbits;

rather, their existence was smeared

across the orbits. This new nonlocal

picture of reality allowed for some

radical new ideas for chemical

The nature of the

chemical bond reflects the

quantum-mechanical

behavior of electrons.

It can be modified

to explain the structure

of molecules.

Quantum mechanics

provides a new way to

describe the behavior

of electrons.

IN CONTEXT

BRANCH

Chemistry

BEFORE

1800 Alessandro Volta lists

metals in decreasing order

of electropositivity.

1852 British chemist Edward

Frankland states that atoms

have definite combining

power, which determines the

formulae of compounds.

1858 August Kekulé shows

that carbon has a valency

of four—it forms four bonds

with other atoms.

1916 American physical

chemist Gilbert Lewis shows

that a covalent bond is a pair

of electrons shared by two

atoms in a molecule.

AFTER

1938 British mathematician

Charles Coulson calculates

an accurate molecular orbital

wave function for hydrogen.

s orbital px orbital

py orbital pz orbital

z

x

x

y

y

z

Electrons orbit an atomic nucleus in

various ways—in shells around the

center (s) or lobes along one axis (p).

Electron orbits