Chemistry, Third edition

MORE ADVANCED IDEAS ABOUT ELECTRONIC STRUCTURE 43

Notice that the 4s and 3d levels do not fill up in a completely regular fashion.

This is because the 3d^5 and 3d^10 configurations are particularly stable, and chrom-

ium and copper would ‘prefer’ to possess these structures, even if the 4s sublevel

remains unfilled.

The electronic structure of all the known elements is given in the back of this book.

Wave nature of electrons

Experiments have shown that electrons sometimes behave as if they were waves.

The idea that the electron can behave as a particle or as a wave is called wave–

particle duality.

Think of a water wave produced by dropping a stone in a pond. Freeze this pic-

ture in your mind and ask yourself the question: where is the water wave? The

answer, of course, is that the water wave consists of many ripples which are spread

out upon the surface of the water. Some ripples are more pronounced than others,

but the wave does not belong to a single spot.

This idea of an ‘uncertainty’ in the position of a wave is carried over into the atom.

If electrons behave like waves we cannot pinpoint their exact position. All we can do

is to consider the probabilityof finding the electron at a point or in a region of space.

Orbitals

Suppose we were able to take a snapshot photograph of the 1s electron in the hydro-

gen atom at an instant. A fraction of a second later we take another snapshot. By

taking thousands of such snapshots, and superimposing all of them on a single frame

we would end up with a single picture showing thousands of points where the elec-

tron had been located. The greater the number of points in a region, the greater is the

probability of finding the electron there in the future.

Such snapshots would show that the volumein which it is 90% probable that the

s electron would be found is spherical. The volume is given the name atomic

orbital, usually abbreviated to orbital. The orbital of a 1s electron is called a 1s-

orbital, and it possesses a radius of about 100 pm (100  10 ^12 m). However, the

singleradius at which the 1s electron is most likely to be found is at 52.9 pm (the

Bohr radius of the n1 shell). But – and this is the crucial difference between the

wave model and the Bohr model – the wave model states there is always a chance

that the electron will be somewhere outside this radius (Fig. 3.12).

All s orbitals are spherical. 2s orbitals have larger radii than 1s orbitals.

Similarly, orbitals containing p electrons are termed p orbitals. There are three

types of p orbital (labelled px, pyand pz), which are normally of equal energy but

which have different directions in space. The shape of a p orbital is often described

as a ‘dumb-bell’ (Fig. 3.13). Orbitals containing d electrons are termed d orbitals.

There are five types of d orbitals, and each is normally of equal energy. The shape

of d orbitals is complicated and will not concern us here.

Each orbital (whether 2pz, 1s or 2s etc.) in an atom holds a maximum of two

electrons. If there are two electrons present, the electrons must be spinning oppos-

ite ways (one anticlockwise, one clockwise) and we say that the electrons are paired.

This is shown using a box for the orbital and arrows for the electrons. Up and

down arrows confirm that their spins are opposite, as follows:

↑↓

100 pm

52.9 pm

Fig. 3.12The orbital for a 1s

electron in the hydrogen atom.

The radius of the sphere in

which it is 90% likely that the

electron will be found, is

about 100 pm. The single

radius at which the 1s

electron is most likelyto be

found is a distance 52.9 pm

from the nucleus. This may be

compared with the Bohr theory

of the atom, where it was

assumed that the electron

wascertainto be found at a

radius of 52.9 pm.