Chemistry, Third edition

38 3 · INSIDE THE ATOM

r 0

n=1

n=2

n=3

n=4

The farther an

electron is from the

nucleus, the higher

is its energy.

Fig. 3.8The Bohr model of
the atom. The electrons are
shown as . The nucleus is
represented as a dot.

BOX 3.1

Quantum theory

Most people have heard of the ‘quantum

theory’ and the mere mention of it is

guaranteed to stop most conversations

in their tracks! Yet, many of the ideas

behind the quantum theory are

beautifully simple.

A quantity is quantized when it is only

allowed to take on certain values. Time

can take any value (2.5 seconds,

123.6787 7 seconds, 0.0143 seconds

etc.) and is not quantized. On the other

hand, the position of a painter on a ladder

is quantized since he can only stand where

there are rungs (Fig. 3.9).

The idea that the energy of electrons is

quantised was first put forward by Max

Planck in 1900. One feature of the

mathematics of the quantum theory is

that it produces ‘magic numbers’ which

can only take on certain values: such

numbers are known as quantum

numbers. For example, in the case of the

hydrogen atom, the energy Eof an

electron is proportional to the principal

quantum numbernof the electron’s

shell. This quantum number can only take

the values, 1, 2, 3, 4 and so on. The fact

that the value of nis restricted to certain

values, means that the value of Eis also

limited to certain values. Similar

relationships apply to all atoms and in

every case the quantization of the

energy of an electron is a natural

consequence of the existence of

quantum numbers.

isotope, will be the most intense. Generalizing, the intensity and m/evalue of peaks in

a mass spectrum are very useful in helping chemists to decide what molecules were

present in the original sample. For more information, see Appendix 2 on the website.

Electronic structure of atoms

Bohr model of the atom

In 1913 Niels Bohr (1885–1962) suggested that the electrons circling the nucleus

could only possess certain energies. In other words, the energy of the electron is

quantized. The key features of the Bohr model (Fig. 3.8) are:

1.Each shell of electrons lies at a definite radius (called the Bohr radius) from the

nucleus. The electrons in each shell are at the same energy.

2.Only some values of Bohr radius are allowed. This means that only certain

energy levels are allowed.

3.Each shell is labelled with a principal quantum numbern. For example, the shell

closest to the nucleus is labelled n1 and in the hydrogen atom its Bohr radius,

r 0 ,is 52.9 pm from the nucleus. The higher the value of n, the higher the energy

of the electron.

4.In order to change from one energy level E 1 to another E 2 , the electron must

gain (or lose) an amount of energy which is exactly equalto the difference in

energy (symbolized E) between the two levels:

EE 2 E 1

5.Each shell can only hold a fixed number of electrons. For example, the first shell

can hold up to two electrons and the second up to eight electrons. (The total

number of electrons allowed in a shell is 2n^2 .)

6.Electrons will occupy as low an energy level as possible. This means that if elec-

trons are fed into the shells of an atom the lowest energy shells are filled first.

3.4

Fig. 3.9Whether he knows it or not,

the position of the painter on the ladder

isquantized.