3.3 Electronic structure  (Page 4/5)

To take this a step further, an atom is at its most stable (and therefore unreactive ) when all its orbitals are full. On the other hand, an atom is least stable (and therefore most reactive ) when its valence electron orbitals are not full. This will make more sense when we go on to look at chemical bonding in a later chapter. To put it simply, the valence electrons are largely responsible for an element's chemical behaviour and elements that have the same number of valence electrons often have similar chemical properties.

One final point to note about electron configurations is stability. Which configurations are stable and which are not? Very simply, the most stable configurations are the ones that have full energy levels. These configurations occur in the noble gases. The noble gases are very stable elements that do not react easily (if at all) with any other elements. This is due to the full energy levels. All elements would like to reach the most stable electron configurations, i.e. all elements want to be noble gases. This principle of stability is sometimes referred to as the octet rule. An octet is a set of 8, and the number of electrons in a full energy level is 8.

Experiment: flame tests

Aim:

Apparatus:

Method:

1. Dip a clean bamboo stick into the methanol
2. Dip the stick into the salt or powder
3. Wave the stick through the flame from the bunsen burner. DO NOT hold the stick in the flame, but rather wave it back and forth through the flame.
4. Observe what happens

Results:

Conclusion:

The above experiment on flame tests relates to the line emission spectra of the metals. These line emission spectra are a direct result of the arrangement of the electrons in metals.

Energy diagrams and electrons

1. Draw Aufbau diagrams to show the electron configuration of each of the following elements:
   1. magnesium
   2. potassium
   3. sulphur
   4. neon
   5. nitrogen
2. Use the Aufbau diagrams you drew to help you complete the following table:

   Element	No. of energy levels	No. of core electrons	No. of valence electrons	Electron configuration (standard notation)
   $\mathrm{Mg}$
   $\mathrm{K}$
   $\mathrm{S}$
   $\mathrm{Ne}$
   $\mathrm{N}$

3. Rank the elements used above in order of increasing reactivity . Give reasons for the order you give.

Group work : building a model of an atom

Earlier in this chapter, we talked about different 'models' of the atom. In science, one of the uses of models is that they can help us to understand the structure of something that we can't see. In the case of the atom, models help us to build a picture in our heads of what the atom looks like.