Electrons occupy particular energy levels.

These energy levels are called shells.

- The first shell can only contain 2 electrons

- The second shell can contain 8 electrons

- The third shell can contain 18 electrons

You should be able to represent the electronic structure of the first twenty elements of the periodic table in the following forms:   (notice that this is the unusual isotope of magnesium with only 11 neutrons, but this does not affetc its electronic arrangement).

Balloons

By setting fire to balloons filled with Hydrogen and Helium, the students show how just one single electron can make a lot of difference to the reactivity. Hydrogen has 1 electron, Helium 2 in their outer shells. Helium’s shell is complete and so very unreactive. Hydrogen’s outer shell is incomplete and very violently reactive.

Exam tip: Learn how to relate reactivity to the numbers of electrons in outer shells. This is made more obvious when the scientists show the students the Alkali Metals; Lithium, Sodium and Potassium, reacting with water.

Exam Questions:

• Explain with reference to the number of electrons in the outer shell and the increasing size of the atoms why Potassium is more reactive than Sodium.

Watch the balloon experiment below here courtesy of Sky Learning www.skylearning.co.uk