An atom can be modelled as a positively charged nucleus, orbited by negatively charged electrons. The positive charge of the nucleus is due to the positively charged protons it contains. For an atom in its natural state, the total negative charge of the electrons, is equal in magnitude to the positive charge of the nucleus. Therefore overall the atom is electrically neutral. The orbits of the electrons are arranged in shells. The first shell is closest to the nucleus and contains a maximum of two electrons. The next outer shell contains a maximum of eight electrons. The next shell also contains a maximum of eight electrons.
Semiconductors are group four elements, i.e. they have four electrons in their outermost shell. The electrons in the outer shell of an atom are called valence electrons. ( Note it is important to emphasise that the element group number refers to the number of electrons in the outer shell of the atom, not the total number of electrons). In the next section we will look at how the atoms of group 4 materials like silicon and germanium bond together and how this affects their electrical properties.