1.02 Outline of atomic theory.

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.

atom

Element groups.

One way of categorising elements, is by the number of electrons in the outer shell of their atoms.

An atom with one electron in it's outer shell is called a group one element.
An atom with two electrons in it's outer shell is called a group two element.
An atom with three electrons in it's outer shell is called a group three element.
An atom with four electrons in it's outer shell is called a group four element.
An atom with five electrons in it's outer shell is called a group five element etc .

element groups

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.