1.02 Outline of atomic theory
An atom can be thought of as a central 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 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.
One way of categorising the atoms of different elements is by the number of electrons in their outer shell.
- 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.
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 point out that the element groups refer to the number of electrons in the outermost shell of the atom, the total number of electrons in the atom can be greater ie for atoms that have inner shells that also contain electrons. Also many atoms have less electrons in their outer shell than the maximum number that the shell can hold. e.g. for group 4 materials silicon and germanium the outer shell can hold a maximum of 8 electrons so the outer shell is only half full. The outer shell of the atom is more stable when the it is completely full.