Electronic Structure
• In atoms electrons repel electrons
– Therefore they arrange themselves to avoid this repulsion
• We need to understand how electrons are arranged because it relates to an atoms physical properties and how it interacts chemically
• There are different models to describe electronic configuration – We will start with the simple Bohr version
Electron Configuration
• Electrons are described as being in electron shells around the nucleus
• Within a shell they have similar energy
• Electron shells closer to the nucleus are smaller and contain fewer electrons.
• These electrons are the most difficult to remove from the atom Electron Configuration
• Often electron shells are called energy levels
• Electrons closer to the nucleus have the lowest energy and the strongest attraction to the nucleus • Electrons in the outer shells are higher in energy but have a weaker attraction to the nucleus
• The number of electrons that can fit in a shell is determined by its size
Electron Shells
• The shells around the nucleus are numbered
1,2,3 . . .
– Where 1 is the inner shell nearest the nucleus
• We can calculate the number of electrons using
2n2 (where n = the shell number)
Electrons in Shells
• Electrons fill up the shells from the inner ones
(i.e. shell 1) to the outer ones
• For nearly all elements the outer most shell contains 8 electrons
– This is called the octet rule
Valence Electrons
• The electrons in the outer most shell are called valence electrons
– They are the most easily removed electrons
– They determine how it reacts chemically
• Elements with similar number of valence electrons tend to have similar chemical properties Bohr Diagram Revision
• Draw a Bohr diagram for an atom of the elements • a) oxygen
• b) neon
Further Electronic Structure
• We can further describe electron shells
– Scientists think of electrons as waves around the nucleus (quantum mechanics)
– These spaces occupied by electrons are known as orbitals – In this model the shells (or energy levels) are numbered 1,2,3 . . .
– Within each
