1. The structure of molecules and material properties
What does a sodium atom look like? (year 9)
valence electron
shells have differing ­ radii
­ electron capacities
­ attraction force from
nucleus
valence shell
1
1.1 The orbital model
The model of electrons travelling on a circular orbit around the nucleus (the so­called Bohr­
modell) did not hold up to Werner Heisenberg's later findings: the location of a very small particle (such as an electron) can never be Werner Heisenberg (1901 ­ 1976)
exactly pinpointed!
Following this discovery (the so­called heisenberg uncertainty principle) many famous chemists (such as Schrödinger, Pauling and Mulliken) tried to figure out a new model, that could accommodate the fact that they no longer knew exactly where the electron was.
Bohr­model
new model
2
Max Born, a Jewish German physicist suggested that instead of an exact location the electron's position needed be described by the probability of it being in a certain place. Using known electron in combination with the heisenberg uncertainty principle, these probabilities can be calculated.
Max Born (1882 ­ 1970)
This model is still widely used by chemists and physicists today and is called the orbital model!
Example: The electron on the left has a 50% chance to be inside the first circle, a 25% chance to be inside the second circle and a 15% chance to be inside the third circle and a 10% chance to be anywhere outside of the circles.
3
Chemists are usually not very interested in how likely it is to find the electron in one certain place. They want to find the places with the highest probability to find the electron. So in most pictures of electron locations, the "bubble" that is shown represents the area with a 90% chance of finding the electron there.
50% + 25% + 15% = 90%
Area of 90% probability
These since "areas of high probability of find and electron" is a bit of an unwieldy term, there is a scientific term for these areas: atomic orbitals.
4
We will be talking about these atomic orbitals a lot throughout the school year. So the definition of an orbital has to be basic knowledge:
An atomic orbital is
an area around the nucleus with a high probability of finding an electron
One important fact about orbitals always has to be remembered: Every orbital can hold only one electron pair! (Pauli exclusion principle)
Quick question: How many different orbitals would we find in a chlorine­atom?
5
Changes between the Bohr model and the orbital model
We used to look at an exact orbit on which the electron travelled around the nucleus. Now we look at an area of high probability (orbital) around the nucleus, in which we can find the electron with a 90% chance.
outermost orbital contains only one electron
4 inner orbitals contain 2 electrons each
6
What does the bond between two flourine atoms look like?
Bohr­model
orbital model
7
1.2 Atomic bonds in the orbital model
Atomic bonds in orbital model are created through overlapping valence electron orbitals (remember: valence electrons are always the electrons on the outermost shell/energy level!) that contain only one electron. The overlapping orbitals create a new molecular orbital in which the two formerly lone electrons form a new electron pair!
Example: Hydrogen molecule (H2)
overlapping single­electron orbitals
molecular orbital with electron pair
8
Examples of molecular orbitals:
9