WebElements
www.webelements.com/
Comprehensive interactive periodic table, detailing standard data, isolation/extraction reaction, background, history, compounds, uses, electronic properties, physical properties, crystal structures and nuclear properties for each element.

The Periodic Table Challenge
www.ilpi.com/genchem/instantquiz.html
An instant quiz. You are given a blank periodic table to fill out with the correct symbols. A good way to learn the periodic table.

The Orbitron
www.shef.ac.uk/chemistry/orbitron/AOs/1s/index.html
Gives clear pictures of the shapes of different atomic orbitals.

The Quantum Theory
http://antoine.frostburg.edu/chem/senese/101/quantum/index.shtml
Delivers information in bullet point format and has a good glossary of quantum mechanic principles. Quantum mechanics is a difficult subject to explain on a simple level - Atkins Quantum Mechanics does a good job at explaining it in detail.

Electrons in Atoms
http://antoine.frostburg.edu/chem/senese/101/electrons/index.shtml
Delivers information in bullet point format.

Quantum Numbers and Electron Configurations
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch6/quantum.html
Further information on quantum numbers and the allowed combinations.

Hydrogen Radial Probability
http://hyperphysics.phy-astr.gsu.edu/hbase/hydwf.html#c1
This site shows the radial distribution functions (graphical representation and equations) for hydrogen.

Ionisation Energy
www.co.uk/atoms/properties/ies.html
Gives a good introduction on ionisation energy trends and has additional links to web pages on atomic orbitals, electronic structures and atomic radii.

Effective Nuclear Charge
www.chm.davidson.edu/ChemistryApplets/AtomicOrbitals/Zeff.html
Interactive display that helps to understand the concept of effective nuclear charge and how this can be used to understand properties of multi-electron atoms.

Aufbau Principle
www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/aufbau-principle.html
Useful method of working out the energy order of atomic orbitals.

Aufbau Principle
www.avogadro.co.uk/light/aufbau/aufbau.htm
Can see the 'building up' principle from hydrogen to zinc.

Electronegativity
www.chemguide.co.uk/atoms/bonding/electroneg.html
A simple introduction and explanation of electronegativity trends.

Electronegativity on Wikipedia
http://en.wikipedia.org/wiki/Electronegativity
History and brief descriptions of the three electronegativity scales.

Electronegativity
www.hull.ac.uk/chemistry/electroneg.php
Detailed descriptions and equations for Pauling, Allred-Rochow and Mulliken-Jaffe electronegativity scales.