What sets an atom of one element apart from the atoms of all other elements?

Spectral Lines (http://www.colorado.edu/physics/2000/quantumzone/index.html)
Background: Atoms and Light Energy (http://imagine.gsfc.nasa.gov/docs/teachers/lessons/xray_spectra/background-atoms.html)
Emission Spectroscopy: Element identification (http://www.800mainstreet.com/spect/emission-flame-exp.html)
ChemTeam: Atomic Structure - Dalton (http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/Dalton.html)