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1. Electrons are negatively charged particles, much smaller than protons & neutrons.
2. The innermost shell is K. They are then labelled sequentially, e.g. K, L, M, etc.
3. Each shell can only hold a fixed number of electrons (K = 2, L = 8, M = 18).
4. Each shell must fill completely before the next outer one can be filled.
5. The innermost shell is filled first because it has the lowest energy.
6. The outermost (valence) shell determines an element’s chemical, electrical and thermal properties.
7. An atom is in its ground state when all of its electrons are in the lowest energy shells.


1. Bushberg, J. T. (2012). The essential physics of medical imaging. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. Find it at Amazon
2. Heggie, J. C., Liddell, N. A., & Maher, K. P. (1997). Applied imaging technology. Melbourne: St. Vincents Hospital.

Ⓒ A. Manickam 2018

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