Electrons occupy shells (orbits) surrounding the nucleus. The shells are lettered K, L, M, N, O… and numbered 1,2, 3, etc., corresponding to energy levels. There is a limit to the number of electrons that can be found in a shell.
The actual number is obtained by a formula 
, where n is the energy level.
Action of Wind
The Constituents of the Atom: Proton, Neutron and Electron
Atomic Number, Mass Number, Isotopes, and Relative Atomic Masses of Atoms
Arrangement of Electrons and the Nucleus
Relative Atomic Mass
Symbols, Formulae and Equations
Empirical Molecular Formulae and Chemical Equations
Action of Running Water
Action of Waves
Classification of Climate
Major Climatic Types
Aspects of Environmental Interactions
Environmental Balance
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