# Electronic structure

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• Created by: Rachh
• Created on: 25-12-12 16:49
• Electronic structure
• Electron shells
• Electrons have fixed energies.
• Electrons move around the nucleus in shells, or energy levels.
• The shells are divided up into sub-shells as the electrons within a shell have different energies.
• The s- sub-shell has 1 orbital so 2 electrons.
• The p sub-shell has 3 orbitals so 6 electrons.
• The d sub-shell has 5 orbitals so 10 electrons.
• The f sub-shell has 7 orbitals so 14 electrons.
• Showing electron configurations
• Sub-shell notation is a way of writing out the configuration of electrons.
• eg an atom of Neon would be written as 1s2 2s2 2p6
• Another way to show it is by drawing arrows in boxes.
• The up and down arrows represent the electrons spinning in different directions.
• Two electrons can only occupy the same orbital if they have opposite spin.
• Energy level diagrams are also used.
• They also show the energy of the electrons in different orbitals.
• These show the number of electrons and their arrangement.
• Working out electron configurations
• Electrons fill up the lowest energy sub-shells first.
• Electrons fill orbitals singly before they start sharing.
• For the configuration of ions from the s and p blocks of the periodic table, add or remove electrons to or from the highest energy occupied sub-shell.
• eg Mg = 1s2 2s2 2p6 3s2 and Mg2+ = 1s2 2s2 2p6
• Nobe gas symbols in square brackets are sometimes  used in electron configurations.
• eg calcium (1s2 2s2 2p6 3s2 3p6 4s2) can be shortened to [Ar] 4s2
• Transition metals:
• Odd ones out:
• Copper (Cu): 1s2 2s2 2p6 3s2 3p6 4s1 3d10
• Chromium (Cr): 1s2 2s2 2p6 3s2 3p6 4s1 3d5
• When transition metals become ions, they lose their 4s electrons before their 3d electrons.
• eg. Fe: 1s2 2s2 2p6 3s2 3p6 4s2 3d6 and Fe3+: 1s2 2s2 2p6 3s2 3p6 3d5
• Electronic structure and chemical properties
• The s-block elements (group 1 and 2) easily lose their outer electrons to form positive ions with an inert gas configuration.
• The elements in groups 5,6, and 7 (p block) can gain 1,2 or 3 electrons to form negative ions with an inert gas configuration.
• Group 0 (the inert gases) have completely filled s and p sub-shells.