Beryllium 4 electrons total 2 in 1s 2 in 2s orbital

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Beryllium: 4 electrons total; 2 in 1s, 2 in 2s orbital Electron configuration: 1s 2 2s 2 (or [He] 2s 2 ) Boron: [He] 2s 2 2p 1 Oxygen: [He] 2s 2 2p 4 Carbon: [He] 2s 2 2p 2 Fluorine: [He] 2s 2 2p 5 Nitrogen: [He] 2s 2 2p 3 Neon: [He] 2s 2 2p 6 (second shell full) Filling orbitals 6
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If the electrons do not fully fill an orbital, follow Hund’s Rule. Put electrons in a sub-shell spin-up until each orbital contains one, then add spin-down electrons. Example: fluorine (1s 2 2s 2 2p 5 ) 2p ____ ____ ____ 2s ____ 1s ____ Hund’s rule 7 All three degenerate p-orbitals are at the same energy level
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Example: oxygen (1s 2 2s 2 2p 4 ) 2p ____ ____ ____ 2p ____ ____ ____ 2s ____ 2s ____ 1s ____ 1s ____ WRONG RIGHT (does not follow Hund’s Rule) Filling orbitals 8
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To follow Aufbau Principle, use designated orbital filling order: 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 7s 7p 8s 8p Orbital filling order 9
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There are some exceptions to the Aufbau Principle – some filled and half-filled subshells are anomalously stable Example: Mo by Aufbau: [Kr] 5s 2 4d 4 Actual: [Kr] 5s 1 4d 5 (two half-filled subshells instead of one full and one 2/5 full) Anomalous electronic configurations 10 4d ____ ____ ____ ____ ____ 5s ____ [Kr] 4d ____ ____ ____ ____ ____ 5s ____ [Kr]
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There are some exceptions to the Aufbau Principle – some filled and half-filled subshells are anomalously stable Example: Ag by Aufbau: [Kr] 5s 2 4d 9 Actual: [Kr] 5s 1 4d 10 (trades [1 full + 9/10 full] for [1 full + 1 half-full]) Anomalous electronic configurations 11 4d ____ ____ ____ ____ ____ 5s ____ [Kr] 4d ____ ____ ____ ____ ____ 5s ____ [Kr]
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