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Atomic Orbitals & Electron Configurations Chemistry 1

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Atomic Orbitals & Electron Configurations 1. Identify the relationships among a hydrogen atoms energy levels, sublevels, and atomic orbitals. 2. Apply the Pauli exclusion principle, the aufbau principle, and Hunds rule to write electron configurations using orbital diagrams and electron configuration notation. 3. Define valence electrons and draw electron-dot structures representing an atoms valence electrons. For the Test: -be able to answer the objectives -know all vocabulary -be able to answer all review/practice questions -study your notes (anything can be asked from them) 2

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Atomic Orbitals Orbitals do not have an exactly defined size, but are just unoccupied spaces available for electrons should the atoms energy increase (or decrease).. As the energy level increases, the orbital becomes larger -n = 1, 2, … 7 The shape of the orbital is represented by the sublevels in that orbital -s, p, d, & f 3

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Atomic Orbitals Each atomic orbital is designated by energy level and sublevel: 1s represents s orbital in the 1 st energy level each suborbital holds 2 electrons, each with opposite spin ~ s = 2 ~ p = 6, 3 suborbitals (2 per suborbital) ~ d = 10, 5 suborbitals (2 per suborbital) ~ f = 14, 7 suborbitals (2 per suborbital) 4

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Electron Configurations electron configuration: arrangement of electrons in an atom. -electrons fill in so they have the lowest possible energy -there are three rules we follow 1. Aufbau principle: each electron occupies the lowest energy orbital available -follows the aufbau diagram all orbitals in a sublevel are equal energy in multi-electron level, energy sublevels within a principle energy level have different energies 7

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in order of increasing energy, the sequence of energy sublevels within a principle energy level is s, p, d, and f orbitals related to energy sublevels within one principle level can overlap orbitals related to energy sublevels within another principle level; begins in n = 3 2. Pauli-exclusion principle: a maximum of 2 electrons may occupy a single orbital, but only if they have opposite spins -arrows pointing up ()and down () represent electron spins -paired electrons in same orbital: represented by. 8

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3. Hunds rule: single electrons with the same spin must occupy each equal energy orbital before additional electrons with opposite spins can occupy the same orbital -s sublevel: max 1 unpaired e- p sublevel: max 3 unpaired e- d sublevel: max 5 unpaired e- f sublevel: max 7 unpaired e- 9

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Orbital Notation/Electron Configurations There are three ways to indicate electron configurations. 1. orbital notation: an unoccupied orbital is represented by a line, ____, with the orbitals principle quantum number and sublevel letter written underneath the line. ___ 1s -an unpaired electron is shown as ____ and 1s paired electrons as shown as ____. 1s 10

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Orbital Notation Practice 1. Mg 2. Al 3. Si 4. P 5. S 6. Cl 7. Ar 8. Cr 9. As 10. Kr 11

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2. electron-configuration notation: eliminates the lines and arrows of orbital notation -the number of electrons in a sublevel is shown by adding a superscript to the sublevel designation; exs. H configuration is 1s 1 (1 electron on the 1s orbital) He is 1s 2 (2 electrons in the 1s orbital) -use the periodic table to write electron configuration notation: s, p, d, and f block. -does not show orbital distributions of electrons 12

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Electron Configuration Notation Practice 1. Mg 2. Al 3. Si 4. P 5. S 6. Cl 7. Ar 8. Cr 9. As 10. Kr 13

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Noble Gas Notation 3. noble-gas notation: shorthand notation using the noble gas elements -noble-gas configuration: outer main energy level fully occupied by eight electrons, except in He -ex: Na 1s 2 2s 2 2p 6 3s 1 is the electron-configuration notation (long form) [Ne]3s 1 is the noble gas notation (short form) because all orbitals are completely filled up to neon, whos orbital notation is 1s 2 2s 2 2p 6 -this notation is only used for elements above neon! (those in n = 3 and above) 14

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Noble Gas Notation Practice 1. Mg 2. Al 3. Si 4. P 5. S 6. Cl 7. Ar 8. Cr 9. As 10. Kr 15

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Exceptions Some transition metals do not follow this trend. -includes groups 6 and 11 (Cr and Cu groups) Cr [Ar]4s 2 3d 4 Cu [Ar]4s 2 3d 9 Instead, we write it so that all s and d orbitals after the noble gas are half filled-increases stablity. Cr [Ar]4s 1 3d 5 Cu [Ar]4s 1 3d 10 Practice with Exceptions: 1. Mo 2. Au 16

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Valence Electrons Only certain electrons determine an elements chemical properties -valence electrons: electrons in the atoms highest most energy level sulfur has 16 electrons, but only 6 valence electrons Valence electrons can be determined by writing electrons configurations: -S[Ne]3s 2 3p 4 6 -Ga[Ar]4s 2 3d 10 4p 1 3(4 th level highest) [Ar] 3d 10 4s 2 4p 1 17

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Electron Dot Structures Chemists use valence electrons to show how atoms are involved in bonding. -electron dot structure: consists of the elements symbol (which represents the nucleus and inner electrons) surrounded by dots (that represent the valence electrons) unpair the electrons first along the four sides of the symbol S 18

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Oxidation Numbers When an atom bonds, it either gains or loses electrons, forming a charge on the atom -oxidation number: charge on an atom after bonding metals lose to form a positive charge Ca +2 loses two electrons nonmetals gain to form a negative charge N -3 gains three electrons Look at the electron dot structure to determine how many it will lose or gain. 19

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