913 - Chapter 3 Atomic Structure and Properties There are...

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Chapter 3- Atomic Structure and Properties Section 3.1 There are two classes of electrons in an atom: Core - Valence- In filled orbitals,tightly bound In unfilled orbitals and in outermost  s orbitals, more loosely bound Examples: Carbon : core = two 1s electrons valence = two 2s and two 2p electrons 1s 2 2s 2 2p 2 core valence filled!
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General Rule:   Valence electrons for “main group” elements are ns # np # (See Table 3.1)  Noble gases have  no  valence electrons!  All core electrons. Section 3.1 Coulomb’s Law  applies to the interaction between  the positively charged nucleus and the  negatively charged electrons.  They are attracted to each other! This is what keeps them together! Valence Electrons
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Section 3.2 How tightly are the electrons held? This depends on what’s in between them and the nucleus! Core electrons “ shield ”  valence electrons Nucleus + Core – Valence – Think about the nucleus +  core electrons vs the  valence electrons The result is called the “Effective Nuclear Charge” (Z eff ) Z eff  = Z nucleus  –  σ (effective charge = nuclear charge – shielding)
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Effective Nuclear Charge (Z ) Section 3.2 Z eff  = Z nucleus  –  σ (effective charge = nuclear charge – shielding) Examples: Helium (He)  - all core electrons - none to shield outer electrons - they shield each other Z  = ~1.7 Carbon (C) - 1s 2  are cores and 2s 2 2p 2  are valence Z nucleus  = +6 from 6 protons eff See Figure 3.2 for more Z eff ’s
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Section 3.2 Trend for Increasing Effective Nuclear Charge (Z eff ) Across a period, add more protons to nucleus  without adding more core electrons to shield  attraction so Z eff  increases. Down a group, the additional core electrons are  more diffuse (spread out) and less effective at  shielding.  They allow the nucleus to pull more.
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Section 3.2 Energy of interaction of an electron and the nucleus (E n ): (n = principle quantum number)
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913 - Chapter 3 Atomic Structure and Properties There are...

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