Electron Configurations and Periodicity. Why is this VERY important? Chemistry is explained through sharing or transferring electrons. This determines which elements can bond and what type of bond they form

How are the electrons arranged in an atom How are the electrons arranged in an atom? What orbitals are preferentially filled by electrons? What is an electron configuration?

Electron Configuration Electrons in Atoms Electron Configuration C. Johannesson

3 RULES FOR THE ARRANGEMENT OF ELECTRONS IN ATOMS. 1) The Pauli Exclusion Principle 2) The Aufbau Principle 3) Hund’s Rule C. Johannesson

A. General Rules Pauli Exclusion Principle Each orbital can only hold TWO electrons with opposite spins. No 2 electrons in the same atom can have the same set of all 4 quantum numbers (electrons can not occupy same space nor have the same spin) Wrong Right

General Rules: Aufbau Principle Electrons fill the lowest energy orbitals first. (means building-up in German) in the ground state, the electrons will fill the atomic orbital of lowest energy first; “Lazy Tenant Rule” C. Johannesson

A. General Rules WRONG RIGHT Hund’s Rule Within a sublevel, place one electron per orbital before pairing them. “Empty Bus Seat Rule” WRONG RIGHT C. Johannesson

1s2 2s2 2p4 O B. Notation 1s 2s 2p 8e- Orbital Diagram Electron Configuration 1s2 2s2 2p4 C. Johannesson

S 16e- 1s2 2s2 2p6 3s2 3p4 S 16e- [Ne] 3s2 3p4 B. Notation Longhand Configuration S 16e- 1s2 2s2 2p6 3s2 3p4 Core Electrons Valence Electrons Shorthand Configuration S 16e- [Ne] 3s2 3p4 C. Johannesson

C. Periodic Patterns s p d (n-1) f (n-2) 1 2 3 4 5 6 7 6 7 © 1998 by Harcourt Brace & Company C. Johannesson

C. Periodic Patterns Period # energy level (subtract for d & f) A/B Group # total # of valence e- Column within sublevel block # of e- in sublevel C. Johannesson

1s1 C. Periodic Patterns 1st column of s-block 1st Period s-block Example - Hydrogen 1s1 1st column of s-block 1st Period s-block C. Johannesson

C. Periodic Patterns p s d (n-1) f (n-2) Shorthand Configuration Core e-: Go up one row and over to the Noble Gas. Valence e-: On the next row, fill in the # of e- in each sublevel. s d (n-1) f (n-2) p C. Johannesson

[Ar] 4s2 3d10 4p2 C. Periodic Patterns Example - Germanium C. Johannesson

D. Stability Full energy level Full sublevel (s, p, d, f) Half-full sublevel C. Johannesson

D. Stability Electron Configuration Exceptions Copper EXPECT: [Ar] 4s2 3d9 ACTUALLY: [Ar] 4s1 3d10 Copper gains stability with a full d-sublevel. C. Johannesson

D. Stability Electron Configuration Exceptions Chromium EXPECT: [Ar] 4s2 3d4 ACTUALLY: [Ar] 4s1 3d5 Chromium gains stability with a half-full d-sublevel. C. Johannesson

D. Stability 1+ 2+ 3+ NA 3- 2- 1- Ion Formation Atoms gain or lose electrons to become more stable. Isoelectronic with the Noble Gases. 1+ 2+ 3+ NA 3- 2- 1- C. Johannesson

O2- 10e- [He] 2s2 2p6 D. Stability Ion Electron Configuration Write the e- config for the closest Noble Gas EX: Oxygen ion  O2-  Ne O2- 10e- [He] 2s2 2p6 C. Johannesson