Electron Configurations: Where are the e–? (probably) p-block s-block d-block H 1s1 He 1s2 P 1s2 2s2 2p6 3s2 3p3 f-block Fe3+ 1s2 2s2 2p6 3s2 3p6 4s2 3d6 3d5 e– lost first are from the highest energy level Aufbau Principle: e– will take the lowest-energy orbital that is available

isoelectronic species: 3p1 Al3+ 1s2 2s2 2p6 3s2 isoelectronic species: 3p1 O2– 1s2 2s2 2p4 2p6 F– 1s2 2s2 2p6 2p5 species having the same e– configuration N3– 1s2 2s2 2p3 2p6 “equal-energy” Hund’s Rule: each degenerate orbital must have one e– before any take a second Friedrich Hund (1896–1997) e.g., the three 3p orbitals are degenerate

Orbital Diagrams …show spins of e– and which orbital each is in O 1s 2s 2p 3s 3p P 1s 2s 2p 3s 3p paired e– unpaired e– valence e–: the s and p e–s (ONLY!) in the outermost energy level

Sections of Periodic Table to Know s-block p-block d-block f-block

Shorthand Electron Configuration (S.E.C.) 1. Put symbol of noble gas that precedes element in brackets. 2. Continue writing e– config. from that point. S [Ne] 3s2 3p4 Co [Ar] 4s2 3d7

In a many-electron atom, each e– is attracted to the nucleus and repelled by the other e–. -- effective nuclear charge, Zeff: the net (+) charge attracting an e– (a measure of how tightly particular e–s are held) Z = atomic number Equation: 3d6 S = # of e– BETWEEN nucleus and e– in question (NOT e– in same subshell) 4s2 Zeff = Z – S 3p6 -- Within a given electron shell, s e–s have the greatest Zeff, f e–s the least. Fe 3s2 2p6 2s2 For Fe, the 3p e–s have Zeff = 26 – 12 = 14; the 3d e–s have Zeff = 26 – 18 = 8. 1s2 nuc.

-- The (+) charge “felt” by the outer e– is always less than the nuclear charge. This effect, due to the core (or kernel) electrons, is called the... screening effect (or shielding effect). K v.e– Li v.e– tougher to remove easier to remove