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Electron Dot Diagrams

• G.N. Lewis idea (UC Berkeley)– Elegantly simple idea, but very instructive– Show each bonding electron as a dot

• As elements brought together, dots merge• Most stable configuration is filled shell

– 2 dots for Hydrogen (2s2 or [He] configuration)– 8 dots for most others (s2+p6, Octet rule)

• Methane example C(4dot) + 4*H (1dot)

– Can have more electron pairs than bonds• “lone pairs” are non-bonding electrons• Lone pairs occupy a geometrical position

– Are part of molecular shape consideration

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Rules of Lewis-Dot diagrams

• Only “valence electrons” considered– Inner core (noble gas configs.) ignored

• They play no part in chemical reactions

– Octet rule applies for >90% of atoms• Hydrogen is main exception with 2 electrons• A few other exceptions

– Starting point is an element’s electrons• Pure element where electrons = protons• Not elements in oxidized or reduced state• “AE” or “Available Electrons”

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Rules of Lewis-Dot diagrams• Electron Counting

– Add up electrons in all elements involved• If product is an ion, must add or subtract electrons• Sulfur (6) + 2 oxygens (6) = 3*6=18 electrons• All require 8 shared electrons 3*8=24• Sharing is 24-18=6

– Each electron is a dot, each pair CAN be a line• Dot plot must meet all the rules (one of these 2 does NOT)

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Octet Rule

• 2 electrons per outer shell for Hydrogen– Hydrogen starts out with single electron - 1s– Diatomic hydrogen provides 2 shared electrons

• 8 electron outer shell for most elements– Some gain electrons, such as Cl(7e) to Cl-(8e)– Others lose electrons, as Na(11e) to Na+(10e)

• Polyatomic Ions share 8 electrons/element– Nitrate, sulfate, phosphate, etc.

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Diatomic HydrogenFormation of H2 via sharing 2 outer electrons, emulating He

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Lewis Structure (electron dot diagram) for ammonia Each of the 3 hydrogen atoms will share its electron with nitrogen to

form a bonding pair of electrons (covalent bond) so that each hydrogen atom has a share in 2 valence electrons (electronic configuration of

helium) and the nitrogen has a share in 8 valence electrons (electron configuration of neon)

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Lone Pair in Ammonia

• NH3 has 8 electrons around the N = ok!

• NH3 has 2 electrons around each H = ok!– Fits the octet and 2 electron rules

• What happens to 2 unbonded N electrons ?– 3 of 5 AE are bonded, 2 are “leftovers”– These 2 are “lone pairs”

• Fulfills the electrical requirements• Non-bonding, help define shape due to repulsion

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Lone Pairs

• Having more electrons than bonds = leftover “lone pair”– Ammonia example

• Nitrogen (5 electrons to share) • Hydrogen (total of 3 electrons to share)• After N-H Bonding, 2 nitrogen electrons “left over”

– Lone pairs• They complete octets of 8 electrons per element• They are part of electronic structure• They are NOT part of molecular structure

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3 of Nitrogen’s 5 valence electrons shared with 3 Hydrogen atoms in Ammonia. “Lone Pair” electrons attract Hydrogen ion

Result is formation of the Ammonium ion

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Each oxygen will share 2 of its valence electrons in order to form 2 bonding pairs of electrons (a double covalent bond) so that each oxygen

will have a share in 8 valence electrons (electronic configuration of neon).

Lewis Structure (electron dot diagram) for the oxygen molecule

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Examples across the chart

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Electronic Configurations of ElementsK

proton Element 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f1 H 12 He 2 Inert Gas3 Li 2 1 Alkali4 Be 2 25 B 2 2 16 C 2 2 27 N 2 2 38 O 2 2 4

9 F 2 2 5 Halogen

10 Ne 2 2 6 Inert Gas11 Na 2 2 6 1 Alkali12 Mg 2 2 6 213 Al 2 2 6 2 114 Si 2 2 6 2 215 P 2 2 6 2 316 Si 2 2 6 2 4

17 Cl 2 2 6 2 5 Halogen18 Ar 2 2 6 2 6 Inert Gas19 Kr 2 2 6 2 6 1 Alkali20 Ca 2 2 6 2 6 2

L M N

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Nitrate Formation from elements1 additional electron needed to fill shells,

meeting octet rule provides (-1) charge to anion

N

O O

O

N

O O

O

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Alternative Nitrate Representations“Ball & Stick” easy to model and understand

“Bubbles” represent electron clouds, size

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Sulfate Formation from elements2 additional electrons needed to fill shells,

meeting octet rule provides (-2) charge to anion

S

O O

O O

S

O O

O O

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Electron Dot Diagrams

• Lines between atoms are 2-electrons– One line equivalent to 2 dots

• 2 lines (double bond) equivalent to 4 dots• 3 lines (triple bond) equivalent to 6 dots

– Can rotate around one line (no interference)• 2 lines (double bond) restricts rotation, planar• 3 lines (triple bond), no rotation, linear

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(Almost) all bonding can be represented by lines and dots

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Each of 4 carbon valence electronsshares orbitals with 1 from Chlorine

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Important Exception

• Ionic bonds electron transfer– Each ion is free agent, unattached to the other

• If not shared, no “double counting” possible– Na + Cl NaCl(s) Na+ + Cl-

– Ions are “Isoelectronic” with [Ne] and [Ar]• Core looks like noble gas, but net charge on ions

– A similar question in the experiment• Metal halides and oxides generally not covalent

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Lewis Diagram for Chloride Ion

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Lewis diagram for Sodium Ion