STARTING DEFINITION: VALENCE ELECTRONS – ELECTRONS IN THE OUTER SHELL OF AN ATOM – THESE ARE THE ONES INVOLVED IN CHEMICAL REACTIONS. FOR EXAMPLE: OXYGEN 1s 2 2s 2 2p 4 valence electrons G.N. LEWIS DEVISED A WAY TO REPRESENT VALENCE ELECTRONS OF ATOMS AND TO ILLUSTRATE THE PART THAT THEY PLAY IN BONDING.

THE 6 VALENCE ELECTRONS OF OXYGEN COULD BE REPRESENTED AS DOTS SURROUNDING THE SYMBOL. START WITH TWO DOTS ON TOP OF THE SYMBOL. THESE REPRESENT THE 2s 2 ELECTRONS. THEN ADD ONE DOT AT 3 O’CLOCK, ONE DOT AT 6 O’CLOCK, AND ONE DOT AT 9 O’CLOCK (THESE ARE THE FIRST THREE ELECTRONS IN THE 2p SUBSHELL). FINALLY, FINISH UP WITH THE REMAINING ELECTRON BACK IN THE 3 O’CLOCK POSITION. THE ORDER CAN BE EXPLAINED BY THE AUFBAU PRINCIPLE AND HUND’S RULE.

THE ABOVE DRAWING SHOWS THE ORDER THAT YOU WOULD ADD THE ELECTRONS. THIS IS ILLUSTRATED FOR THE MAIN GROUP ELEMENTS IN THE THIRD PERIOD: HERE’S A QUICK TUTORIAL:

LEWIS DOT STRUCTURES ARE VERY USEFUL IN SHOWING HOW BONDING OCCURS. CONSIDER POTASSIUM, K, IN AN IONIC REACTION: K + Cl  K + Cl - K 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 K + 1s 2 2s 2 2p 6 3s 2 3p 6 4s o POTSSSIUM LOSES AN ELECTRON TO FORM THE POTASSIUM ION. IN LEWIS DOT STRUCTURE, WE WOULD WRITE THAT AS K + OR [K] +

FOR CHLORINE IN THAT IONIC REACTION Cl 1s 2 2s 2 2p 6 3s 2 3p 5 Cl - 1s 2 2s 2 2p 6 3s 2 3p 6 octet CHLORINE GAINS AN ELECTRON TO FORM THE CHLORIDE ION (ANION). LEWIS DOT FORMULA IS: [ ] -

LEWIS DOT STRUCTURES CAN ALSO BE USED TO PREDICT THE FORMULAS FOR IONIC COMPOUNDS. PROBLEM: USE LEWIS STRUCTURES TO PREDICT THE FORMULA FOR THE COMPOUND THAT FORMS BETWEEN CALCIUM AND CHLORINE.

THE RULES FOR WRITING LEWIS DOT STRUCTURES: 1)COUNT THE TOTAL NUMBER OF VALENCE ELECTRONS 2)WRITE THE SKELETON STRUCTURE, AND PUT THE ELEMENTS THAT NEED THE MOST ELETRONS IN THE MIDDLE 3)USE 2 VALENCE ELECTRONS TO FORM EACH BOND IN THE SKELETON STRUCTURE 4)MAKE OCTETS BY DISTRIBUTING THE REMAINNG ELECTRONS IN PAIRS.

LEWIS THEORY CAN ALSO BE USED TO PEDICT TYPES OF BONDS IN COVALENT BONDING. WE WILL WORK THE FOLLOWING EXAMPLES: H 2 O Cl 2 O 2 N 2

LEWIS THEORY ACCOUNTS FOR WHY COVALENT BONDS ARE HIGHLY DIRECTIONAL. A COVALENT BOND LINKS JUST ONE SPECIFIC PAIR OF ATOMS IN THE SPACE BETWEEN THEM. THIS IS IN CONTRAST TO IONIC BONDS, WHICH ARE NONDIRECTIONAL AND HOLD TOGETHER AN ENTIRE ARRAY OF ATOMS. THE FUNDAMENTAL UNITS OF COVALENT COMPOUNDS ARE MOLECULES.