WHY DO ATOMS BOND

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WHY DO ATOMS BOND?
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VALENCE ELECTRONS

• ONLY CERTAIN ELECTRONS, CALLED VALENCE ELECTRONS,
  DETERMINE THE CHEMICAL PROPERTIES OF AN ELEMENT
• THE VALENCE ELECTRONS ARE THE ELECTRONS IN
  THE OUTERMOST ORBITALS OF THE ATOM
• GENERALLY THOSE ORBITALS ASSOCIATED
  HIGHEST ENERGY
  LEVEL

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VALENCE ELECTRONS
THE OUTERMOST ELECTRONS OF AN ATOM ARE THE ONES
MOST AFFECTED BY THE APPROACH OF ANOTHER ATOM.
THE FILLED INNER SHELL ELECTRONS ARE NOT GREATLY
AFFECTED BY REACTIONS WITH OTHER ATOMS,
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ELECTRON-DOT STRUCTURES

• CHEMISTS OFTEN REPRESENT THE VALENCE ELECTRONS
  VISUALLY WITH A SIMPLE SHORTHAND METHOD
• AN ATOMS ELECTRON-DOT STRUCTURE CONSISTS OF THE
  ELEMENTS SYMBOL, WHICH REPRESENTS THE NUCLEUS
  AND THE CORE ELECTRONS
• SURROUNDING THE SYMBOL ARE DOTS REPRESENTING THE
  ATOMS VALENCE ELECTRONS

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ELECTRON-DOT STRUCTURES

• IN WRITING AN ATOMS ELECTRON-DOT STRUCTURE, DOTS
  THAT REPRESENT VALENCE ELECTRONS, ARE PLACED ONE
  AT A TIME ON THE FOUR SIDES OF THE SYMBOL
• THEY MAY BE PLACED IN ANY SEQUENCE
• AND THEN PAIRED UP UNTIL ALL ARE USED
• THE MAXIMUM NUMBER OF DOTS AROUND ANY GIVEN
  ELEMENT IS 8

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Be



B



C





N






O






F






Ne






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ITS REALLY VERY EASY TO TELL HOW MANY VALENCE
ELECTRONS AN ATOM HAS AND THEREFORE HOW MANY DOTS
TO USE IN ITS DOT DIAGRAM.
I
II
III
IV
V
VI
VII
VIII
Na
Mg
Al
Ar
Si
P
S
Cl
YOU JUST NEED TO COUNT ITS COLUMN
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HOW DO WE KNOW WHEN TWO ELEMENTS CAN COMBINE TO
FORM A COMPOUND?

• ONE WAY TO UNDERSTAND BONDING DEALS WITH THE
  FORCES OF REPULSION AND THE ATTRACTION
• REPULSION BETWEEN THE NEGATIVE ELECTRON CLOUDS OF
  EACH ATOM
• ATTRACTION BETWEEN THE POSITIVE NUCLEI AND THE
  NEGATIVE ELECTRON CLOUDS

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• WHEN TWO ATOMS APPROACH EACH OTHER CLOSELY ENOUGH
  FOR THEIR ELCTRON CLOUDS TO OVERLAP
• THE ELECTRONS OF ONE ATOM REPEL THE ELECTRONS OF
  THE OTHER
• AND THE SAME HAPPENS WITH THE NUCLEI OF THE TWO
  ATOMS

AS THE OPTIMUM DISTANCE IS ACHIEVED THAT BALANCES
THESE FORCES, THERE IS A RELEASE OF ENERGY.
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WITH A SMALL ENERGY DIFFERENCE THE BOND FORMED IS
VERY WEAK, BUT IF THE ENERGY DIFFERENCE IS LARGE
THEN WE SPEAK OF A CHEMICAL BOND.
THERE ARE TWO TYPES OF STRONG CHEMICAL BONDS
IONIC BONDS AND COVALENT BONDS.
THE ATOMS INVOLVED AND THE ENERGY DIFFERENCE IN
THEIR BONDING DECIDES WHICH TYPE OF BOND FORMS.
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THE FORMATION OF A CHEMICAL BOND IS A FAVORED
PROCESS
BY REARRANGING THE ELECTRONS SO THAT EACH ATOM
RESEMBLES A NOBLE GAS, THESE ATOMS BECOME MORE
STABLE PAIRED UP.
SOMETIMES TO DEVELOP THE EC OF A NOBLE GAS THERE
IS A TRANSFER OF ELECTRONS BETWEEN TWO ATOMS AND
SOMETIMES A SHARING OF ELECTRONS BETWEEN TWO ATOMS
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WHICH TYPE OF BOND THAT FORMS CAN ALSO BE
DETERMINED BY EXAMINING THE AMOUNT OF DIFFERENCE
THE ATOMS HAVE IN TERMS OF ELECTRONEGATIVITY.
ELECTRONEGATIVITY IS A MEASURE OF THE
ABILITY AN ATOM HAS TO ATTRACT ELECTRONS IN A
CHEMICAL BOND.
THE GREATER THE DIFFERENCE IN ELECTRONEGATIVITIES
THE MORE IONIC THE BOND IS.
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THE TAKE HOME LESSON WITH ELECTRONEGATIVITY IS
THIS
THE CLOSER THE ATOMS ARE ON THE PERIODIC TABLE,
THE MORE EVENLY THEY SHARE THEIR ELECTRONS, AND
THEREFORE MORE LIKELY TO FORM COVALENT BONDS
THE FARTHER APART THEY ARE ON THE PERIODIC TABLE,
THE LESS EVENLY THEY SHARE THEIR ELECTRONS, AND
THEREFORE MORE LIKELY TO FORM IONIC BONDS.
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COVALENT BONDS
IN A COVALENT BOND THE ELECTRONEGATIVITY
DIFFERENCES IN THE 2 ATOMS INVOLVED IS NOT
EXTREME, SO THE ELECTRONS THAT ARE INTERACTING
ARE SHARED IT MAY NOT BE AN EQUAL SHARING, BUT
AT LEAST THE ELECTRONS ARE BEING SHARED.
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COVALENT BONDS
LETS LOOK AT THE MOLECULE Cl2

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each atom must have 8 valence e's
Cl
Cl
NOTICE 8 ELECTRONS IN EACH VALENCE SHELL!!!
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COVALENT BONDS
HOW ABOUT THE MOLECULE HCl?

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SO WHATS THE BOTTOM LINE?
TO BE STABLE THE TWO ATOMS THAT ARE INVOLVED IN
THE COVALENT BOND SHARE THEIR ELECTRONS IN ORDER
TO RESEMBLE A NOBLE GAS
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COVALENT BONDING LEADS TO THE FORMATION OF
MOLECULES, WHICH CAN BE REPRESENTED BY A A
MOLECULAR FORMULA
C6H12
A MOLECULAR FORMULA TELLS HOW MANY OF EACH ATOM
ARE PRESENT IN THE COMPOUND BY USING SUBSCRIPTS
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IN AN IONIC RELATIONSHIP THE ELECTRONEGATIVITY
DIFFERENCE IS EXTREME, SO THE ATOM WITH THE
STRONGER PULL DOESNT REALLY SHARE THE
ELECTRON INSTEAD THE ELECTRON IS ESSENTIALLY
TRANSFERRED FROM THE ATOM WITH THE LEAST TO THE
ATOM WITH THE MOST ATTRACTION
LET'S EXAMINE THIS "TRANSFER"
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AN IMPERFECT ATOM
I WANT TO BE PERFECT!

Na
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WHAT DOES SODIUM NEED TO DO TO BECOME LIKE NEON?

IT COULD LOSE AN ELECTRON!
Na
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WHAT DOES SODIUM NEED TO DO TO BECOME LIKE NEON?

IT COULD LOSE AN ELECTRON!
Na
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WHAT DOES SODIUM NEED TO DO TO BECOME LIKE NEON?
1

NOW SODIUM IS MORE STABLE!
1
Na
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ANOTHER IMPERFECT ATOM
I WANT TO BE PERFECT!
Cl
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WHAT DOES CHLORINE NEED TO DO TO BECOME LIKE A
NOBLE GAS, LIKE ARGON?
IT COULD GAIN AN ELECTRON!
Cl
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WHAT DOES CHLORINE NEED TO DO TO BECOME LIKE A
NOBLE GAS, LIKE ARGON?
IT COULD GAIN AN ELECTRON!
Cl
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WHAT DOES CHLORINE NEED TO DO TO BECOME LIKE
ARGON?

-1
-1
Cl
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SO WHATS THE BOTTOM LINE?
AN ATOM WILL LOSE OR GAIN ENOUGH ELECTRONS TO
ANOTHER ATOM IN ORDER TO LOOK LIKE A NOBLE GAS.
EIGHT IS GREAT!!!
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THINK ABOUT IT
FROM OUR PREVIOUS EXAMPLE SODIUM HAD AN EXTRA
ELECTRON AND GAVE IT AWAY TO CHLORINE, WHICH WAS
IN NEED OF ANOTHER ELECTRON. THIS FORMED A BRAND
NEW COMPOUND, NaCl IN WHICH BOTH ELEMENTS WERE
MORE STABLE
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AN ELECTRON IS TRANSFERRED FROM THE SODIUM ATOM
TO THE CHLORINE ATOM

Cl
Na
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BOTH ATOMS ARE HAPPY, THEY BOTH LOOK LIKE A NOBLE
GAS.
THIS IS AN IONIC COMPOUND
NOTICE 8 ELECTRONS IN EACH VALENCE SHELL!!!
-1
1
Cl
Na
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OCTET RULE
AN ATOM WILL LOSE, GAIN, OR SHARE ELECTRONS IN
ORDER TO ACHIEVE A FULL SET OF VALENCE ELECTRONS.
THE OPTIMUM NUMBER OF VALENCE ELECTRONS IS 8
(WITH THE EXCEPTION OF ATOMS THAT CAN LOOK LIKE
HELIUM)
Ionic Bond
Covalent Bond
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WOW! HOW COOL IS THAT!