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CHEMICAL BONDS A Dog’s Tale about an Element’s Search for Happiness (chemical stability)

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Presentation on theme: "CHEMICAL BONDS A Dog’s Tale about an Element’s Search for Happiness (chemical stability)"— Presentation transcript:

1 CHEMICAL BONDS A Dog’s Tale about an Element’s Search for Happiness (chemical stability)

2 CHEMICAL BONDS The forces that hold atoms together…often driven by the Octet Rule and stability. (Mutual electrical attraction between nuclei and valence electrons of different atoms that binds them.)

3 Chemical Bonds Sometimes it helps to think of bonds (which you can't see) in terms of familiar things you can see. This is called an analogy. Sometimes it helps to think of bonds (which you can't see) in terms of familiar things you can see. This is called an analogy. Let's use the natural attraction of dogs to bones as an analogy to the attractions that cause chemical bonds.

4 Chemical Bonds The negatively charged electrons determine how two or more atoms will interact when they are brought near each other. In a sense, the atoms fight over the available electrons in much the same way two or more dogs will fight over bones. The negatively charged electrons determine how two or more atoms will interact when they are brought near each other. In a sense, the atoms fight over the available electrons in much the same way two or more dogs will fight over bones. The Dog Bone Analogy works quite well for several types of atomic bonds. The Dog Bone Analogy works quite well for several types of atomic bonds. Dogs are the atoms. Bones are the electrons.

5 Octet Rule Atoms tend to gain, lose, or share electrons in order to acquire a full set of valence electrons. 8 (most of the time) 2 (1 st energy level) or Happy dogs!

6 IONIC BONDS One big greedy thief dog! Ionic bonding can be best imagined as one big greedy dog stealing the other dog's bone. One big greedy thief dog! Ionic bonding can be best imagined as one big greedy dog stealing the other dog's bone.

7 IONIC BONDS The bone represents the electron that is up for grabs. When the big dog gains an electron he becomes negatively charged, and the little dog who lost the electron becomes positively charged. When the big dog gains an electron he becomes negatively charged, and the little dog who lost the electron becomes positively charged. These two ions (that's where the name ionic comes from) are attracted very strongly to each other as a result of the opposite charges.

8 IONIC BONDS Chemical bonding that results from transfer of electrons and electrical attraction between cations and anions. Form between metals (on the left side of the Periodic Table) and non-metals (on the right side of Periodic Table) Form between metals (on the left side of the Periodic Table) and non-metals (on the right side of Periodic Table) Metal loses electron(s) and becomes positively charged…a cation. Metal loses electron(s) and becomes positively charged…a cation. Non-metal gains electron(s) and becomes negatively charged…an anion. Non-metal gains electron(s) and becomes negatively charged…an anion. The oppositely charged ions are held together because they are strongly attracted to each other. The oppositely charged ions are held together because they are strongly attracted to each other. Determine difference in electronegativity Determine difference in electronegativity –See page 161 –Difference of greater than 1.7 is often considered ionic –Ionic character greater than 50%

9 IONIC BOND? Cs and F? Cs and F? Page 161 electronegativity chart Page 161 electronegativity chart F en = 4.0 Cs en = is the electronegativity difference 3.3 is the electronegativity difference  A bond between Cs and F is ionic, greater than 1.7

10 IONIC COMPOUNDS Expressed as “formula units” rather than molecules because they form networks of ions attracted to each other…not independent neutral units. Expressed as “formula units” rather than molecules because they form networks of ions attracted to each other…not independent neutral units. Characteristics: Characteristics: –Crystalline structure –High melting points –Brittle –Dissolve in water –This separates the ions by breaking the ionic bonds…separated ions move freely making solution of ionic compounds good conductors of electricity. –Also conductive when melted.

11 Formula unit is NaCl, 1 sodium and 1 choride, the simplest ratio Ionic compounds form orderly arrangements of the ions, called crystal lattices.

12

13 COVALENT BONDS The dogs share the bones. There are two types of sharing: UNEQUAL EQUAL

14 COVALENT BONDS Bonds formed when electrons are shared   Covalent bonds form between non-metals (on the right side of the Periodic Table)   The electrons are shared by the atoms.   Electrons may be shared equally or unequally.   Molecules are formed because atoms sharing electrons must be near each other

15 COVALENT BONDS  Enormous variety of size, shape, physical, and chemical properties.  often lower melting point,  not necessarily crystalline  do not conduct electricity,  may or may not dissolve in water

16 POLAR COVALENT BONDS Unevenly matched dogs that are willing to share. These bonds can be thought of as two or more dogs that have different desire for bones. They share unequally.

17 POLAR COVALENT BONDS The bigger dog has more strength to possess a larger portion of the bones. Sharing still takes place but is an uneven sharing. The bigger dog has more strength to possess a larger portion of the bones. Sharing still takes place but is an uneven sharing.

18 POLAR COVALENT BONDS  In a covalent bond, the electrons are shared between atoms to fulfill the Octet Rule for both.  In a polar covalent bond, the electrons are shared unequally. They are attracted more to the more electronegative element.  The electrons spend more time near the more electronegative element, making it seem more negative and the other end of the molecule seem more positive…or “polar” Determine difference in electronegativity Determine difference in electronegativity –See page 161 –Difference of is often considered polar covalent –Ionic character 5-50%

19 POLAR COVALENT BOND? Unequal sharing of electrons Unequal sharing of electrons Partial negative charge Partial positive charge

20 POLAR COVALENT BOND? H and S? H and S? Page 161 Page 161 S en = 2.5 H en = is the electronegativity difference 0.4 is the electronegativity difference  A bond between H and S is polar covalent,.3-1.7

21 Polar molecules and polar bonds Partial positive charge towards less electronegative element Partial negative charge towards element with greater electronegativity. Two sides to the molecules/bonds S

22 Polar bonds lead to polar molecules

23 Polamolecules Polamolecules Calm, quiet, humble on the sideline Fearless, crazed, maniacal on the field Two sides to Troy Polamalu

24 Polar molecules and polar bonds Partial positive charge towards less electronegative element Partial negative charge towards element with greater electronegativity. Two sides to the molecules/bonds S

25 NON-POLAR COVALENT BONDS Covalent Bonds: Dogs of equal strength share equally. Covalent bonds can be thought of as two or more dogs with equal attraction to the bones. Covalent Bonds: Dogs of equal strength share equally. Covalent bonds can be thought of as two or more dogs with equal attraction to the bones. They share equally.

26 NON-POLAR COVALENT BONDS Since the dogs are identical, then the dogs share the bones evenly. Since one dog does not have more of the bone than the other dog, the bone is equally shared between both dogs. One dog does not have more than the other.

27 NON-POLAR COVALENT BONDS  In a covalent bond, the electrons are shared between atoms to fulfill the Octet Rule for both.  In a non-polar covalent bond, the electrons are shared equally.  The charge is evenly distributed between both atoms. The molecule is non-polar meaning one side does not have more charge than the other. Determine difference in electronegativity Determine difference in electronegativity –See page 161 –Difference of less than.3 is often considered nonpolar covalent –Ionic character less than 5 %

28 NON-POLAR COVALENT BOND? Cl and Br? Cl and Br? Page 161 for en Page 161 for en Cl en = 3.0 Br en = is the electronegativity difference 0.2 is the electronegativity difference  A bond between Cl and Br is non=polar covalent, less than 0.3

29 NON-POLAR COVALENT BOND? Equal sharing of electrons Equal sharing of electrons Equal distribution of charge Equal distribution of charge

30 Diatomic Elements They pair up! Br 2 Br 2 I 2 I 2 N 2 N 2 Cl 2 Cl 2 H 2 H 2 O 2 O 2 F 2 F 2 The diabolical DIATOMIC BrINClHOF TWINS!

31 METALLIC BONDS Mellow dogs with plenty of bones to go around. They don’t have to worry about sharing!

32 METALLIC BONDS These bonds are best imagined as a room full of puppies who have plenty of bones to go around and are not possessive of any one particular bone. This allows the electrons to move through the substance with little restriction. The model is often described as the "kernels of atoms in a sea of electrons.”

33 METALLIC BONDs  Electrons are “delocalized”  Electrons move freely around and between atoms in the network of empty orbitals. The electrons move through the substance with little restriction. The model is often described as the "kernels of atoms in a sea of electrons.”

34 METALLIC BONDS  The free movement of electrons explains the properties of metals…  Conductivity: free movement of electrons  Malleability & ductility: bonding is the same in all directions  Luster: energy released as electrons move easily between orbitals

35 We will look more closely at the different types of bonds to understand how millions of different compounds form from only about 100 different elements!

36 Lewis Dot of Ionic Compounds (electrons taken, not shared) Write the dot structures of the neutral atoms Write the dot structures of the neutral atoms Write the ions created by the loss or gain of electrons, in brackets. The metals have no dots. The non-metals have a full outer shell. Write the ions created by the loss or gain of electrons, in brackets. The metals have no dots. The non-metals have a full outer shell. Show the ion charges as superscripts. Show the ion charges as superscripts. Show the ratio of ions needed to create a net charge of zero. Show the ratio of ions needed to create a net charge of zero.

37 Criss-Cross Method to Determine Ratio of Ions Write the ion symbols and charges (superscripts). Get this info from periodic table for elements, polyatomic ion list for polyatomic ions. Write the ion symbols and charges (superscripts). Get this info from periodic table for elements, polyatomic ion list for polyatomic ions. Criss-cross the charge (numbers only) to subscripts. This shows the ratio of ions required for a net charge of zero. Criss-cross the charge (numbers only) to subscripts. This shows the ratio of ions required for a net charge of zero. Simplify. Mg 2+ N 3- Simplify. Mg 2+ N 3- Mg 3 N 2

38 Charges of ions from PT group ve or 8 charge /

39 Why does it work? The goal is a net charge of 0. The charges, or oxidation numbers, must add up to zero. Mg 3 N 2 3 x 2 + = x 3 - = net charge 0 net charge

40 Lewis Dot Structures of Covalent Bonds(shared electrons) Lewis dot structures show atoms near each other sharing pairs of electrons to create bonds and satisfy octet rule for both atoms. Lewis dot structures show atoms near each other sharing pairs of electrons to create bonds and satisfy octet rule for both atoms. They may share up to 3 pairs of electrons They may share up to 3 pairs of electrons –1 SHARED PAIR :SINGLE BOND –2 SHARED PAIRS ::DOUBLE BOND –3SHARED PAIRS :::TRIPLE BOND It does not matter which atom the electrons come from because they are shared. It does not matter which atom the electrons come from because they are shared.

41 Covalent bonds usually follow the octet rule Put all of the valence electrons in a pot and distribute them between the atoms so each atom has a full octet by sharing up to 3 pairs of electrons. Put all of the valence electrons in a pot and distribute them between the atoms so each atom has a full octet by sharing up to 3 pairs of electrons. : single bond :: double bond :: triple bond


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