Bonding

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Bonding

• Forces of attraction that hold atoms together
  making compounds

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Chemical symbols

• Symbols are used to represent elements
• Either one capital letter, or a capital letter
  with a lower case letter
• Know names and symbols of elements
• 1 30, plus
• Rb, Cs, Sr, Ba, Ag, Au, Cd, Hg, Pt, Ga, Ge, As,
  Sn, Pb, Se, Br, I, and U

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Basic idea...

• All chemical bonds form because they impart
  stability to the atoms involved
• lower energy greater stability

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Quick review

• All types of chemical bonds involve electrons
• Valence electrons, the electrons in the outermost
  occupied energy level of an atom, are usually the
  electrons involved in bonding

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• The representative elements have the same number
  of valence electrons as their family number in
  the American system
• Example Mg, column IIA, 2 valence electrons
• The transition metals all have two valence
  electrons
• ns2(n-1)dx

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• Lewis dot structures are used to represent the
  valence electrons
• each dot represents a valence electron
• no more than 8 dots total
• no more than 2 dots on a side

.
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Lewis dot structures of representative elements
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The Octet Rule

• Atoms will gain, lose, or share electrons in
  order to achieve an ns2np6 valence configuration
  8 valence electrons

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cations

• Atoms that have lost one or more electrons
• Become positive () ions
• Usually the metals
• Only have 1,2 or 3 valence electrons

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anions

• Atoms that have gained one or more electrons
• Become negative (-) ions
• Usually nonmetals
• Have 5, 6, or 7 valence electrons
• Gain enough electrons to end up with 8 valence
  electrons

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• Yellow metals
• Green semimetallics
• Blue purple nonmetals

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Ionic Bonding

• Metals lose electrons easily, nonmetals have a
  strong attraction for more electrons
• metal atoms will lose electrons to nonmetal
  atoms, causing both to become ions

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• Metals, having lost one or more electrons, become
  cations ()
• Nonmetals, having gained one or more electrons,
  become anions (-)
• Opposites attract the cations and anions are
  held together electrostaticly
• termed ionic bonds

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In summary...

• Ionic bonds are electrostatic attractions between
  cations and anions that are formed when
  electron(s) are transferred from the metal to the
  nonmetal.

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Ionic Compounds

• Only exist as compounds as solids
• Held together by ionic bonds
• - A strong attractive force
• High melting points
• - Must break the bonds to melt the solid
• - The higher the melting point, the stronger
  the ionic bonds

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Ionic Compounds

• 4. brittle solids
• 5. nonconducting as solids
• - No charges can flow
• 6. conduct electricity as liquids or aqueous
• - Ions are free to move

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Ionic Compounds

• As solids, exist in a 3-D repeating pattern
  called a crystal lattice
• the lattice energy is the energy lowering
  (stability) accomplished by the formation from
  free ions
• Also a measure of the energy required to break
  apart the ionic compound once formed
• The greater the lattice energy, the stronger the
  force of attraction

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Ionic compound crystalline solid
Cation ()
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Ion dissociation

• Many ionic compounds will dissolve in water if it
  results in more stability (lower E) than in the
  solid ionic compound
• the ions dissociate from each other
• Ex CaCl2(s) H2O ? Ca2(aq) 2Cl-(aq)

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Ionic Bond Strength

• A measure of the attractive force between the
  ions
• smaller atoms stronger ionic bonds
• fewer atom ratio stronger bond
• evidence melting points

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Compare the melting points

• KCl 776oC
• KI 723oC
• smaller atoms result in stronger ionic bonds

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Compare the melting points

• CaCl2 772oC
• NaCl 800oC
• fewer atoms result in stronger ionic bonds

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Periodic trends
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Sizes of atoms

• Periodic trend atomic radii increase moving down
  a group
• Increasing energy level
• Periodic trend atomic radii decrease moving left
  to right in a period
• The charge felt by the valence electrons becomes
  larger

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Sizes of atoms

• There is a general decrease in atomic radius from
  left to right, caused by increasing positive
  charge in the nucleus.

• Valence electrons are not shielded from the
  increasing nuclear charge because no additional
  electrons come between the nucleus and the
  valence electrons.

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• For metals, atomic radius is half the distance
  between adjacent nuclei in a crystal of the
  element.

• For elements that occur as molecules, the atomic
  radius is half the distance between nuclei of
  identical atoms.

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Atomic Radius
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Atomic Radius

• Atomic radius generally increases as you move
  down a group.

• The outermost orbital size increases down a
  group, making the atom larger.

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Ionization energy

• The energy needed to remove a valence electron
  from an atom
• A measure of how tightly the electrons are being
  held
• periodic trend
• increases from the bottom up
• increases left to right

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• In general, metals have lower IE than nonmetals
• alkali metals are the lowest IE family
• noble gases are highest IE family

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Ionization energy

• The energy required to remove the first electron
  is called the first ionization energy.

• First ionization energy increases from left to
  right across a period.
• First ionization energy decreases down a group
  because atomic size increases and less energy is
  required to remove an electron farther from the
  nucleus.

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Ionization energy
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Ionization energy
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Electron affinity

• A measure of how strongly an element would like
  to gain an electron
• periodic trend
• increases from the bottom up
• increases left to right
• ignore the noble gases

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• Atoms that lose electrons easily have little
  attraction for additional electrons (and vice
  versa)
• metals have low IE, low EA
• Nonmetals have high IE, high EA
• Octet rule when atoms react, they tend to
  strive to achieve a configuration having 8
  valence electrons
• This results in some form of bond formation

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Periodic trends

• As you move from left to right along a period
• Atoms get
• . Smaller
• Ionization energy goes
• . Up
• Electron affinity goes
• . Up

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Periodic trends

• As you move down a group/family
• Atoms get
• . Larger
• Ionization energy goes
• . Down
• Electron affinity goes
• . Down

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Check your understanding
The lowest ionization energy is the ____.
A. first B. second C. third D. fourth
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Bonding

• Forces of attraction that hold atoms together
  making compounds

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Types of compounds

• All compounds are made of two or more elements
  held together by chemical bonds
• Ions of opposite charges are held together by
  ionic bonds
• Ionic bonding is non-directional
• There are no ionic molecules
• Formulas of ionic compounds show the ratio of
  cation to anion
• Ionic compounds only exist in the solid state, in
  a 3-D crystal lattice

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Covalent Bonding

• Covalent bonding involves the sharing of electron
  pairs
• usually between two high EA, high IE nonmetals
• both want more e-s, neither is willing to lose
  the e-s they have

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Covalent Bonding

• A nonmetal will form as many covalent bonds as
  necessary to fulfill the octet rule
• example C, with 4 valence e-s, will form 4
  covalent bonds
• results in 8 valence e-s around the carbon atom
  at least part of the time
• double and triple covalent bonding is a
  possibility

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Types of compounds

• Covalent compounds are made of two or more
  elements held together by covalent bonds
• Covalent bonding is directional
• Between two individual atoms
• A group of covalently bonded atoms is referred to
  as a molecule
• Covalent compounds are also referred to as
  molecular compounds

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When does the octet rule fail?
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H, He and Li

• Helium strives for 2 valence electrons
• 1s2 configuration
• Hydrogen will sometimes will share its one
  electron with another atom, forming a single
  covalent bond
• Lithium will lose its lone valence electron,
  gaining the 1s2 configuration of He

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Be

• Be will sometimes lose its 2 valence electrons,
  gaining the Is2 configuration of He
• Be will sometimes form 2 covalent bonds, giving
  it 4 valence electrons
• nuclear charge of 4 cannot handle 8 valence
  electrons

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B

• Boron will often make three covalent bonds using
  its three valence electrons
• nuclear charge of 5 cannot handle 8 valence
  electrons in a stable manner

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organometallic compounds

• Some metals will form covalent compounds with
  nonmetals
• Hg, Ga, Sn, and others
• The octet rule is not followed for the metals,but
  is for nonmetals
• Form 2 or more covalent bonds

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P, S, Cl, Se, Br, I

• Elements in the third period and lower have empty
  d orbitals
• there is room for more than 8 valence electrons
• These elements will at times make more than 4
  covalent bonds

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Rules for Drawing structural formulas

• 1) Determine the central atom, place the other
  atoms evenly spaced around the outside
• 2) Count the total number of valence electrons
• 3) Draw single bonds between the central atoms
  and each of the outside atoms

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• 4) Complete the octet on the outside atoms by
  placing electrons in pairs around the outside
  atoms (lone pairs)
• 5) Place any remaining electrons on the central
  atom in pairs
• 6) If the central atom does not have its minimum
  number of electrons (usually 8), form double
  bonds by moving lone pairs off of the outside
  atoms and drawing them as bonding pairs
• never make double bonds until ALL of the
  electrons are used first!!