Covalent Bonding

1
Covalent Bonding

• Chapter 9

2
The Covalent BondSection 9.1

• Why do atoms bond?
• To achieve full outer electron shells
• Octet Rule- atoms gain, lose, or share electrons
  to achieve the electron configuration of noble
  gases
• Gain and Lose? IONIC BONDING
• Share ? COVALENT BONDING

3
What is a covalent bond?

• A chemical bond that results in the SHARING of
  valence electrons
• Occurs between 2 or more nonmetals

4

• A molecule is formed when two or more atoms bond
  covalently
• Examples sugars, DNA, proteins, fats,
  carbohydrates, cotton, synthetic fibers

5
Formation of a covalent bond

• REMEMBER Hydrogen (H2), Nitrogen (N2), Oxygen
  (O2), Fluorine (F2),Chlorine (Cl2) Bromine (Br2)
  and Iodine (I2) occur in nature as diatomic
  molecules

6
Covalent Bonding

• An attractive force occurs between the protons of
  one atom and the electrons of the other atom
• When a single pair of electrons is shared, such
  as in the hydrogen molecule, a single covalent
  bond forms

7
Lewis Structures

• Use electron-dot diagrams to show how electrons
  are arranged in molecules

8

• Group 7A (Halogens) have 7 VE
• One more VE is necessary
• A single covalent bond will form
• Group 6A have 6 VE
• Two more VE are necessary
• Two covalent bonds will form
• Group 5A have 5 VE
• Three more VE are necessary
• Three covalent bonds will form
• Group 4A have 4 VE
• Four more VE are necessary
• Four covalent bond will form

9

• Sigma Bonds- single covalent bonds- when electron
  pairs are centered between two atoms
• Multiple Bonds
• In many molecules, atoms attain noble gas
  configuration by sharing more than one pair of
  electrons between two atoms
• Carbon, Nitrogen, Oxygen, and Sulfur most often
  form multiple bonds

10
Strength of Covalent Bonds

• The strength of covalent bonds depends on how
  much distance separates both nuclei
• The distance between the two bonding nuclei at
  the position of maximum attraction is called bond
  length
• Determined by the size of the atoms and how many
  electron pairs are shared
• Bond length decreases as the number of bonds
  increases (triple bond has a shorter bond length
  than a single bond)

11
Energy Changes

• An energy change accompanies the forming or
  breaking of a bond between atoms in a molecule.
• Energy is released when a bond forms
• Energy must be added to break the bonds of a
  molecule
• The amount of energy required to break a specific
  covalent bond is called bond dissociation energy

12

• Bond dissociation energy indicates the strength
  of a chemical bond because a direct relationship
  exists between bond energy and bond length
• In chemical reactions, bond in reactant molecules
  are broken and new bonds are formed as product
  molecules form

13

• Endothermic reactions occur when a greater amount
  of energy is required to break the existing bonds
  in the reactants than is released when the new
  bonds from in the product molecules
• Exothermic reactions occur when more energy is
  released forming new bonds than is required to
  break bonds in the initial reactants

14
Checkpoint

• What is a covalent bond? How does it differ from
  an ionic bond?
• What type of elements form covalent bonds?
• Draw the Lewis Structures for each of these
  molecules
• PH3
• H2S
• CCl4

15
Naming Molecules Section 9.2

• Naming Binary Molecular Compounds
• The first element in the formula is always named
  first, using the entire element name.
• The second element in the formula is named using
  the root of the element and adding the suffix
  ide.
• Prefixes are used to indicate the number of atoms
  of each type that are present in the compound.

16
(No Transcript)
17
Practice Naming

• CCl4
• As2O3
• CO
• SO2
• NF3

18
Naming Acids

• Binary Acids
• Use the prefix hydro- to name the hydrogen part
  of the compound
• The rest of the name consists of a form of the
  root of the second element plus the suffix ic,
  followed by the word acid.
• Examples
• HCl- Hydrochloric Acid
• HCN? Hydrocyanic acid (even though there are more
  than 2 elements present, if no oxygen is present-
  the acid is named as a binary)

19
Oxyacids

• Acids that contain an oxyanion (polyatomic ion
  that contains oxygen)
• The name of the oxyacids consists of a form of
  the root of the anion, a suffix, and the word
  acid
• If the anion suffix is ate, it is replaced with
  the suffix ic
• If the anion suffix is ite, it is replaced with
  the suffix ous.

20
Practice Problems

• HI
• HClO3
• HClO2
• H2SO4
• H2S

21
Checkpoint

• Write the molecular formula for each of the
  following compounds
• Disulfur trioxide
• Iodic acid
• Dinitrogen monoxide
• Hydrofluoric acid
• Phosphorus pentachloride
• What is the difference between a binary acid and
  oxyacid?
• Complete the following table ?

22
Molecular Structures Section 9.3

• Structural Formula- uses letter symbols and bonds
  to show relative positions of atoms
• Lewis Structure Procedure
• 1. Predict the location of certain atoms
• Hydrogen is always a terminal, or end, atom.
  Because it can share only one pair of electrons,
  hydrogen can be connected to only one other atom
• The atom with the least attraction got shared
  electrons in the molecule is the central atom.
  This element usually is the one closer to the
  left on the periodic table. The central atom is
  located in the center of the molecule, and all
  other atoms become terminal atoms.

23

• 2. Find the total number of electrons available
  for bonding. This total is the number of valence
  electrons in the atoms in the molecule.
• 3. Determine the number of bonding pairs by
  dividing the number of electrons available for
  bonding by two.
• 4. Place one bonding pair (single bond) between
  the central atom and each of the terminal atoms.

24

• 5. Subtract the number of pairs you used in step
  4 from the number of bonding pairs you determined
  in step 3. The remaining electron pairs include
  lone pairs as well as pairs used in double and
  triple bonds. Place lone pairs around each
  terminal atom bonded to the central atom to
  satisfy the octet rule. Any remaining pairs are
  assigned to the central atom.

25

• 6. If the central atom is not surrounded by 4
  electron pairs, it does not have an octet. You
  must convert one or two of the lone pairs on the
  terminal atoms to a double bond or a triple bond
  between the terminal atom and the central atom.
  These pairs are still associated with the
  terminal atom as well as with the central atom.
  Remember that, in general, carbon, nitrogen,
  oxygen, and sulfur can form double or triple
  bonds with the same element or with another
  element.

26
Space-filling Structure
Lewis Structure
Ball-and-stick molecular model
27
Examples

• NH3
• PO4-3

• CO2
• BH3

28
Resonance Structures

• Resonance is a condition that occurs when more
  than one valid Lewis structure can be written for
  a molecule or ion
• Nitrate Ion Resonance Structures

29
Exceptions to the Octet Rule

• A small group of molecules has an odd number of
  valence electrons and cannot form an octet around
  each atom. (i.e. NO2 , ClO2, and NO)
• Some compounds form with fewer than eight
  electrons present around an atom. (Example
  Boron)
• When one atom donates a pair of electrons to be
  shared with an atom or ion that needs two
  electrons to become stable, a coordinate covalent
  bond forms

30

• 3. Some central atoms contain more than eight
  valence electrons (expanded octet)
• Examples PCl5, SF6, and XeF4

31
Molecular ShapeSection 9.4

• The shape of the molecule determines many of its
  physical and chemical properties.
• Molecular shape is determined by the overlap of
  orbitals that share electrons
• Valence Shell Electron Pair Repulsion model or
  VSEPR model
• Based on an arrangement that minimizes the
  repulsion of shared and unshared pairs of
  electrons around the central atom

32
VSEPR Model

• The angle formed by any two terminal atoms and
  the central atom is a bond angle.
• Shared electron pairs repel one another
• Lone pairs of electrons occupy a slightly larger
  orbital than shared electrons
• Shared bonding orbitals are pushed slightly
  together by lone pairs

33
Look at the VSEPR cheat sheet ?
34
Hybridization

• A hybrid results from combining two of the same
  type of object, and it has characteristics of
  both.
• Hybridization- a process in which atomic orbitals
  are mixed to form new, identical hybrid orbitals.
  

35
Practice Problems

• Determine the molecular geometry and bond angle
  for the following
• BF3
• NH4
• OCl2
• BeF2
• CF4

36
Electronegativity and Polarity Section 9.5

• Electron affinity is a measure of the tendency of
  an atom to accept an electron

37

• The character and type of a chemical bond can be
  predicted using the electronegativity difference
  of the elements that are bonded
• Polar Covalent- Unequal sharing
• Nonpolar Covalent- Equal sharing

Generally- Ionic bond form when the
electronegativity difference is greater than 1.70
38
Polar Covalent Bonds

• Polar covalent bonds form because not all atoms
  that share electrons attract them equally
• The shared pair of electrons is pulled toward one
  of the atoms
• Partial charges occur at the ends of the bond
• Partially negative?
• Partially positive ?
• The resulting polar bond is referred to as a
  dipole (two poles)

39
Molecular Polarity

• Molecules are either polar or nonpolar, depending
  on the location and nature of the covalent bonds
  they contain.
• A polar molecule has a partial negative charge on
  one side, while the other side of the molecule
  has a partial positive charge

40
Polar Molecule or not?

• Compare H2O and CCl4

41
Solubility of polar molecules

• The ability of a substance to dissolve in another
  substance is known as the physical property
  solubility
• The bond type and the shape of the molecules
  present determine solubility
• Likes dissolve likes
• Polar compounds are usually soluble in polar
  substances
• Nonpolar molecules only dissolve in nonpolar
  substances

42
Properties of Covalent Compounds

• Lower melting and boiling points (indicating weak
  bond strength)
• Many are liquids or gases at room temperature
• Do not conduct electricity
• Many do not dissolve in water (polar)

43
Practice Problems

• Decide whether each of the following molecules is
  polar or nonpolar
• SCl2
• H2S
• CF4
• CS2