Chemical Bonding and Molecular Geometry

1
Chemical Bonding and Molecular Geometry

• Lewis Symbols and the Octet Rule
• Ionic Bonding
• Covalent Bonding
• Molecular Geometry

2
Introduction

• Salt vs. Sugar

solutions conduct solutions dont
electricity conduct electricity
electrolyte non-electrolyte
ionic molecular
3
Chemical Bonds

• Sugar and salt differ in the type of attractive
  forces between the atoms/ions in the compound.
• Chemical bond strong attractive force that
  exists between atoms (or ions) in a compound
• ionic bonds
• covalent bonds
• metallic bonds

4
Chemical Bonds

• Ionic Bond the electrostatic force of
  attraction between oppositely charged ions in an
  ionic compound
• metal cation ()
• non-metal anion (-)
• The Na and Cl- ions in a salt (NaCl) crystal are
  held together by electrostatic attraction.

5
Chemical Bonds

• Covalent Bonds the attractive force between
  atoms in a molecule that results from sharing of
  one or more pairs of electrons
• non-metals
• H2O O
• H H
• Cl2 Cl Cl

H-O and Cl-Cl bonds result from sharing of
electrons
6
Lewis Symbols

• Electron configuration for sodium
• Ne3s1
• The 3s electron of a sodium atom is a valence
  electron.
• electrons residing in the incomplete outer shell
  of an atom
• involved in chemical bonding and ion formation

7
Lewis Symbols

• You must be able to determine the number of
  valence electrons for the main group elements.
• For main group elements, the number of valence
  electrons for an element group number of the
  element
• N (group 5A) has 5 valence electrons
• Br (group 7A) has 7 valence electrons

8
Lewis Symbols

• Since valence electrons are involved in the
  formation of chemical bonds, its important to
  keep track of them.
• electron-dot symbols or Lewis symbols
• simple way to depict valence electrons and track
  them during the formation of chemical bonds

9
Lewis Symbols

• Lewis symbol has two components
• chemical symbol for the element
• dot for each valence electron
• dots are placed on all 4 sides of the chemical
  symbol
• all four sides of the symbol are equivalent
• up to 2 dots (electrons) per side

10
Lewis Symbols

• Example Draw the Lewis symbol for oxygen.

Chemical symbol O Group number 6A of
valence electrons 6
11
Lewis Symbols

• Example Draw the Lewis symbol for silicon.

Chemical symbol Si Group number 4A of
valence electrons 4
12
Lewis Symbols

• Example Draw the Lewis symbol for argon.

Chemical symbol Ar Group number 8A of
valence electrons 8
13
Octet Rule

• The noble gases are particularly stable because
  their outer shell is full of electrons.
• With the exception of He, all noble gases have 8
  valence electrons.
• ns2np6
• Octet Rule Atoms tend to gain, lose, or share
  electrons until they are surrounded by 8 valence
  electrons

14
Octet Rule

• The octet rule can be used to predict the charge
  of ions formed by main group elements as well as
  the structure of molecular compounds.
• There are many exceptions to the octet rule.

15
Ionic Bonding

• Ionic compounds form when
• Electrons are transferred from one atom to
  another
• 2 Na (s) Cl2 (g) ? 2 NaCl (s)
• 2 Na ? 2 Na 2 e-
• Cl2 2 e- ? 2 Cl-
• Resulting cation and anion are attracted to each
  other due to opposite charges
• Energy is released when ions form a solid array
  or lattice

16
Ionic Bonding

• The stability of the solid array or lattice is
  given by the lattice energy.
• The energy required to completely separate a mole
  of a solid ionic compound into its gaseous ions
• The greater the lattice energy, the more stable
  the ionic compound will be
• Harder to separate the ions

17
Ionic Bonding

• Lattice energy depends on the charges of the ions
  and the size of the ions.
• As charge of the ions increases, lattice energy
  increases
• As size of the ions increases, lattice energy
  decreases.

18
Covalent Bonding

• Octet Rule Atoms tend to gain, lose or share
  electrons until they are surrounded by eight
  valence electrons.
• When ionic compounds are formed, electrons are
  gained or lost.
• When molecular compounds are formed, electrons
  are shared.

19
Covalent Bonding

• Molecular compounds are held together by covalent
  bonds that result from the sharing of electrons.
• Simplest example of a covalent bond is
• H H

20
Covalent Bonding

• When 2 H atoms approach each other, electrostatic
  interactions occur between their respective
  electrons and their nuclei.
• The two nuclei repel each other
• The two electrons repel each other
• The nuclei and the electrons attract each other.

21
Covalent Bonding
electron
attraction
nucleus
Repulsion
22
Covalent Bonding

• The attractions between the nuclei and the
  electrons cause the electron density to
  concentrate between the two nuclei.
• The atoms in H2 are held
• together by the electrostatic attraction of the
  two nuclei for the concentration of negative
  charge between them.

23
Covalent Bonding

• The shared pair of electrons between the two
  nuclei act as glue to hold the atoms together
  in the H2 molecule.

24
Covalent Bonding

• Lewis structures (also called electron-dot
  structures) can be used to represent the covalent
  bonds that are present in a molecule.
• Symbol for each atom
• Bond between atoms depicted using a solid line
• Unshared electron pairs are shown around the
  appropriate atom

25
Covalent Bonding

• The formation of H2

H H ? H H or H H

• The Lewis structure for Cl2

26
Covalent Bonding

• The Lewis structure for HF

27
Covalent Bonding

• The bond between H and F in HF is called a single
  bond
• sharing of one pair of electrons
• In some molecules, atoms attain an octet of
  electrons by sharing more than one pair of
  electrons.
• Double bond
• Triple bond

28
Covalent Bonding

• Double bond two electron pairs are shared
  between atoms
• depicted using two lines to represent the two
  shared electron pairs
• O C O or O C O

Carbon dioxide
29
Covalent Bonding

• Triple bond three electron pairs are shared
  between atoms
• depicted using three lines to represent the 3
  pairs of shared electrons
• N N N N or N N

Nitrogen (N2)
30
Covalent Bonding

• In some molecular compounds, the bonding
  electrons are shared equally between the atoms in
  the molecule
• H2 Cl2 N2
• Nonpolar covalent bond bonding electrons are
  shared equally

31
Covalent Bonding

• In many molecular compounds, however, one atom
  attracts the bonding electrons more strongly than
  the other.
• H
• O
• H

d
Oxygen attracts electrons more strongly than
hydrogen.
d-
d
32
Covalent Bonding

• Polar covalent bond
• a chemical bond in which the electrons are not
  shared equally
• one atom attracts the bonding electrons more
  strongly
• If the difference in relative ability to attract
  electrons is large enough, an ionic bond is
  formed.

33
Covalent Bonding

• You can think of chemical bonds as existing along
  a continuum

Polar Covalent Bonds
Nonpolar Covalent Bonds
Ionic Bonds
34
Covalent Bonding

• The ability of an atom in a molecule to attract
  electrons to itself is called electronegativity.
• Range 0.7 (Cs) - 4.0 (F)
• As electronegativity increases, the attraction
  that an atom has for electrons increases.

35
Covalent Bonding

• Trends to know
• The four most electronegative elements are
• F (4.0)
• O (3.5)
• N (3.0)
• Cl (3.0)

36
Covalent Bonding

• The difference in electronegativity between two
  atoms can be used to determine the relative
  polarity of the chemical bond between them.
• The greater the difference in electronegativity
  between two atoms, the more polar their bond.

37
Covalent Bonding

• Difference in
• Type of Bond Electronegativity
• Ionic gt 2.0
• Polar covalent 0.5 to lt2.0
• Nonpolar covalent lt 0.5

38
Covalent Bonding

• Water contains polar covalent bonds
• H
• O
• H

d

• Electronegativity
• O 3.5
• H 2.1

d-
Difference 1.4
d
In a water molecule, the electron density is
drawn away from the H atoms and towards the more
electronegative O atom.
39
Covalent Bonding

• Example Which of the following bonds is the
  most polar Cl-S, Cl-P, or Al-Cl?

The most polar bond will be the one in which the
atoms have the greatest difference in
electronegativity.
Electronegativity decreases as elements move
toward the left (away from the halogens).
Al-Cl