Chemistry Lecture 4

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Two Classes of Bonding
Ionic Bonding Result when electrons are completely transferred from the atoms of one element to those of another element.
Covalent Bonding Result from the sharing of electrons between atoms of different elements.
(American Chemis ago, Linus Pauling developed the most common scale of Electronegativity (EN) values for the elements.
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Electronegativity Values
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Electronegativity Values
• Non-metals are more electronegative than metals. • The most electronegative element is Fluorine, with Oxygen a close second. • When Oxygen bonds with elements, other than Fluorine, it always pulls electrons towards itself. To denote the electropositive and electronegative aspect of a molecule, we can use the following notation. E.g. for HF, or
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The Continuum of Bonding
Q. Is an X—Y bond ionic or covalent?
Answer: Slightly both! Better to ask: Q. To what extent is the X—Y bond ionic or covalent?
Q. What about metallic bonding?
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Metallic Bonding
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Metallic Bonding
Covalent Compounds Result from the sharing of electrons between atoms of different elements.
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Covalent Compounds
E.g. H2O (Water)
• Form when elements share electrons, which usually occurs between non-metals. • Two Hydrogen atoms (non-metal atoms) each share an electron with one Oxygen atom (non-metal atom).
o The LiF(s) bond would be considered ionic. o The HF(g or l) bond would be considered to be polar covalent. o The F2 (g) bond is considered to be mostly non-polar covalent.
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The Continuum of Bonding Across a Period
Sodium Chloride
Silicon tetrachloride
Chlorine
N.B.
Why the properties of the period 3 chlorides change from solid to liquid to gas, as ΔEN becomes smaller, will become more apparent when we study intermolecular forces.
Phosphorous trichloride
Disulfur dichloride
Chlorine
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The Continuum of Bonding Across a Period
Sodium Chloride
Silicon tetrachloride
Chlorine
N.B.
As ΔEN becomes smaller, the bond becomes more covalent, and the properties of the period 3 chlorides change from those of a solid consisting of ions to those of a gas consisting of individual molecules.
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The Continuum of Bonding
ΔEN > 2.0 0.3-2.0 < 0.3 Ionic Character Mostly Ionic Polar Covalent Non-Polar Covalent
N.B. Electron sharing occurs to some extent in every bond, even one between an alkali metal and a halogen.
• The bond in LiF has more ionic character than the HF bond, which has more ionic character than the F2 bond.
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The Continuum of Bonding
Q. To what extent is the X—Y bond ionic or covalent?
> 2.0
0.3-2.0
Mostly Ionic
Polar Covalent
< 0.3
Non-Polar Covalent
For example • ΔEN for LiF(s) is: 4.0 – 1.0 = 3.0; • ΔEN for HF(g or l) is: 4.0 – 2.1 = 1.9; • ΔEN for F2 (g) is: 4.0 – 4.0 = 0
The Continuum of Bonding
ΔEN > 2.0 0.3-2.0 < 0.3 Ionic Character Mostly Ionic Polar Covalent Non-Polar Covalent
For example • ΔEN for LiF(s) is: 4.0 – 1.0 = 3.0; • ΔEN for HF(g or l) is: 4.0 – 2.1 = 1.9; • ΔEN for F 2 (g) is: 4.0 – 4.0 = 0
Summary of Lecture 3
1. 2. 3. 4. The Periodic Table Valency Ionic compounds Electronegativity
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Aims of Lecture 4
1. 2. 3. 4. The continuum of bonding Metallic bonding Covalent bonding Lewis structures
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The Continuum of Bonding
Q. To what extent is the X—Y bond ionic or covalent?
• A greater electronegativity difference (ΔEN) results in larger partial charges and a higher partial ionic character. For example • ΔEN for LiF(s) is: 4.0 – 1.0 = 3.0; • ΔEN for HF(g or l) is: 4.0 – 2.1 = 1.9; • ΔEN for F 2 (g) is: 4.0 – 4.0 = 0
o Thus, the bond in LiF has more ionic character than the HF bond, which has more ionic character than the F2 bond .
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The Continuum of Bonding
ΔEN Ionic Character
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Why Partially Ionic or Partially Covalent? Electronegativity
•
A very important concept in chemistry.
• It is the relative ability of a bonded atom to attract the shared electrons in a chemical bond.
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Two Classes of Molecules/Compounds
Ionic Compounds Result when electrons are completely transferred from the atoms of one element to those of another element.
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The Continuum of Bonding
N.B. • ΔEN values can be used to divide chemical bonds into ionic, polar covalent or non-polar covalent. • We will look at the terms polar covalent and non-polar covalent in more detail in future lectures, so do not worry if you do not understand these terms yet. 13