Ch 3

• Chemical bond


  The force of attraction between any two atoms in a compound
• This attractive force overcomes the repulsion of the positively charged nuclei of the two atoms participating in the bond
• Interactions involving valence electrons are responsible for the chemical bond
• Lewis symbol


  A way to represent atoms using the element symbol and valence electrons as dots
• As only valence electrons participate in bonding, this makes it much easier to work with the octet rule
• The number of dots used corresponds directly to the number of valence electrons located in the outermost shell of the atoms of the element
• Writing Lewis symbols

  1. Place one dot on each side until there are four dots around the symbol

  2. Then add a second dot to each side in turn

  3. The number of valence electrons limits the number of dots placed

  4. Each unpaired dot (unpaired valence electron) is available to form a chemical bond
• Principal Types of Chemical Bonds

  • Ionic
  • Covalent
• Ionic bond


  Attractive force due to the transfer of one or more electrons from one atom to another
• Covalent bond


  Attractive force due to the sharing of electrons between atoms
• Some bonds have characteristics of both types and are not easily identified as one or the other
• Ionic bonding

  • Representative elements form ions that obey the octet rule
  • Electrons are lost by a metal and they are gained by a nonmetal
  • Each atom achieves a "noble gas" configuration
  • 2 ions are formed - a cation and anion, which are attracted to each other
  • Ions of opposite charge attract each other creating the ionic bond
• Ionic Bonding: NaCl

  1. Na + Cl → NaCl

  2. Sodium has a low ionization energy (it readily loses its electron)

  3. When sodium loses the electron, it gains the Ne configuration

  4. Chlorine has a high electron affinity

  5. When chlorine gains an electron, it gains the Ar configuration
• Ionic bonding

  • Metals tend to form cations because they have low ionization energies and low electron affinities
  • Nonmetals tend to form anions because they have high ionization energies and high electron affinities
  • Ions are formed by the transfer of electrons
  • The oppositely charged ions formed are held together by an electrostatic force
  • Reactions between metals and nonmetals tend to form ionic compounds
• Ion Arrangement in a Crystal

  1. As a sodium atom loses one electron, it becomes a smaller sodium ion

  2. When a chlorine atom gains that electron, it becomes a larger chloride ion

  3. Attraction of the Na cation with the Cl anion forms NaCl ion pairs that aggregate into a crystal lattice
• Covalent Bonding

  1. Consider the formation of H2

  2. H + H → H2

  3. Each hydrogen has one electron in its valance shell

  4. If it were an ionic bond it would look like this

  5. However, both hydrogen atoms have an equal tendency to gain or lose electrons

  6. Electron transfer from one H to another usually will not occur under normal conditions
• Covalent Bond


  • Each atom attains a noble gas configuration by sharing electrons
  • The shared Electron pair is called a covalent bond
• Covalent bonds form between atoms with similar tendencies to gain or lose electrons
• Compounds containing covalent bonds are called covalent compounds or molecules
• The diatomic elements have completely covalent bonds (totally equal sharing)


  • H2, N2, O2, F2, Cl2, Br2, I2
• Each fluorine is surrounded by 8 electrons – Ne configuration
• Polar Covalent Bonding


  Bonds made up of unequally shared electron pairs
• Polar Covalent Bond

  1. Hydrogen is somewhat positively charged

  2. Fluorine is somewhat negatively charged

  3. The two electrons between H and F are not shared equally

  4. The electrons spend more time with fluorine

  5. This sets up a polar covalent bond

  6. A truly covalent bond can only occur when both atoms are identical
• Polar Covalent Bonding in HF

  1. Fluorine is electron rich

  2. Hydrogen is electron deficient

  3. This results in unequal sharing of electrons in the pairs = polar covalent bonds
• Electronegativity


  A measure of the ability of an atom to attract electrons in a chemical bond
• Elements with high electronegativity have a greater ability to attract electrons than do elements with low electronegativity
• The difference in electronegativity determines the extent of bond polarity
• Electronegativities of Selected Elements

  • The most electronegative elements are found in the upper right corner of the periodic table
  • The least electronegative elements are found in the lower left corner of the periodic table
• Electronegativity Calculations

  1. The greater the difference in electronegativity between two atoms, the greater the polarity of their bond

  2. H-F . . . 4.0 − 2.2 = 1.8

  3. H-Cl . . . 3.2 − 2.2 = 1.0

  4. The HF bond is more polar than the HCl bond
• Nomenclature


  The assignment of a correct and unambiguous name to each and every chemical compound
• Two naming systems

  • Ionic compounds
  • Covalent compounds
• Formula

  The representation of the fundamental compound using chemical symbols and numerical subscripts
• The formula identifies the number and type of the various atoms that make up the compound unit
• The number of like atoms in the unit is shown by the use of a subscript
• Presence of only one atom is understood when no subscript is present
• Ionic Compounds

  • Metals and nonmetals usually react to form ionic compounds
  • The metals are cations and the nonmetals are anions
  • The cations and anions arrange themselves in a regular three-dimensional repeating array called a crystal lattice
  • Formula of an ionic compound is the smallest whole-number ratio of ions in the substance
• Writing Formulas of Ionic Compounds from the Identities of the Component Ions


  1. Determine the charge of each ion

  2. Metals have a charge equal to group number

  3. Nonmetals have a charge equal to the group number minus eight

  4. Cations and anions must combine to give a formula with a net charge of zero

  5. It must have the same number of positive charges as negative charges
• Writing Names of Ionic Compounds from the Formula of the Compound 1

  1. Name the cation followed by the name of the anion

  2. A positive ion retains the name of the element; change the anion suffix to -ide
• Writing Names of Ionic Compounds from the Formula of the Compound 2

  1. If the cation of an element has several ions of different charges (as with transition metals) use a Roman numeral following the metal name

  2. Roman numerals give the charge of the metal
• Common Nomenclature System


  • Use -ic to indicate the higher of the charges that ion might have
  • Use -ous to indicate the lower of the charges that ion might have