Unit 3 Part I: Chemical Bonds
1. What is a chemical bond?
 Chemical bonds are the forces that hold atoms together. They are a result of either the
attraction between a positive and negative ion, or the attraction between the nucleus
and electrons.
*Electrostatic force
2. Octet Rule
 Valence electrons are involved in forming chemical bonds.
*The valence electrons determine the reactivity of various elements.
 Octet rule: Atoms tend to gain, lose, or share electrons in order to acquire a full set of
valence electrons (like noble gasses!)
o Most stable
o ns2np6

Exceptions
o Period 1: Stable with only 2 electrons (H and He)
o Pseudo noble gas notation: Atom loses or gains electrons to form an outer
energy level containing full s, p, and d sublevels.
 Transition metals
 Example: Zn
Examples:
3. Types of Chemical Bonds
1. Ionic bonds: Transfer electrons
2. Covalent bonds: sharing electrons
o Nonpolar covalent
o Polar covalent
3. Metallic bonds: Sharing delocalized electrons
 Types of bonds are determined by electronegativity. If the difference in
electronegativity between two atoms is
o <0.5 forms a nonpolar covalent bond
o 0.5-1.7 forms a polar covalent bond
o ≥1.7 forms an ionic bond
Examples
4. Ionic Bonds
 Ionic bond is a kind of chemical bond between a cation and an anion
 It is the electrostatic force that holds oppositely charged particles together
 Ionic compounds are compounds held together by ionic bonds
 Ions
o Some elements lose electrons to reach a stable octet. They form ___________.
 Metals tend to lose electrons (low EN)
o Some elements gain electrons to reach a stable octet. They form ___________.
 Nonmetals tend to gain electrons (high EN)
o
Examples
Examples: How would the following elements combine to form ionic compounds?
5. Properties of Ionic Compounds
The types of chemical bond determines the physical properties of the compound
 Solids at room temperature
 Very strong bonds
 High melting point
o Ionic bonds are very strong so lots of energy is needed to break them apart
 Hard, brittle
 Conducts electricity when in liquid state or when dissolved in water
o For a material to conduct an electri current, there must be charged particles that
can move.
o Ionic compounds in a liquid state or an aqueous solution have ions that are free
to move
o Electrolyte = aqueous solution of an ionic compound that conducts electricity
 Has a geometric shape called crystal lattice
o Ionic compounds form crystals
o Positive and negative ions are packed into a regular, repeating pattern that
balances the attraction and repulsion between ions
o This results in a crystal lattice: a 3-dimensional geometric arrangement of
particles
 Each positive ion is surrounded by negative ions
 Each negative ion is surrounded by positive ions
6. Energy of ionic bonds
The attraction of positive and negative ions form a more stable system
 When ionic bonds form, energy is released = exothermic
 Lattice (formation) energy = energy released when one mole of an ionic compound is
formed from atoms in its gaseous state
o More negative lattice energy is, the strong the force of attraction
o Energy needed to break apart ionic compound into gaseous ions is called the
lattice dissociation energy and is opposite in sign (positive)
 Smaller ions, closer together, lattice is energy is more negative
 Greater the charge of the ion, lattice energy is more negative
 Ionic radii
o Cations are always smaller than their neutral atom
o Anions are always bigger than their neutral atom
o Moving down a group: ionic radii increases
o Moving across a period: ionic radii of cations decreases, atomic radii of anions
decrease
Examples: Which compound has the more negative lattice energy? Why?
Examples: Order the following compounds from lowest to highest melting point.