Unit 3: Bonding and Nomenclature

Unit 3: Bonding and Nomenclature

Content Outline: Molecules (3.3)

I.  Molecule

A.  The term refers to any electrically neutral group of atoms that are bound together.

1.  It can be 2 or more atoms of the same element or composed of different elements.

For example: N2 (nitrogen gas) or NH3 (Ammonia)

A.  Molecular Compound This term is used to refer to a chemical compound (2 or more atoms) whose simplest units are molecules.

For example: H20 (water) or O2 (Oxygen gas) or C6H12O6 (Glucose)

B.  This term is usually used with molecules that are bound together using covalent bonds.

C.  These molecules can possess single bonds (-), double bonds (=), or even triple bonds (Ξ).

1.  The purpose of “creating” the bonds is to achieve the lowest possible Potential Energy state by filling the valence shell with 8 electrons (except for H, He, Li, and Be) so that a molecule behaves “like” a Noble gas.

a.  8 electrons in the outer valence shell is referred to as the Octet Rule.

b.  It is a combination of the “s” orbital (2 electrons) and the “p” orbital (6 electrons).

2.  These “shared electrons” are from overlapping orbitals in the valence shells.

II.  Ionic Compound

A.  These molecules are composed of positive and negative ions that are combined in a lattice (3D cube) like structure that looks “like” a crystal (crystalline).

1.  The ions alternate (positive- negative) so as to maintain neutrality and reduce repulsive forces between like charged ions.

2.  The attractive strengths vary between the atoms is due to the size and charges associated with the ions in the compound.

a.  These ionic compound strengths are measured using Lattice Energies

b.  This is the amount of energy released when one mole of an ionic crystalline compound is formed from gaseous ions.

B.  The electrical charges are balanced, so as to be neutral.

C.  Ionic compounds are hard but brittle (crumbly).

D.  In the solid state, they cannot conduct electricity (as the atoms can’t move); but when dissolved in water, they are great conductors of electricity (the ions cans move freely).

III.  Chemical Formula

A.  This shows the relative number of atoms of each element in a chemical compound using symbols and subscripts.

For example: C12H22O11 (Sucrose)

B.  Common prefixes for numbers:

Di – 2 Penta – 5 Octa – 8 Mono-1

Tri – 3 Hexa – 6 Nona – 9

Tetra – 4 Hepta – 7 Deca – 10

IV.  Molecular Formula

A.  This type of chemical formula shows the types (Chemical symbol) and numbers (subscripts) of atoms combined in a single molecule of a molecular compound.

B.  For example: H2O or O3

V.  Formula Unit

A.  This type of chemical formula is used with ionic compounds to establish the simplest collection of atoms from which an ionic compound. It is like Molecular weight. Calculated using Atomic Masses and measured in moles.

For example: NaCl or KCl or CaF2 (There could be thousands of atoms, but this is the simplest ratio.)

VI.  Empirical Formula

A.  This type of chemical formula shows the kinds of atoms and their relative numbers in a substance in the smallest whole number ratios.

For example: H2O2 à HO OR C6H12O6 à CH2O (Take the lowest subscript number and divide it into the other subscripts.)

VII.  Energy and Molecules

A.  The natural tendency is to achieve the lowest possible Potential Energy state and thus behave “like” a Noble gas element.

B.  Energy is released in bond formation between atoms.

C.  Energy is required in the breaking of a bond between atoms.

1.  The energy to make or break a bond is referred to as bond energy.

2.  It is reported as kiloJoules/mole (kJ/mol)

3.  The octet rule applied to the number of bonds an element can make.

D.  Endothermic – Energy is absorbed from the surrounding environment.

E.  Exothermic – Energy is released to the surrounding environment.

F.  The energy of a chemical bond is inversely proportional with the nucleus distance.

1. This is referred to as Bond Length Energies.

1.  High Potential energy = very little distance apart. (There exists lots of repulsion between like charges of the protons in each nucleus.)

2.  Low Potential Energy = very far apart. (There is very little repulsion by the nucleus protons.)

3.  Do not confuse with Coulombs Forces that deal with one atom’s own electrons and protons. These are between two different atoms.

Please help students “see the difference “between the two atomic forces as the wording is critical to clear understanding because they all sound so much alike.