CLASS CONCEPTS

1. Introduction to Chemistry

2. The Periodic Table

3. Quantum Numbers

4. Electron Configuration

5. Chemical Families

6. Oxidation Numbers

7. Chemical Formulas

8. Chemical Names

9. Formula Mass

10. Percentage Composition

11. Reaction Types

12. Balancing Equations

13. The Mole Concept

14. Solution Concentration

15. Stoichiometry

16. Kinetic Theory

17. The Gas Laws

18. Enthalpy & Heat

19. Reaction Rates

20. Acids & Bases

21. pH Scale

22. Salts

23. Net Ionic Equations

24. Redox Reactions

25. Organic Chemistry

26. Nuclear Chemistry

6. Oxidation Numbers and Chemical Bonding

 
Oxidation Numbers

An element's oxidation number, sometimes called valence, is the number of electrons gained or lost when forming compounds. This characteristic is controlled by the electrons in the outer energy level (valence electrons).

Atoms gain or lose electrons to get eight electrons in their outer shell.

Elements with a positive oxidation number (usually metals) lose electrons when forming compounds. Elements with a negative oxidation number (usually nonmetals) gain electrons when forming compounds.

All chemical reactions occur between electrons in the outer energy level of atoms.

The table below shows the elements and oxidation numbers that can be read from the periodic table.

Group # Elements Oxidation #
1 all +1
2 all +2
3 - 12 Ag

Zn

+1

+2

  Other elements in group have more
than one possible oxidation #
13 Al +3
14 C and Si +4 or −4
  Other elements in group have more
than one possible oxidation #
15 N and P −3
  Other elements in group have more
than one possible oxidation #
16 O and S −2
  Other elements in group have more
than one possible oxidation #
17 all −1
18 all 0
 

Stability in Atoms

Certain electron arrangements are more stable than others.

  • The most stable atoms - their outer energy level is full.
     
  • Somewhat stable atoms - the sublevels of their outer energy level are half-filled.
The Octet Rule - the maximum number of electrons in the outer energy level is 8. Atoms will form compounds to reach eight electrons in their outer energy level.
  • Atoms with less than 4 electrons in their outer level tend to lose electrons to form compounds.
     
  • Atoms with more than 4 electrons in their outer level tend to gain electrons to form compounds.
     
  • A helium atom only has one energy level (#1), which has only one sublevel (s). The 1s level can only hold two electrons. Helium has those two electrons, which makes the atom's outer energy level full. Because of this, helium is said to conform to the octet rule.
 

Chemical Activity

  • Metals increase in chemical activity as you go from right to left on a horizontal row and from top to bottom in a vertical column.

    The most active metal on the Periodic Table is Francium.

    chemical activity
  • Nonmetals increase in chemical activity as you go from left to right on a horizontal row and from bottom to top in a vertical column.

    The most active nonmetal on the Periodic Table is Fluorine.

    chemical activity
  • Group 18 (Noble Gases) on the Periodic Table are chemically inert nonmetals.
 
Chemical Bonding

The tendency of an atom to attract electrons is electron affinity. This characteristic determines the type of bond formed between atoms.

  • Metals generally have a low electron affinity.
     
  • Nonmetals generally have a high electron affinity.
Ionic bondsWWW - electrons are transferred between atoms.
  • One atom gains electrons while another atom loses electrons.
     
  • The atoms become ions - one with a positive charge, the other with a negative charge.
     
  • The force of attraction between these opposite charges holds the ions together in an ionic bond.
     
  • Ionization - the process of removing an electron from an atom to form an ion.
     
  • Ionization energy - the energy needed to remove an electron from an atom.
Covalent bondsWWW - electrons are shared between atoms.
  • Atoms with a high electron affinity usually form covalent bonds.
     
  • Since both bonding atoms have a high attraction for the electrons, the shared electrons spend most of their time between the two atoms.
     
  • The force of attraction between both nuclei and the shared electrons hold the atoms together in a covalent bond.
     
  • Multiple pairs of electrons may be shared between atoms, forming multiple bonds. The number of pairs of electrons shared is expressed by bond order.WWW
Chemical bonding is often expressed as percentages of ionic characteristics and covalent characteristics.
  • The closer atoms are to each other on the periodic table, the more covalent their bond characteristics.
     
  • The farther apart atoms are on the periodic table, the more ionic their bond characteristics.
Metallic bondsWWW - electrons are distributed equally through a metallic crystal.
  • The electrons in metals can be thought of as being property of all the atoms in the crystal.
     
  • The positive nuclei are surrounded by a "sea" of electrons that are all attracted by the nuclei at the same time.
     
  • This characteristic is what makes metals such good conductors of electricity.
Hydrogen bondsWWW - a special type of bond between a hydrogen atom in a covalently bonded molecule and an unshared electron in a nearby molecule. Since the hydrogen atom is so small, it can get very close to other atoms - causing electrons in those atoms to be attracted to it.
This is especially important with small, polar molecules like water.WWW Hydrogen bonding is responsible for the very high specific heat capacity (ability to absorb large amounts of heat with little temperature change) of water, its relatively high boiling point, for water often being called the universal solvent, and for the formation of water clusters.WWW

Combined Volumes

 
Lewis StructuresWWW

Lewis dot diagrams were first used by Gilbert Newton LewisWWW in 1902 to represent valence electron arrangements in covalently bonded molecules.

The chemical symbol represents all the subatomic particles in the nucleus of the atom, plus all the electrons that are NOT in the outer energy level.

Each dot represents a valence electron.

  • Dots are placed on the four sides of the chemical symbol; top, bottom, left, and right.
     
  • Each side can accommodate up to two electrons, like an orbital.
     
  • All sides of the symbol are equivalent, which means that the choice of which side to place two electrons versus one electron is arbitrary.

Notice that the number of dots (valence electrons) in each diagram above is the same as the group number of the element.

 
Diatomic moleculesWWW are two atoms of the same element covalently bonded. When found alone in nature, these elements exist only as two covalently bonded atoms.

  • Hydrogen - H2
  • Nitrogen - N2
  • Oxygen - O2
  • Florine - F2
  • Chlorine - Cl2
  • Bromine - Br2
  • Iodine - I2
 

Polyatomic Ion - a group of covalently bonded atoms that have an overall electric charge. They act like a single ion when combining with other atoms to form compounds.

The Lewis diagram above shows a group 5 of atoms sharing electrons (covalently bonded) to have 8 valence electrons. There are a total of 32 electrons valence electrons in the diagram.

An oxygen atom is known to have 6 valence electrons.
4 X 6 = 24
A phosphorus atom is known to have 5 valence electrons.
1 X 5 = 5
That's a total of 29!
The group of atoms has 3 EXTRA electrons!

See a chart of the polyatomic ions used in this class.

 

Chemical Bonding