Humboldt State University ® Department of Chemistry

Richard A. Paselk

	Science 331
Fall 2004	Lecture/Activity	Office: SA560a
Notes: 3 November		Phone: x 5719 Home: 822-1116
		e-mail: rap1

Chemical Bonds

Atoms and molecules can be held together by Strong bonds or Weak bonds. We are first going to look at strong bonds.

Covalent vs. Ionic compounds: There are two kinds of strong bonds: ionic bonds and covalent bonds.

• In covalent compounds atoms have a definite relationship to each other, they are "married." Thus for water, H₂O, the smallest particle is a water molecule containing one oxygen and two hydrogens.
• In ionic compounds ions of opposite charge attract each other, but there is no definite attachment. Thus in sodium chloride crystals each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions and they are equally attracted by each - there is no one-to-one relationship.

We will begin our discussion with ionic bonds since they are easier to understand.

With the representative elements bond formation generally results in the formation of "octets" of electrons in the outermost shell.

Ionic bonds and Ionic Compounds:

• Formation: 2 Na + Cl₂ Æ NaCl. Can think of as composed of two equations, the oxidation of Na and the reduction of Cl:
  • Na + energy Æ Na⁺ + e^-
  • Cl + e^- Æ Cl^- + energy
• Lewis Structure
• Crystal Structure (model)
• Ionic Compound Lewis Structure Examples:
  • Potassium bromide
  • Aluminum chloride

Covalent Compounds and Covalent Bonds:

• Formation: H₂ + Cl₂ Æ 2 HCl. In this case can consider that we get two equations each involving a homo dissociation to give radicals, that is atoms with unpaired electrons:
  • H₂ Æ 2 H^.
  • Cl₂ Æ 2 Cl^. These radical then combine to form a bond with these two electrons shared between the two atoms.
• Lewis Structure:

Note that some ions are also covalent "compounds." That is we have molecules which have gained or lost electrons relative to teh protons in the atoms. Nitrate ion (NO₃^-), sulfate ion (SO₄^2-) and ammonium ion (NH₄⁺) are examples.

• Covalent Compound Lewis Structure Examples:
  • Water
  • Ammonia
  • Ammonium ion
  • Methane
  • Hydrogen sulfide
  • Carbon dioxide
  • Carbon monoxide

Chemical Equations, Atoms, and Matter

Remember:

• Atomic nuclei are not affected by chemical processes.
• Mass is conserved in chemical processes.

This means that for any chemical reaction we must have the same number of atoms of each type at the beginning and at the end!! Thus the term, "Chemical equation" - the two sides are equal in terms of atoms (though they will have been rearranged).

Let's look at some chemical reactions.

Hydrogen gas plus oxygen gas gives water  H2 + O2 -> H2O      Now we need to have the same number of each kind of atoms on each side. Multiplying hydrogen and water by 2 will give us:  2 H2 +  O2 -> 2H2O      2x2=4H + 1x2=2O = 2x2=4H +2x1=2O     And eveything is balanced!

Let's try another reaction

Methane gas (natural gas, CH4) gas plus oxygen gas gives carbon dioxide plus water  CH4 + O2 -> CO2  +  H2O  Now we need to have the same number of each kind of atoms on each side. Let's count the atoms: Carbons  1x1=1C = 1x1=1C Hydrogens  1x4=4H            1x2=2H Oxygens      1x2=2O    1x2=2O  +  1x1=1O Carbon is balanced (thus the = sign), but hydrogen and oxygen are not. The best strategy for balancing is to start with the molecule containing atoms other than oxygen, in this case, methane. Comparing the two sides we again see we have the same number of carbons, but we need two more hydrogens on the right, so multiply the water by two:  CH4  +  O2  -> CO2  +  2 H2O Carbons  1x1=1C      =  1x1=1C     Hydrogens  1x4=4H      =       2x2=4H Oxygens       1x2=2O     1x2=2O     2x1=2O Comparing the two sides again, we now have the same number of carbons, and the same number of hydrogens, but we need two more oxygens on the left, so multiply the oxygen gas by two:   CH4 +   2 O2  ->  CO2 +   2 H2O  Carbons  1x1=1C      =  1x1=1C      Hydrogens  1x4=4H      =       2x2=4H  Oxygens       2x2=4O  =   1x2=2O     2x1=2O voila! We have now balanced the entire equation. We have different substances on the two sides, but the same number of atoms of each type - elements and mass are conserved!

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© R A Paselk

Last modified 3 November 2004