| Chem 107 |
Fundamentals of Chemistry |
Fall 2008 |
| Lecture Notes: 25 September |
© R. Paselk 2005 |
|
| |
|
|
| PREVIOUS |
|
NEXT |
Mole Calculations, cont.
Recall that when we look at the Periodic Chart the atomic masses have two meanings for chemists and for us:
- atomic mass = mass in atomic mass units for one average atom of that element = amu/atom;
- atomic mass = mass in grams for one mole of that element = g/mol.
For example, for the equation:
2H2 + O2 
2 H2O
We have two meanings:
- Two molecules of hydrogen combine with one molecule of oxygen to give two molecules of water; thus 2 (2 amu) + 32 amu = 2 (18 amu), or 4 amu + 32 amu = 36 amu
- Two moles of hydrogen combine with one mole of hydrogen to give two moles of water; thus 2 (2 g) + 32 g = 2 (18 g), or 4 g + 32 g = 36 g.
2Al + 3S Al2S3
- Two atoms of aluminum combine with three atoms of sulfur to give one "formula unit" of aluminum sulfide; thus 2(27.0 amu) + 3(32.1 amu) = 150.3 amu, or 54 amu + 96.3 amu = 150.3 amu.
- Two moles of aluminum combine with three moles of sulfur to give one mole of aluminum sulfide; thus 2(27.0 g) + 3(32.1 g) = 150.3 g, or 54 g + 96.3 g = 150.3 g.
Chemical Formulae and Molar Masses: For a compound such as calcium chloride, CaCl2, one mole of the compound will contain one mole of Ca and two moles of Cl. Note that the formula tells how many atoms are in the compound, it does not indicate how they are arranged, nor does the subscript 2 imply the chlorines are associated with each other, that is they are not "Cl2."
Chemical Formulae and Molar Masses: Let's look at the example again, but do some calculations. Example
How many moles of Cl- are there in 2.36g CaCl2?
Moles Cl- = 4.25 x 10-2
Lewis Dot Structures-a Simple Model for Bonding
Lewis Structures for Atoms: Just show inner "kernel" where symbol stands for nucleus and all inner shell electrons.
- Only good for Representative elements.
- For ions the charge is always shown.
- For metal ions such as calcium the Lewis Structure simply becomes the symbol for the ion.
- For negative ions such as we see for oxygen (2-) we enclose the ion and its electrons in brackets to indicate that the electrons are all "owned" by the oxygen - it does not share.
Examples:
- Sodium:
- Sodium ion:
- Phosphorus:
- Bromine:
- Bromide ion:
- Sodium chloride:
Chemical Nomenclature
Recall the elements you must know, shown on the Periodic Table below:
Periodic Table of the Elements
| IA |
IIA |
|
IIIA |
IVA |
VA |
VIA |
VIIA |
VIIIA |
| 1 |
2 |
13 |
14 |
15 |
16 |
17 |
18 |
| H |
He |
| Li |
Be |
|
B |
C |
N |
O |
F |
Ne |
| Na |
Mg |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
Al |
Si |
P |
S |
Cl |
Ar |
| K |
Ca |
|
Ti |
V |
Cr |
Mn |
Fe |
Co |
Ni |
Cu |
Zn |
Ga |
Ge |
As |
Se |
Br |
Kr |
| Rb |
Sr |
|
|
|
|
|
|
|
|
Ag |
Cd |
|
Sn |
|
|
I |
Xe |
| Cs |
Ba |
|
|
|
W |
|
|
|
Pt |
Au |
Hg |
|
Pb |
|
|
|
|
You may find the table of elements on the Chemistry Department web site useful in organizing these elements into more memorable categories - but you only need to memorize those noted above.
Covalent vs. Ionic compounds:
- 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 atom and two hydrogen atoms.
- In ionic compounds ions of opposite charge attract each other, but there is no definite attachment, just a constant ratio. 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.
This distinction will be important in some aspects of naming chemical compounds.
IUPAC vs traditional names
There are two common naming systems:
The IUPAC/Stock system
This is the modern, systematic scheme developed by the International Union of Pure and Applied Chemists.
Important nomenclature information is available on the Chemistry Department General Chemistry Supplement pages.
- In this system the positively charged elements in a compound are named as the element name followed by the "apparent charge" (later we will see this really refers to something called the oxidation number which often = charge) written in Roman numerals enclosed in brackets. Thus iron two-plus = iron (II) and iron three-plus = iron (III). Elements which exhibit only one charge do not require that the charge be shown. Thus sodium one-plus = Na, magnesium two-plus = Mg and aluminum three-plus = Al. (Recall that the alkali metals are always plus one or zero and the alkaline earth metals are always plus two or zero.) Look at the Table of Ions for additional ions.
- For negatively charged single atom ions the ending -ide is placed on the beginning of the element name. Thus chlorine one-minus becomes chloride, and similarly we see fluoride, bromide and iodide, while the two-minus charge ions of Group VIA become oxide, sulfide, etc.
Recognize these traditional names for metal ions:
- Fe(II) = ferrous
- Fe(III) = ferric
- Cu(I) = cuprous
- Cu(II) = cupric
- Hg(I) = mercurous {Hg22+)
- Hg(II) = mercuric
© R A Paselk
Last modified 25 September 2008
