Chem 109 - General Chemistry - Spring 2015
Lecture Notes 7: 2 February
Chemical Periodicity
, cont.
Last time looked at the Periods (rows) in the Periodic Table, today we look a bit further and discuss the Groups or columns in the Table.
Note that elements go from metallic(Group 1) to non-metallic (Group VIII) as you proceed from left to right across the table.
Groups
- The vertical columns of elements on the table sharing a family resemblance of properties (e.g. Li - Fr) are called groups. For example:
- IA (1) = alkali metals;
- IIA (2) = Alkaline earth metals;
- VIIA (17) = Halogens (note the generic symbol of X standing for any halogen);
- Group VIII (18) is known as the Noble Gases, or sometimes the Inert Gases because until the 1960's they had no known compounds. Very unreactive and only known compounds are with very reactive elements like F and O, and even they don't form compounds with smaller Noble gases such as He and Ne.
Note the numbering of the groups. The numbers from 1 - 18 are the internationally accepted numbers. We will also use the I - VIII "American" numbering system for the representative elements.
Terms etc.:
- Representative elements: the elements of the s-block and p-block ("tallest" columns, blue and green on the table below).
- Transition metal elements: the elements of the d-block (yellow in the table below).
- Inner-transition metal elements: The Lanthanides and Actinides (not shown on the table below)
Periodic Table of the Elements
| IA |
IIA |
|
IIIA |
IVA |
VA |
VIA |
VIIA |
VIIIA |
| H |
He |
| Li |
Be |
|
B |
C |
N |
O |
F |
Ne |
| Na |
Mg |
IIIB |
IVB |
VB |
VI |
VIIB |
VIIIB |
IB |
IIB |
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 |
Sb |
|
I |
Xe |
| Cs |
Ba |
|
|
|
W |
|
|
|
Pt |
Au |
Hg |
|
Pb |
Bi |
|
|
|
Stoichiometry: Moles, Atoms, & Formulae
Stoichiometry is the quantitative study of the composition of compounds (e.g. determining the ratios of atoms in a molecule) and/or the ratios of substances in chemical reactions.
The Mole
This is the SI unit of amount of substance. 1 mole = the number of carbon atoms in 12 g of 12C. This number, called Avogadro's Number, has been measured as 6.022 x 1023 mol-1 (current value: 6.022 141 99 x 1023mol-1). Notice that this number can refer to anything (a mole of eagles, a mole of pennies, etc.). In each case we are talking about 6.022 x 1023 items or entities.
For chemists a mole has two common uses:
- It refers to Avogadro's Number of entities.
- It refers to the atomic weight (or formula weight or molecular weight) of a substance expressed in grams. Thus a mole of sodium is 22.99 g of sodium (which contains 6.022 x 1023 atoms of sodium!).
Note that Avogadro's number, 6.022 x 1023 is thus the conversion factor from amu's to grams!
Mole Samples Demo
Atomic Mass
- Determination by Mass Spectrometry: see text, pp 83. Basically have an "ion gun" to provide a focused beam, then a magnetic selection system (recall how we could move electron beam in Crooke's tube) to select/determine mass, and finally a detection system (measures current from ions). Mass spectrometry today is a key tool in environmental chemistry analysis and monitoring as well as a super-sensitive system for detecting isotopes such as 14C.
Recall that atomic masses have two meanings:
- At the microscopic scale (atoms, ions and molecules) it is the mass in amu's of a single atom etc.
- At the macroscopic scale (visible amounts of stuff) it is the mass in grams of a mole of atoms etc.
Examples:
- What is the mass of 27 atoms of oxygen
- in amu's? (27 atoms)(16.00amu/atom) = 432.0 amu
- in grams? (27atoms)(16.00amu/atom)(1g/[6.022 x x 1023amu]) = 7.174 x 10-22g
- Given 3.45 grams of copper
- how many moles of copper is this? (0.0543 mole)
- how many atoms of copper are there in this sample? (3.45g)(1mol/63.55g)(6.022 x 1023atom/mol) = 3.27 x 1022atoms
- A 2.34 mole sample of sulfur contains
- how many grams of sulfur? (2.34mol)(32.06g/mol) = 75.0 g
- how many atoms of sulfur? (4.52 x 1025)
© R A Paselk
Last modified 4 February 2015
