Chem 109 - General Chemistry - Spring 2013

Lecture Notes 6: 4 February

Chemical Periodicity

Look at the Periodic Chart.

Periods

The rows are referred to as periods. The pattern arises due to a repetition or periodicity of chemical properties.
- Combining ratios with hydrogen. 1 - 4 - 1: LiH, BeH₂, BH₃, CH₄, H₃N, H₂O, HF.
- Ions:
  1. Group IA always +1
  2. Group IIA always +2
  3. Group IIIA commonly +3, Al always+3
  4. Group IV +4 or -4 (usually covalent)
  5. Group VA commonly -3
  6. Group VIA commonly -2
  7. Group VIIA commonly -1
- Metal oxides are basic combined with water (e.g. NaOH, KOH), non-metal oxides are acidic combined with water (e.g. SO₂ and water gives H₂SO₃, one of the important acids in acid rain).
  - IA & IIA oxides are basic, combining with water to give compounds such as NaOH and Mg(OH)₂, while VIA & VIIA oxides form acids with water such as H₂SO₄ and HClO₄. Intermediate groups show transitional behavior, e.g. carbon froms a weak acid, nitrogen strong acid, aluminum a weak base, phosphorous a weak acid, sulfur a strong acid.
- Properties go from metallic in Group IA to non-metallic in VIIIA.

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 I Xe

Cs Ba W Pt Au Hg Pb

**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			I	Xe
Cs	Ba				W				Pt	Au	Hg		Pb

Stoichiometry

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!).