Back to Chemistry 1405 Concept Reviews

Early Laws of Chemistry

Law of Conservation of Mass: The fundamental law of chemistry discovered by Antoine Lavoisier which states that matter is neither created nor destroyed during chemical reactions.

Law of Definite Proportions: The law discovered by Joseph Proust which states that a particular compound will always be formed from the same elements in the same fixed proportions regardless of what chemical processes were used to form the compound.

Law of Multiple Proportions: The law discovered by John Dalton which states that elements can sometimes combine in different proportions to form different compounds which have very different properties. Additionally, when two elements can form more than one compound, then the ratios of the masses of the second element which combine with a fixed mass of the first element will always be ratios of small whole numbers.

Precepts of Dalton’s Atomic Theory

1. Each element is composed of extremely small particles called atoms.

2. All atoms of the same element are identical, but the atoms of one element are different from the atoms of all other elements.

3. Atoms of one element cannot be changed into atoms of a different element by chemical reactions. In addition, atoms are neither created nor destroyed in chemical reactions.

4. Compounds are formed when atoms of more than one element combine. A given compound will always have the same elements combined in the same exact ratio.

Isotopes

Isotopes: Atoms that have the same number of protons (i.e.-the same element) but different numbers of neutrons.

Isotope Notation	Useful Equations
	$LaTeX: Mass\:Number\:=Protons+Neutrons$

The Mole: A Chemist’s “Dozen”

The mole refers to a specific number. That number is , which is also known as Avogadro’s number. Just as a dozen is 12 of something, a mole is of something. For chemists, this number is useful for expressing measurable amounts of atoms or molecules. 1 mole of an element has a mass equal to the atomic mass number of grams. These two mole relationships can be expressed as follows:

$LaTeX: 1\:mol=6.022\times10^{23}\:atoms\:\left(or\:molecules\right)$ $LaTeX: 1\:mol=Molar\:Mass\:\left(in\:grams\right)$

Molar Mass of a Compound: The molar mass of a compound is the mass of 1 mole of that compound. Molar mass is calculated by taking the sum of all the molar masses of each atom in the compound. See the following example on how to calculate molar mass:

Example: Find the molar mass of Al₂(SO₄)₃:

Al: 2 x 26.98 = 53.96
S: 3 x 32.07 = 96.21
O: 12 x 16.00 = 192.00
342.17 g/mol

Mole Conversion Relationships

The Periodic Table

Periodic Law: When the elements are arranged in order of increasing mass, certain sets of properties recur periodically.

Period: A row in the periodic table. Each successive row repeats the series of the special properties of different element types.

Group: A column in the periodic table. Elements in the same group tend to have similar properties. Certain groups have special names that are often derived from the specific chemical properties of elements in the group.

1411 Chapter 2 img 3.jpg

Representative elements: Elements that are labeled with an “A” and are sometimes simply called “A-group elements”. These are called “representative” because they represent the broad spectrum of physical and chemical properties present among the many elements in the periodic table.

Metals: Elements that tend to be shiny, malleable (able to be hammered into sheets), ductile (able to be drawn or “pulled” into wires), and conduct heat and electricity well.

Nonmetals: Elements that tend to be dull, brittle, and do not conduct heat or electricity well as solids or liquids. Most non-metals are in the gas phase at normal temperatures.

Ions: Atoms that have become charged by either gaining electrons (anions) or losing electrons (cations).