Atoms and Elements: Fundamental Laws and Atomic Theory

Atoms and Elements Unit 2

Introduction

This unit introduces the foundational concepts of atoms and elements, focusing on the laws governing chemical reactions and the development of atomic theory. Understanding these principles is essential for further study in general chemistry.

Law of Conservation of Mass

Definition and Explanation

• Law of Conservation of Mass: The total mass remains constant during a chemical reaction.

• The mass of the substances before the reaction equals the mass of the substances after the reaction.

Example

• When sodium (Na) reacts with chlorine gas (Cl2) to form sodium chloride (NaCl):

Reactant	Mass (g)
Na	7.7
Cl2	11.9
Total	19.6

Product	Mass (g)
NaCl	19.6

• Mass of reactants = Mass of product

Law of Definite Proportions (Law of Constant Composition)

Definition and Explanation

• All samples of a given compound, regardless of their source or preparation method, have the same proportions of their constituent elements.

Example: Water

• Decomposition of 18.0 g of water yields 16.0 g of oxygen and 2.0 g of hydrogen.

• The mass ratio of oxygen to hydrogen is:

$\frac{16.0\,\text{g O}}{2.0\,\text{g H}} = 8:1$

Law of Multiple Proportions

Definition and Explanation

• When two elements (A and B) form more than one compound, the masses of element B that combine with 1 g of element A can be expressed as a ratio of small whole numbers.

Example: Carbon Oxides

Compound	Mass Ratio (O:C)
Carbon dioxide (CO2)	2.67:1
Carbon monoxide (CO)	1.33:1

• The ratio between these two mass ratios: $\frac{2.67}{1.33} = 2$

Dalton’s Atomic Theory

Key Postulates

• Each element is composed of tiny particles called atoms.

• All atoms of a given element have the same mass and other properties that distinguish them from atoms of other elements.

• Atoms combine in simple, whole-number ratios to form compounds.

• In a chemical reaction, atoms only change the way they are bound together with other atoms; they are not created or destroyed.

Properties of Electrical Charge

Fundamental Properties

• Opposite charges attract: Positive and negative electrical charges attract one another.

• Like charges repel: Positive charges repel positive charges; negative charges repel negative charges.

• Charge magnitude: Positive and negative charges of exactly the same magnitude sum to zero when combined.

Charge Type	Interaction
Positive (+) & Negative (-)	Attract
Positive (+) & Positive (+)	Repel
Negative (-) & Negative (-)	Repel

$+1 + (-1) = 0$

Additional info: These notes cover the foundational laws and theories of atomic structure and chemical reactions, which are essential for understanding the behavior of matter in chemistry.