Atomic Theory Evolution and Chemical Fundamentals

Dalton's Atomic Theory and Postulates

In 1808, the English scientist John Dalton (1766-1844) published his ideas about the constitution of matter. These postulates can be summarized using modern language:

• Matter is comprised of extremely small particles called atoms, which are indivisible and indestructible.
• Each chemical element has atoms that are all identical, but different from those of other chemical elements.
• According to Dalton, a chemical compound is formed by the union of atoms of different elements of the same type and always in the same proportion.

Chemical Elements, Compounds, and Electrolysis

• Chemical element: A substance that cannot be decomposed into other simpler substances because it consists of a single type of atom.
• Composite (Compound): Those substances which can be decomposed into other simpler substances because they include more types of atoms of an element.
• Electrolysis: The process of breaking a compound in its entirety by using electricity.

The Thomson Plum Pudding Model

Thomson found that electrons are very small particles with a negative charge and proposed an atomic model based on this new discovery: "The atoms are formed by electrons with a negative charge, which are distributed through a sphere of matter of positive charge so globally that the atom is electrically neutral."

Atomic Number and Mass Number

• The atomic number (Z) is the number of protons an atom has. A chemical element is characterized by the fact that all its atoms have the same number of protons in the nucleus.
• The mass number (A) is the total number of particles in the nucleus of an atom, i.e., the sum of protons and neutrons.

The difference between the mass number and the atomic number is the number of neutrons: N = A - Z.

Understanding Isotopes and Ions

Isotopes are atoms that have the same atomic number but a different mass number. Two isotopes belong to the same chemical element because they have the same atomic number.

Ions are atoms that have lost or gained electrons in the shell:

• Cations: If a neutral atom loses an electron, it has an excess of positive charge and is transformed into a positive ion or cation.
• Anions: If a neutral atom gains electrons, it is negatively charged with an excess of negative charge and becomes an anion.

The Rutherford Atomic Model

Later, it was discovered that atoms contain two more subatomic particles in addition to electrons: protons and neutrons. Protons have the same electrical charge as electrons but of the opposite sign (positive) and a much larger mass. Neutrons have no electric charge, and their mass is nearly identical to that of the proton.