Chemistry Chapter: Atomic Concepts

Ancient Indian and Greek Philosophers

• Ancient Indian and Greek philosophers pondered the unseen form of matter and its divisibility
• Indian philosopher, Maharishi Kanad, proposed that matter could be divided into smaller and smaller particles (Parmanu), eventually reaching indivisible particles
• Greek philosophers Democritus and Leucippus suggested that matter could be divided into indivisible particles called atoms

Law of Conservation of Mass

• Lavoisier and Proust demonstrated that mass does not change during a chemical reaction
• Mass is neither created nor destroyed in a chemical reaction

Law of Constant Proportions

• Compounds always contain the same elements in the same proportions by mass, regardless of the source or method of preparation

Dalton's Atomic Theory

• Dalton proposed that all matter is made of atoms
• Atoms are indivisible particles that cannot be created or destroyed in a chemical reaction
• Atoms of the same element are identical in mass and chemical properties
• Atoms of different elements have different masses and chemical properties
• Atoms combine in simple whole number ratios to form compounds

Atoms and Molecules

• Atoms are the fundamental building blocks of matter
• Atoms cannot exist independently and are joined together to form molecules or ions
• Molecules are groups of bonded atoms that can exist independently
• Elements combine to form compounds
• The smallest unit of an element that can exist independently is an atom
• Compounds are materials made of more than one element that are bonded together

Atomic Mass

• John Dalton introduced the concept of atomic mass
• A relative scale for comparing masses of all atoms with an atomic mass unit (amu or u) was created, which relates elements to the atomic mass of carbon-12 isotope
• The molecular mass of a compound is the sum of the atomic masses of all atoms in a molecule

Chemical Formulas

• Chemical formulae represent the composition of compounds using symbols and subscripts
• Symbols represent elements and subscripts represent the number of atoms of each element in a molecule or formula unit
• Valency or charge is used in writing formulas
• When a compound consists of a metal and a nonmetal, the metal's symbol is written first.
• In compounds with polyatomic ions, the formula of the ion is enclosed in parentheses, and the number of those ions is written outside the parentheses