SCH3U Unit 1: Matter, Chemical Trends and Chemical Bonding History of the Atom SCH3U Unit 1: Matter, Chemical Trends and Chemical Bonding

The History of the Atom Atomic theory has evolved significantly over time to our modern theory 400 BC: Democritus If matter was divided into smaller and smaller pieces, eventually a point would be reached where it could no longer be divided Atoms (greek – atomos) – small indivisible particles

Dalton’s Atomic Model 1808: John Dalton Described atoms as solid, indestructible particles that make up all matter (“billiard ball model”).

Dalton’s Atomic Model The main postulates of Dalton’s theory: All matter is made of tiny, indivisible particles called atoms All the atoms of any given element are identical in mass and size The atoms of any given element are different masses and size of atoms of any different elements Atoms of different elements combine in small whole number ratios to form compounds Atoms remain unchanged in chemical reactions. They only join together or separate from one another.

Dalton’s Atomic Model Dalton’s theory helped to explain two previously discovered laws: The Law of Conservation of Mass: Matter cannot be created nor destroyed. The mass of the products is equal to the mass of the reactants. The Law of Definite Proportions: The elements of a compound are always present in the same proportion by mass.

Thomson’s Atomic Model 1897: J.J Thomson Discovered the electron, which was found to have a negative charge He proposed the atom was made of negatively charged electrons embedded in a sphere of positive charge (“raisin bun model”)

Rutherford’s Atomic Model 1910: Ernest Rutherford Performed the gold foil experiment He directed alpha-particles (positively charged particles from a radioactive source) at a sheet of gold foil He predicted that based on Thomson’s model, all the particles would pass through the foil undeflected

Rutherford’s Atomic Model

Rutherford’s Atomic Model Most of the atom is made up of empty space An atom contains an equal number of positively charged protons and negatively charged electrons The dense, positive nucleus of the atom accounts for most of its mass The nucleus contains protons (and neutrons, later discovered by James Chadwick in 1932) Very small, negative electrons orbit the nucleus

Rutherford-Bohr Model 1913: Neils Bohr Bohr extended Rutherford’s theory by stating that electrons occupy specific energy levels surrounding the nucleus

Modern Atomic Theory Atom: the smallest particle of an element that has all of the chemical properties of that element Molecule: the smallest particle of a compound that has all of the chemical properties of that compound

Subatomic Particles Protons Electrons Neutrons Charge Positive Negative Neutral Mass 1.67 x 10-24g (1amu) 9.11 x 10-28g (0.0005 amu) 1 amu Location Nucleus Orbits the nucleus Discovered 1886 Eugen Goldstein 1897 JJ Thomson 1932 James Chadwick

Modern Atomic Theory All matter is made up of tiny particles called atoms. Each atom is made of subatomic particles including protons, electrons and neutrons The atoms of any given element remain unchanged during a chemical reaction Atoms of the same element have the same properties, which differ from properties of different elements Atoms of different elements combine in specific proportions to form compounds