Atomic Theory Timeline 400 B.C. 350 B.C 1743-1794 1800 1900 Democritus Aristotle Lavoisier Dalton Thomson 1903 1907 1913 Rutherford Millikan Bohr

Democritus atoms are particles in space (the void) In 400 B.C. Democritus believed all existence consisted of two things: atoms (coined the term atom from the Greek word ‘atomos’ meaning indivisible) the void (space) Democritus’ theory: atoms are particles in space (the void) atoms always in motion; make life predictable matter could not be divided indefinitely; atoms are indivisible all things we see differ because of the shape, size, and arrangement of atoms

Aristotle In 350 B.C. Aristotle and his followers dismissed the idea of atoms Instead modified an earlier theory that matter was made up of four ‘elements’: earth fire water air Even though Aristotle was wrong his theory persisted for 2000 years!

Lavoisier During his lifetime, 1743-1794, Antoine Lavoisier established the Law of Conservation of Matter Law= in a chemical reaction, the mass of the reactants is equal to the mass of the products Theory stated that mass is concrete and measurable, rather than ‘magical’

Dalton In 1800 John Dalton was examining gases dissolved in water and decided that matter was made of individual particles with spaces between them Devised a chemical atomic theory: all matter is made of atoms atoms of an element are identical each element has different atoms atoms of different elements combine in constant ratios to form compounds atoms are rearranged in reactions

Thomson In 1897, looking at charged particles, J.J Thomson found particles that had a charge that did not change regardless of the metal they came from… He called these charged particles electrons (negatively charged particles) Later he proposed a model for the composition of the atom called the ‘raisin bun’ or ‘plum pudding model’ although he was proved to be incorrect

Adding Electrons to the Model Materials, when rubbed, can develop a charge difference. This electricity is called ‘cathode rays’ when passed through an evacuated tube These rays have small mass and are negative Thomson noted that these negative subatomic particles were a fundamental part of all atoms 1) Dalton’s ‘Billiard Ball’ model (1800-1900): atoms are solid and indivisible

2) Thomson’s ‘Plum Pudding’ Model (1900): negative electrons in a positive framework 3) The Rutherford Model (around 1910): atoms are mostly empty space negative electrons orbit a positive nucleus

Rutherford Used radiation to unlock clues to the atom; alpha particle (α) -positive charge and beta particle(β) -negative charge Ernest Rutherford short alpha (α) particles at gold foil Most particles passed through; therefore, atoms are mostly empty Some positive alpha deflected or bounced back! Thus, a ‘nucleus’ is positive and holds most of an atom’s mass

Millikan In 1907 Robert Millikan found that all electrons are identical Millikan was able to find the charge of a single electron

Bohr Around 1913 Niels Bohr discovered that electrons orbit the nucleus in ‘shells’ Electrons can be bumped up to a higher sell if hit by an electron or a photon of light It is when electrons fall back down energy level(s) that they release a photon

There are two types of spectra : continuous spectra line spectra These jumps down from shell to shell account for the line spectra seen in gas discharge tubes (through spectroscopes)