9 Science Notes Week 9: History of Atomic Theory PDF

# 9 Science Notes Week 9: ## History of Atomic Theory - Democritus vs Aristotle (ancient Greece) - Alchemy became the basic theory of understanding nature through the four elements - fire, air, water and earth. ## John Dalton (late 1700's) - His research developed the theory of atoms and made symb...

# 9 Science Notes Week 9: ## History of Atomic Theory - Democritus vs Aristotle (ancient Greece) - Alchemy became the basic theory of understanding nature through the four elements - fire, air, water and earth. ## John Dalton (late 1700's) - His research developed the theory of atoms and made symbols for the elements and compounds - Theory: - All elements are composed of submicroscopic invisible parts called atoms. - Atoms of the same element have the same general properties - Atoms of different elements combine in whole number ratios to form - Chemical reactions involve the rearrangements of atoms ## J. J. Thompson (late 1800s) - English Physicist - Provided first evidence that atoms are made of even smaller parts - Was able to prove that atom have negative and positive charge - Found electrons - Discovered that atoms are neutral - The Plum Pudding Model - suggested that atoms were like a plum pudding where the positively charged material is the cake and the electrons are the fruit. ## Ernest Rutherford (1871-1937) - Discovered that some part of the atom must contain a large amount of positive charge - This model accepted that: - The nucleus contains protons and neutrons - Protons carry electric charge - Neutrons are neutral (no charge) - The mass of the atom is due to the nucleus because protons contain little mass - Electrons move around outside the nucleus - Electrons have negative charge - Atoms are mostly empty space ## Niels Bohr (1885-1962) - The Bohr model describes how electrons move in a specific area of space called atom shells that proposes: - Each shell can contain a limited number of electrons - Electrons orbit around the nucleus and the distance depends on the amount of its energy - When an electron gains energy, it moves to an outer shell of higher energy - When the electron moves back to its lower-energy shell, the excess energy is given off as light - The number of electrons in an outer shell is called valency - The number of electrons in the valence shell of an atom determines the chemical properties of the element and how it will bond with other elements - The light can be seen through a spectroscope as coloured lines and the pattern that is observed is called an emission spectrum which is unique to each element ## Quantum -mechanical model - Determines the allowed energies an electron can have and its level, it cannot be in between - Electrons behave as waves and particles - Electrons don't have fixed orbits, but exist in regions called orbitals - Uncertainty exists because we cannot know exactly where it is and how fast it is travelling - We can only predict where an electron will be - Pauli rule: no electron can have the same properties as another within an atom - Electrons have a property called spin which means it can move up and down