The ATOM
The Atom Named by Democritus (460-370 BCE, Greece) The smallest indivisible particle of matter ‘Atomos’ means indivisible
But Aristotle (384-322 B.C.) thought differently.. He thought matter was infinitely divisible. (No smallest particle). A magic knife could cut forever.. Five elements – Earth, Water, Air, Fire, Aether The Atomic Theory was rejected ... For 2000 years.
Then the alchemists invented chemistry and we started learning.. Substances were reacted together and carefully studied. Reactions occur in simple ratios (for example, one part oxygen and two parts hydrogen react to make water) John Dalton (1766-1844, England) observed this and concluded – ‘there must be particles of different elements’ – The atom was reborn.
Dalton’s Atomic Model (1800) - Matter is composed of small and indivisible particles (atoms). All atoms of the same element are identical. Atoms of different elements are different from each other. Atoms can be combined to form compounds. Millions of compounds can be made.
Thomson’s Atomic Model (1856-1940) Experimented with cathode ray tubes Discovered Electrons Negatively charged particles part of the atom Can detach from the rest of the atom Indivisible?????? Dalton’s model now modified to include electrons
How did Thomson discover the electron? The beam of light was attracted by a positive charge and repelled by a negative charge. Conclusion:____________________________ ______________________________________ The beam could cause a small pinwheel to turn Conclusion: ____________________________ The beam cast a shadow in the shape of an obstacle. Final conclusion: ________________________
Rutherford’s Atomic Model (1871-1937) (British, but he worked in Montreal for a while.) He studied radiation from the newly discovered element radium (Marie Curie discovered it). Studied 3 forms of radiation: Alpha: positively charged Beta: negatively charged Gamma: neutral charge
Rutherford’s Gold Foil Experiment (1911) He aimed alpha particles at a thin piece of gold. Most went through, some were bounced back.
Rutherford’s conclusion: The atom must be almost entirely empty space (because most of the particles went right through the gold foil). At the very center of the atom is a positively charged nucleus (because some of the positively charged particles were bounced back). Electrons must orbit around the nucleus like planets around a star (because the atom is electrically neutral so there must be negative electrons to balance the positive nucleus.
The Rutherford atomic model
Two questions remained... Why doesn’t the negatively charged electron crash into the positively charged nucleus? Niels Bohr (1885-1962) realized that the electrons have energy that keeps them orbiting at a distance from the nucleus (kind of like the moon’s kinetic energy keeps it from crashing into the earth).
Electrons are in energy levels Electrons can gain energy and jump to higher levels (orbitals). When they drop back down they release the extra energy as light. The Rutherford-Bohr Model contains protons in the nucleus, and orbitals.
The second question.. Why don’t the positive charges of the nucleus repel? Why doesn’t the nucleus explode?
James Chadwick (1891-1974) Discovered there are two types of particle in the nucleus; the positively charged proton and the neutral neutron. The neutron is the glue that holds the nucleus together. This is the model of the atom that we use in this course. The Simplified Atomic model It contains protons, neutrons and electrons.