Models of the Atom

Review of Ancient Models of Matter Leucippus and Democritus - the idea of the “atom” – the indivisible particle Empedocles and Aristotle - the 4-element model Aristotle, Aquinas and The Church – the 4-element model persists for over 2000 years. The Alchemists – develop new chemical lab equipment and techniques but do not apply the Scientific Method

Enter the Scientists The great breakthroughs made by Galileo, Copernicus and Isaac Newton made way for a revolution in pure science in Europe. Great scientists such as Boyle, Lavoisier, Priestly and Scheele began to make strides in the field of Chemistry. Scientists such as Francis Bacon and Rene Descartes developed the process we know as the Scientific Method. The first scientist to apply scientific methodology to the study of matter and atoms was John Dalton

John Dalton Dalton was a schoolteacher and later a professor of Mathematics and Natural philosophy (Science) His strongest interests were in the field of Meteorology (Dalton’s Law of Partial Pressures) but he also made contributions in Chemistry He began to make measurements of the masses of elements and created a simple table of elements. He then went on to conduct experiments with chemical reactions and determined the masses and ratios of elements in the newly formed compounds.

Dalton’s Chemical Formulae

Dalton’s Atomic Theory In 1803, Dalton suggested an atom is a solid featureless sphere. Billiard Ball Model of the Atom Dalton made two assertions about atoms: atoms of each element are all identical to one another but different from the atoms of all other elements atoms of different elements can combine to form more complex substances.

Five main points of Dalton's atomic theory Elements are made of extremely small particles called atoms. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. Atoms cannot be subdivided, created, or destroyed. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. In chemical reactions, atoms are combined, separated, or rearranged.

Problems with Dalton’s Model Atoms exhibit electrical properties – charged atoms, static electricity, etc… The true nature of atoms could not be determined since atoms were extremely small and no known methods of testing individual atoms was known

J.J. Thomson In 1897, J.J. Thomson conducted experiments using a cathode ray tube. Thomson’s was able to create a stream of particles that flowed through the tube and fluoresced when they hit a detectoe screen Using magnets Thomson was able to bend the flow of particles.

JJ Thomson’s Model of the Atom The particles were called “electrons” because of their electrical properties Electrons were determined to have very small masses and a negative charge (they were repelled by the negative force of a magnet)

J.J. Thomson’s Model of the Atom Atoms contain particles called electrons Electrons have a small mass and a negative charge The rest of the atom is a sphere of positive charge Electrons are embedded in this sphere, so that the resulting atoms are neutral or uncharged Thomson compared the (negative) electrons to plums embedded in a positively charged pudding

J.J. Thomson’s Model of the Atom Hence the Plum Pudding Model or Raisin Bun Model

Ernest Rutherford In 1895, the German scientist Roentgen discovered X-rays and started a rush of work to study the electromagnetic radiation emitted by radioactive elements. In the early part of the 20th century, many scientists such as the Curies, Becquerel, Geiger, and Rutherford studied these invisible rays. Scientists began using these minute waves to study the properties of atoms.

Ernest Rutherford Rutherford decided to test the model of his mentor, Thomson. In 1911, Ernest Rutherford conducted his famous gold foil experiment. He aimed alpha particles (positively charged particles) at a thin sheet of gold foil PREDICTION: Particles would pass straight through the gold foil

The Gold Foil Experiment PREDICTED OBSERVATION: A small number of alpha particles bounced almost straight back from the gold foil OBSERVED

Ernest Rutherford’s Model of the Atom An atom has a tiny, dense, positive core called the nucleus (which deflected the alpha particles and contains protons) The nucleus is surrounded mostly by empty space, containing rapidly moving negative electrons (through which the alpha particles passed unhindered) Ernest Rutherford

Ernest Rutherford’s Model of the Atom Rutherford’s atom had a tiny, dense, positive nucleus orbited by electrons. Most of the atom’s volume is empty space

James Chadwick – the real Jimmy Neutron In 1932, Chadwick conducted experiments that suggested that at least half of the mass of the nucleus was unaccounted for. Chadwick’s research suggested the existence of a neutral particle in the nucleus with similar mass to the proton. James Chadwick – the real Jimmy Neutron

Updated Model of the Atom

Modern Development of the Model of the Atom

A Problem with the Planetary Model Classical Newtonian Physics suggest that any object travelling in a curved path (even if it is moving at a constant speed) is said to be accelerating. An electron accelerating around the nucleus would continuously emit electromagnetic radiation and lose energy Therefore, it would eventually fall into the nucleus and the atom would collapse However, not consistent with real-world observations – atoms are stable

Assigned Questions p. 15 UC # 1, 2, 3, 4, 5, 6 Learning Checkpoint Assigned Questions p. 15 UC # 1, 2, 3, 4, 5, 6