Matter Unit Learning Goal #2: Summarize the major experimental evidence that led to the development of various models, both historic and current.

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460 B.C. – Democritus Democritus believed that all matter was composed of atoms that could be divided no further. Each atom was separated by a distinctive amount of space Democritus also described atoms as solid, possessing no clear internal structure and varying greatly in terms of size, shape, and weight The differences in these characteristics explained the differences in the properties of the matter around us. Unfortunately for Democritus, and mankind in general, his ideas were largely ignored for the next 2000 years.

350 B.C. - Aristotle Modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. This theory was wrong. However, his theory persisted for 2000 years.

1800 – John Dalton John Dalton is usually credited with developing the first coherent atomic theory. Dalton's theory can be summarized as follows: Matter is composed of small particles called atoms. All atoms of an element are identical, but are different from those of any other element. During chemical reactions, atoms are neither created nor destroyed, but are simply rearranged. Atoms always combine in whole number multiples of each other. For example, 1:1, 1:2, 2:3 or 1:3. His ideas account for the law of conservation of mass (atoms are neither are created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

1897- J.J. Thomson Thomson’s 'Raisin in the Pudding' model of the atom He discovered the electron and proposed a model for the structure of the atom. Thomson knew that electrons had a negative charge and thought that matter must have a positive charge. His model looked like raisins stuck on the surface of a lump of pudding.   Positive Sphere Negative Charges

Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

1910 – Ernest Rutherford In 1909, Rutherford set a fellow scientist, Hans Geiger, and a student, Ernest Marsden, to find out the structure of the atom. They devised a system that allowed alpha particles (the nuclei of helium atoms) to be shot at a very thin piece of gold foil and the trajectory of the particles monitored.

Rutherford’s Gold Foil Experiment Alpha particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

Try it Yourself! In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

The Answers Target #1 Target #2

Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions: The nucleus is small The nucleus is dense The nucleus is positively charged

1912 – Neils Bohr In 1912 a Danish physicist, Neils Bohr came up with a theory that said the electrons do not spiral into the nucleus and came up with some rules for what does happen. Bohr came up with two rules which agreed with experiment: RULE 1: Electrons can orbit only at certain allowed distances from the nucleus. RULE 2: Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low- energy orbit to a high-energy orbit.

Quantized energy levels Electron moves in a circular orbit Electron jumps between levels by absorbing or emitting photon of a particular wavelength

Discovered the Neutron. James Chadwick 1932 Discovered the Neutron.

Modern Atomic Theory - Quantum Theory. Many scientists have contributed to the atomic theory since those listed above. Quantum theory has shown us that the electrons although they are particles also exhibit properties of waves. Now we think of the atom as a nucleus that is surrounded by probability clouds. The clouds represent the most probable locations of electrons. We still refer to these clouds as orbitals. The size and shapes of the orbitals may be calculated mathematically by using the equations for the waves.

Quantum Mechanical Model Electrons are located in specific energy levels. There is no exact path around the nucleus. The model estimates the probability of finding an electron in a certain position.

The Electron Cloud The electron cloud represents positions where there is probability of finding an electron. The cloud is less dense where there is less probability of finding an electron.

Modern Atomic Theory Element have a characteristic average mass which is unique to that element. Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions! All matter is composed of atoms Atoms of any one element differ in properties from atoms of another element