Atoms were originally thought to be invisible

Transcription

1 Atomic Molecular Theory this is a theory, remember what that means. Start with the structure of the Atom: Atoms were originally thought to be invisible Today we define the atoms as the smallest particle that still retains the chemical properties of the elements As the exploration went deeper, it was found that the atom contained sub-particles o Protons o Neutrons o Electrons The first discovery came in the late 1800s by passing and electric current through various gases at low pressure. Cathode ray tubes Streams of negatively charged particles were found to emanate from cathode tubes, causing fluorescence.

2 Robert Millikan (University of Chicago) determined the charge on the electron in 1909, with the Millikan oil drop experiment. Scientists used this information plus more experimentation since then to obtain a more accurate representation of the electron s charge (9.109X10-31 kg or 1/1837 or the mass of a hydrogen atom) since Millikan s experiment. Based on the discovery of the electron two inferences were made: 1. Because atoms are electrically neutral, they must contain positive sub-atomic particles to balance charge. 2. Since atoms are so much heavier then electrons, atom must contain other particles in order to account for the mass differentiation. Thomson suggested the plum pudding model (after the English dessert). Suggested that the negative charge was evenly distributed throughout the atom with the positive charge This arrangement could be compared to a watermelon; the seeds are spread throughout the melon but do not contribute much to the mass.

3 However, it was later disproved with new experimentation, but very important theory to stepping forward in the AMT New Zealander Ernest Rutherford was the one to theorize that there was a nucleus. He shot α particles at a thin sheet of gold foil and observed the pattern of scatter of the particles However, he would not have been able to shot alpha particles through the gold foil without the knowledge of radioactivity. Radioactivity: a spontaneous emission of radiation be an atom It was first observed by Henri Becquerel. Marie and Pierre Curie also studied it. Rutherford discovered that there are three different types of radiation Alpha particles Beta particles Gamma particles Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. Most of the volume of the atom is empty space. This was because of the large amount of scattering. After a few month of experimentation he determined that if the nucleus was the size of a marble then the size of the atom would be the size of a football field.

4 So he figured out where the nucleus was, but where are the electrons? His student Niels Bohr proposed a model in which the electrons surrounded the positively charged nucleus. This theory was explained with the nucleus being represented by the sun and the electrons by the plants. The other subatomic particles.protons were discovered by Rutherford in Neutrons were discovered by James Chadwick in Protons and electrons are the only particles that have a charge. Protons and neutrons have essentially the same mass. The mass of an electron is so small we ignore it.

5 Symbol of Elements Elements are symbolized by one of two letters. Examples B Boron, Cl Chlorine, H Hydrogen All atoms of the same element have the same number of protons, which is called the atomic number, Z. In the example above Z is 6, B is 5, H is 1. The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom. For the example above the amu is 12. Isotopes