Krista+and+Miranda

1. History of the atom

J.J. Thomson (12-18-1856-8-30-1940) Discovered the electron. This led to his atomic model, in which negatively charged electrons are embedded in a sphere of positive electric charge. First Experiment: Thomson constructed a Crookes tube with an electrometer set to one side, out of the direct path of the cathode rays. Thomson could trace the path of the ray by observing the phosphorescent patch it created where it hit the surface of the tube. Thomson observed that the electrometer registered a charge only when he deflected the cathode ray to it with a magnet. He concluded that the negative charge and the rays were one and the same. Second Experiment: He investigated whether or not the rays could be deflected by an electric field. Previous experimenters had failed to observe this, but Thomson believed their experiments were flawed because their tubes contained too much gas. Thomson constructed a Crookes tube with a near-perfect vacuum. At the start of the tube was the cathode from which the rays projected. The rays were sharpened to a beam by two metal slits - the first of these slits doubled as the anode, the second was connected to the earth. The beam then passed between two parallel aluminum plates, which produced an electric field between them when they were connected to a battery. The end of the tube was a large sphere where the beam would impact on the glass, created a glowing patch. Thomson pasted a scale to the surface of this sphere to measure the deflection of the beam. When the upper plate was connected to the negative pole of the battery and the lower plate to the positive pole, the glowing patch moved downwards, and when the polarity was reversed, the patch moved upwards. Third Experiment: Thomson measured the mass-to-charge ratio of the cathode rays by measuring how much they were deflected by a magnetic field and how much energy they carried. He found that the mass to charge ratio was over a thousand times lower than that of a hydrogen ion (H+), suggesting either that the particles were very light and/or very highly charged.

2. Atomic Structures The atom is a basic unit of matter that consists of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons. The electrons of an atom are bound to the nucleus by the electromagnetic force. Atoms are composed of three types of particles: protons, neutrons, and electron. Both the protons and neutrons reside in the nucleus and electrons reside in orbitals around the nucleus. Atoms of different elements have different numbers of protons and neutrons in their nucleus. More importantly, they have different numbers of electrons around the nucleus. It is the interaction of the orbiting electrons with those of other atoms that give rise to the various chemical properties of different elements. www.wikipedia.com