Structure of the Nuclear Atom Atoms can be broken down into smaller, more fundamental particles Electrons: negatively charged subatomic particles. Thomson (English physicist) discovered electrons in 1897.

How did he discover electrons? passed electric current through gases at low pressure sealed the gases in glass tubes fitted at both ends with metal disks called electrodes electrodes were connected to a source of high-voltage electricity one electrode, the anode, became positively charged other electrode, the cathode, became negatively charged a glowing beam formed between the electrodes, called a cathode ray. Cathode rays are attracted to metal plates that have a positive electrical charge. Plates that carry a negative electrical charge repel the rays.

Cathode Ray Tube Opposites attract and like charges repel, Thomson proposed that a cathode ray is a stream of tiny negatively charged particles moving at a high speed. These particles were named electrons Thomson also showed that cathode rays did not depend on the kind of gas in the tube or the type of metal used. Electrons must be parts of the atoms of all elements

More Discoveries Robert Millikan: carried out experiments that allowed him to find the quantity of charge carried by an electron. He also determined the ratio of the charge to the mass of an electron. He used these values to determine an accurate value for the mass of an electron (9.11 x10-28 g) Goldstein : observed in a cathode ray tube that there were rays traveling in the opposite direction than the cathode rays. Concluded that they were positive particles, and called them protons Chadwick: discovered another subatomic particle, the neutron; subatomic particles with no charge but with a mass nearly equal to that of a proton

Millikan Oil Drop Experiment Virtual Chem Lab

Black Box Activity

Atomic Nucleus: After the discovery of subatomic particles, scientists then wondered how the particles were put together in an atom Rutherford: tested a theory to see how an atom is composed directed a narrow beam of alpha particles at a very thin sheet of gold foil. (Alpha particle are helium atoms that have lost their two electrons and have a double positive charge because of the two remaining protons) Scientists thought that the electrons were evenly distributed throughout the atom and uniformly filled with positively charged material. According to this, the alpha particles should have passed easily through the gold, with only a slight deflection due to the positive charge thought to be spread out in the gold atoms.

Rutherford Experiment But, to everyone’s surprise, the great majority of alpha particles passed straight through the gold atoms, without deflection. A small fraction however bounced off the gold foil at very large angles, even straight back. Based on his results, he proposed that the atom is mostly empty space. He concluded that all the positive charge and almost all mass are concentrated in a small region that has enough positive charge to account for the great deflection of some of the alpha particles.

Rutherford’s Experiment http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

Properties of Subatomic Particles Nucleus: the central core of an atom, composed of protons and neutrons mass of atom is due mainly to the number of protons and neutrons assigned charges of +1 and -1, they are relative charges Absolute charge is the same Properties of Subatomic Particles Particle Symbol Relative electrical charge Relative mass (Mass of proton =1) Actual mass (g) Electron e- 1- 1/1840 9.11x10-28 Proton P+ 1+ 1 1.67x10-24 Neutron n◦

Review How small is the nucleus compared to the entire atom? Housefly sitting on second base in a baseball stadium represents the nucleus, the rest of the atom would be the size of the stadium What are the charges of the three main subatomic particles? Electrons Protons Neutrons

Review What must be true of the number of electrons and protons in electrically neutral atoms? The number of protons and electrons must be the same The number of protons must be greater than the number of neutrons The number of protons must be greater than the number of electrons The number of protons must equal the number of neutrons

Review Describe Thomson’s, Millikan’s, and Rutherford’s contributions to atomic theory. Passed electric current through sealed glass tubes filled with gases. The resulting glowing beam was described as a stream of tiny negatively charged particles moving at high speeds. Concluded that electrons must be parts of atoms of all elements. (T) Determined the charge and mass of the electron (M) Gold-foil experiments indicated that the atom had a positively charged, dense nucleus which is tiny compared to the atom as a whole (R)