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  1. Review

  2. Isotopes • J.J. Thomson saw that there were two different neon atoms in the cathode ray tube • These atoms were different in mass, but the same chemically • Atoms of the same element that have a different mass are known as Isotopes

  3. Isotopes • Contain the same number of protons • Contain a different number of neutrons • Two definitions for isotopes: • Atoms of the same element with different amount of neutrons • Atoms of the same element with difference in mass

  4. Isotope Uses • Carbon dating • Can determine the amount of carbon-14 in a sample to find the age of rock • Smoke Detectors • When smoke is present, conductivity in detector changes from small amount of radioactive material • Cancer treatments • Radiopharmaceuticals

  5. Henry Moseley

  6. Henry Moseley • Found that the wavelength of X-rays depended on the number of protons in the nucleus of the atom • Also found that the number of protons is always the same in an element • Atomic number: number of protons in an element • Protons: positive charge, mass: about 1 amu, symbol p+

  7. Looking at the Numbers • Nucleons: particles that make up the atomic nucleus • Mass Number: the total number of nucleons (protons and neutrons) in an atom • Neutrons: neutral charge, mass: about 1 amu, symbol n0 • If you know this, you can figure out the number of protons, neutrons, and the mass number of anything!

  8. Writing Isotopes • There are two main ways that isotopes can be written for an element. 126I Hydrogen-3 Isotopes of Neon Neon-20: 10 neutrons Neon-21: 11 neutrons Neon-22: 12 neutrons

  9. Atoms are Neutral • The number of protons must equal the number of electrons for the atom to be neutral • Isotopes change the number of neutrons

  10. Let’s Practice! • An atom of vanadium contains 23 electrons. How many protons does it contain? • An atom of silver contains 47 protons. What is its atomic number? • How many electrons, neutrons, and protons are found in a copper atom of mass number 65?

  11. Atomic Mass • On the periodic table, the AVERAGE atomic mass is given at the bottom • This number represents the average mass of all of the isotopes for that particular element • This number also involves the abundance of these particular isotopes in nature.

  12. Atomic Mass Calculations • Calculate the average atomic mass of gold with 50% of gold being gold-197 and 50% being gold-198.