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How Atoms Differ

How Atoms Differ. Atomic Number (Z). Identity of each element is determined by the number of protons in the nucleus. The number of protons in the nucleus is the atomic number. In a normal atom, the number of electrons equals the number of protons. Atomic Mass (A).

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How Atoms Differ

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  1. How Atoms Differ

  2. Atomic Number (Z) • Identity of each element is determined by the number of protons in the nucleus. • The number of protons in the nucleus is the atomic number. • In a normal atom, the number of electrons equals the number of protons.

  3. Atomic Mass (A) • …is equal to the number of protons and the number of neutrons in the nucleus.

  4. Isotopes • Atoms of the same element having different atomic masses. • Atoms of the same element having different numbers of neutrons. • Hydrogen has three isotopes: Protium – 0 neutrons Deuterium – 1 neutron Tritium – 2 neutrons

  5. Isotopes - Naming • It is often important to identify one isotope from another. This is done in two ways. • Hyphen notation. This gives the element name (which you can use to determine the atomic number) followed by the atomic mass which you can use to determine the number of neutrons. • Hydrogen – 2

  6. Isotopes - Naming • It is often important to identify one isotope from another. This is done in two ways. • Nuclide or nuclear symbol. Atomic mass (A) 4 2 He Atomic number (Z)

  7. Some Important Isotopes • Carbon – 14 • Carbon - 12

  8. Average Atomic Mass • An average sample of hydrogen in nature contains 99.985% protium, 0.015% deuterium and a miniscule amount of tritium. • Protium has an atomic mass of 1.007825 amu. • Deuterium has an atomic mass of 2.014102 amu.

  9. Average Atomic Mass • Multiply the masses of each isotope by its percentage. These are weighted values. • Add all of these values together. (Do NOT divide by the number of isotopes. This is unnecessary.)

  10. Average Atomic Mass 1.007825 * .99985 = 1.0076738 amuprotium • 1.00798 amu is a weighted average because most of the hydrogen is protium. .00015 * 2.014102 = 0.0003021 amu deuterium 1.0076738 + 0.0003021 = 1.00798 amu

  11. Average Atomic Mass • 1.00798 amu is a weighted average because most of the hydrogen is protium. • Multiply the masses of each isotope by its percentage. These are weighted values. • Add all of these values together. (Do NOT divide by the number of isotopes. This is unnecessary.)

  12. Average Atomic Mass • The normal way to determine the average mass of something would be to add the two masses and simply divide by two.

  13. Average Atomic Mass • The normal way to determine the average mass of something would be to add the two masses and simply divide by two. 1.007825 + 2.014102 = 3.021927 3.071927 / 2 = 1.5109635 amu

  14. Sample Problem • What is the atomic mass of silicon if 92.21% of its atoms have mass 27.977u, 4.70% have mass 28.976 u and 3.09% have mass 29.974? • 28.1 u • .9221 x 27.997 = 25.82 u • .0470 x 28.976 = 1.36 u • .0309 x 29.974 = 0.926 u

  15. Sample #2 • Neon has two isotopes. Neon-20 has a mass of 19.992 u and neon-22 has a mass of 21.991 u. In any sample of 100 neon atoms, 90 will be neon-20 and 10 will be neon-22. Calculate the average atomic mass of neon. • 20.192 u • (.90 x 19.992) + (.10 x 21.991) • 17.9928 + 2.1991

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