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The Atom

The Atom. Lesson 2 - Isotopes. Different Forms of the Same Element. In any specific element, the # of protons is always constant. Unlike the number of protons, the number of electrons and neutrons can vary within atoms of an element without changing the identity of the element.

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The Atom

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  1. The Atom Lesson 2 - Isotopes

  2. Different Forms of the Same Element • In any specific element, the # of protons is always constant. • Unlike the number of protons, the number of electrons and neutrons can vary within atoms of an element without changing the identity of the element. • Ex. Carbon (C) ALWAYS has 6 protons, but it can have anywhere from 6-8 neutrons and 2-10 electrons

  3. Isotopes • Isotopes are atoms of the same element (same number of protons) but with different number of neutrons • Carbon has three isotopes 12 C 6 p+, 6 no 6 Notice how the atomic # (# of protons) does NOT change but the mass number does. 13 C 6 p+, 7 no 6 14 C 6 p+, 8 no 6

  4. The atomic mass on the periodic table is an average of all the known isotopes of each element. • It is not the mass of any individual atom. • To determine the mass of a specific isotope, you need to add the number of protons to the number of neutrons. This is mass number.

  5. Practice • A lithium atom has 3 protons, 3 electrons, and 3 neutrons. • A = • A nitrogen atom has 8 neutrons • A = • An unknown element has 92 protons and 143 neutrons. • Element = A = 6 15 235 U

  6. Representing Isotopes • Option # 1 • Top number is mass number (A), bottom number is atomic number (Z) • U • K • C • Option #2 • U – 235 • K - 40 • C – 14 • Only the mass number (A) is listed, the atomic number (Z) can be found on the Periodic Table 235 92 40 19 14 6

  7. Finding average atomic mass • To find the average atomic mass of an element you need two pieces of information: • The mass numbers of the different isotopes (this is NOT the average mass found on the periodic table) • Neon has three: Ne - 20 , Ne - 21, Ne - 22 • The relative abundance of each isotope • Of all the neon measured, 90.60% is Ne-20, 0.200% is Ne-21, and 9.200% is Ne-22

  8. Finding average atomic mass • Even though it is the least massive, Ne- 20 accounts for the vast majority of Neon.

  9. Examples • Gallium-69 has a relative abundance of 60.11% and Gallium-71 has a relative abundance of 39.89%. What is the average atomic mass of Gallium?

  10. Examples • Thallium has two stable isotopes, Thallium-203 and Thallium-205. Thallium-203 has a relative abundance of 29.52%.Thallium-205 has a relative abundance of 70.48%. What is the average atomic mass of Thallium?

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