The Atom Lesson 2 - Isotopes.

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Presentation transcript:

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. Ex. Carbon (C) ALWAYS has 6 protons, but it can have anywhere from 6-8 neutrons and 2-10 electrons

Isotopes C 6 p+, 6 no C 6 p+, 7 no C 6 p+, 8 no 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

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.

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

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

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

Finding average atomic mass Even though it is the least massive, Ne- 20 accounts for the vast majority of Neon. Isotope Mass Abundance Mass Contribution Ne-20 20 amu 90.60% 18.12 Ne-21 21 amu 0.20% 0.042 Ne-22 22 amu 9.200% 2.024 Avg. mass = 20.19 amu

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? Isotope Mass Abundance Mass Contribution Ga-69 69 amu 60.11% 41.48 Ga-71 71 amu 39.89% 28.32 Avg. mass = 69.80 amu

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? Isotope Mass Abundance Mass Contribution Tl-203 203 amu 29.52% 59.93 Tl-205 205 amu 70.48% 144.5 Avg. mass = 204.4 amu