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 Notice how the atomic # (# of protons) does NOT change but the mass number does.

4. Determining the mass number of isotopes 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 number of a specific isotope you need to add the number of protons to the number of neutrons.

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 = 235 15 6 U

6. Representing Isotopes Option # 1 U K C Option #2 U – 235 K - 40 C – 14 Only the mass number is listed, the atomic number can be found on the P.T. 92 235 40 14 6 19

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 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. IsotopeMassAbundanceMass Contribution Ne-2020 amu90.60%18.12 Ne-2121 amu0.20%0.042 Ne-2222 amu9.200%2.024 Avg. mass =20.19

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? 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?