Journal 10/16/14 What does AMU mean? Objective Tonight’s Homework To become more familiar with the structure of an atom Finish the practice problems. Structure of the Atom Atoms are made of 3 particles: Positively-charged, heavy protons, found in the middle, or nucleus. Neutral, heavy neutrons, also found in the nucleus. Very light negatively-charged electrons, found far out from the nucleus. Structure of the Atom If we look at the nucleus of an atom, we have protons and neutrons. Protons determine the identity of an element. You can change neutrons or electrons without changing what element an atom is, but if you change protons, you change what it is. Carbon always has 6 protons. Copper always has 29. Sulfur always has 16. Gold always has 79. Structure of the Atom We call the number of protons than at element has it’s “atomic number”. Structure of the Atom Since atoms are so small, they weigh almost nothing. So instead of measuring their weight in grams, we measure it in AMU (atomic mass units). Scientists defined the weight of hydrogen as 1 AMU since it’s the lightest element. An element’s AMU tells us how many times heavier than hydrogen that element is. To measure an element’s AMU, we add together the number of protons and neutrons in the element’s nucleus. For most light elements, the number of neutrons and protons are about equal. Notes on Reading the Periodic Table We can do some simple math to figure stuff out about isotopes. Atomic mass = (M1)(P1)+(M2)(P2)+(M3)(P3)+ … Where: M1 = mass (protons + neutrons) of an isotope P1 = percent that this isotope is of all atoms of this element We can repeat this for other isotopes. (M2 and P2. M3 and P3. Etc, etc Notes on Reading the Periodic Table Example: Carbon has 2 isotopes: - C-12, with a mass of 12.00 amu, makes up 98.89% of all carbon - C-13, with a mass of 13.00 amu, makes up 1.11% of all carbon. What's the average atomic mass?” Notes on Reading the Periodic Table Example: Carbon has 2 isotopes: - C-12, with a mass of 12.00 amu, makes up 98.89% of all carbon - C-13, with a mass of 13.00 amu, makes up 1.11% of all carbon. What's the average atomic mass?” Atomic Atomic Atomic Atomic mass mass mass mass = = = = (M1)(P1)+(M2)(P2) (12)(0.9889)+(13)(0.0111) (11.87)+(0.14) 12.01 Atomic Mass Practice 1. The element copper has naturally occurring isotopes with mass numbers of 63 and 65. The relative abundance and atomic masses are 69.2% and 30.8% respectively. Calculate the average atomic mass of copper. 2. Calculate the average atomic mass of sulfur if 95.00% of all sulfur isotopes are Sulfur-32, 0.76% are Sulfur-33 and 4.22% are Sulfur-34. 3. The four isotopes of lead are shown below, each with its percent by mass abundance and the composition of its nucleus. Using the following data, first calculate the approximate atomic mass of each isotope. Then calculate the average atomic mass of lead. 82p 122n 1.37% 82p 124n 26.26% 82p 125n 20.82% 82p 126n 51.55% 4. Calculate the average atomic mass of bromine. One isotope of bromine has an atomic mass of 78.92amu and a relative abundance of 50.69%. The other major isotope of bromine has an atomic mass of 80.92amu and a relative abundance of 49.31%.