Sometimes we run into the issue of having the same element that weighs something different. Like people, it still identifies the same way, but its MASS is different. Why? Look at the above image to find out.
Incidentally, this means something for the periodic table values as well. Mass numbers are an average. Its like going to PE class and looking at the charts that tell you what you should weigh, based on your height. Elements do the same thing, their weight is an average based on protons + neutrons. To find out how we do this visit the links below. You will also simulate this in the lab.
OBJECTIVE: use isotopic composition to calculate average atomic mass of an element
1. Be able to determine how many neutrons are in an isotope
2. Be able to calculate atomic mass from %composition
Incidentally, this means something for the periodic table values as well. Mass numbers are an average. Its like going to PE class and looking at the charts that tell you what you should weigh, based on your height. Elements do the same thing, their weight is an average based on protons + neutrons. To find out how we do this visit the links below. You will also simulate this in the lab.
OBJECTIVE: use isotopic composition to calculate average atomic mass of an element
1. Be able to determine how many neutrons are in an isotope
2. Be able to calculate atomic mass from %composition
Homework helper:
Section 1: If the total mass number is written after the element name, and the element is identified by the number of protons, then the number of neutrons is equal to the mass number minus the atomic number. Example: Potassium-39 has 39mass - 19protons = 20 nuetrons
Section 2: If elements are identified by the number of protons, and their mass is protons plus neutrons. Example: 76 protons and 114 neutrons is Osmium-190. (76 = Osmium, 76+114 = 190)
Section 3: To get the average atomic mass (just like in your lab) multiply each isotopes mass by its percentage, then add them together. Example: Rubidium has two common isotopes, 85Rb and 87Rb. If the abundance of 85Rb is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of rubidium?
(85 x 72.2%) + (87 x 27.8%) = 85.56 amu. (amu = atomic mass units)
Section 1: If the total mass number is written after the element name, and the element is identified by the number of protons, then the number of neutrons is equal to the mass number minus the atomic number. Example: Potassium-39 has 39mass - 19protons = 20 nuetrons
Section 2: If elements are identified by the number of protons, and their mass is protons plus neutrons. Example: 76 protons and 114 neutrons is Osmium-190. (76 = Osmium, 76+114 = 190)
Section 3: To get the average atomic mass (just like in your lab) multiply each isotopes mass by its percentage, then add them together. Example: Rubidium has two common isotopes, 85Rb and 87Rb. If the abundance of 85Rb is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of rubidium?
(85 x 72.2%) + (87 x 27.8%) = 85.56 amu. (amu = atomic mass units)