Atomic Mass Mrs. Cook. Atomic Mass - The average relative mass of all naturally occurring isotopes of an element. relative mass – The mass of one object.

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Presentation transcript:

Atomic Mass Mrs. Cook

Atomic Mass - The average relative mass of all naturally occurring isotopes of an element. relative mass – The mass of one object compared to a similar object.

The mass of atoms is compared to carbon 12. An atomic mass unit (amu) is defined as the mass equal to 1/12 the mass of a carbon 12 atom. An atom of carbon 12 then has a mass of exactly amu *Because 12C has 6p and 6n, the mass of a proton and a neutron are about 1 amu.

What is the mass of carbon on the periodic table? amu It is not exactly 12, because carbon exists as three isotopes, carbon 12, carbon 13, and carbon 14. Carbon 13 and carbon 14 are heavier, so increase the average atomic mass.

Atomic Masses are a weighted average. That means that the more common isotopes are counted more in the average.

To calculate atomic mass: 1. Change the percent abundances to a decimal. (Divide by 100.) 2. Multiply the mass of each isotope by the decimal abundance of the isotope. 3. Add the results of step 2.

Remember: A single atom or isotope does not have the mass listed on the periodic table. The periodic table is the AVERAGE of all isotopes. We can use the average because very rarely do scientists have a sample of a single isotope.

Example: Magnesium occurs in nature in three isotopic forms: (78.70% abundance), (10.13% abundance), and (11.17% abundance). The relative masses of these three isotopes are , , and amu, respectively. Calculate the atomic mass of magnesium for these data.

Solution: 1. Change the percent abundances to a decimal 78.70% = % = % = Multiply mass by decimal abundance amu x = amu amu x = amu amu x = amu

3. Add the results of Step amu amu amu amu amu Solution: *Notice that you keep all sig figs until the end*

Calculate the atomic mass for chlorine, given the following: Mass number Exact weight Percent abundance % %

Solution amu x = amu amu x = amu amu amu This is what “showing your work” looks like.

What is the atomic mass of an element if it has three naturally occurring isotopes. One has a mass of amu and make up % of the atoms. The second has a mass of amu and a natural abundance of 4.67%. The third has a mass of amu and makes up 3.10 % of the atoms. Which element is this?

amu x = amu amu x = amu amu x = amu amu It is Silicon! 28.1 amu Solution

Atomic Mass of Candium 1. Obtain a bag of candium. 2. Separate the “atoms” into different isotopes. 3. Count the number of each isotope present. 4. Calculate the percent of each isotope: % abundance = number of atom of the isotope X 100 Total # of atoms 5. Find the mass of each isotope by massing ten atoms and dividing the mass by Calculate the average “atomic” mass by multiplying the mass of each isotope by the percent abundance, as a decimal and adding the results.

Data: Total number of atoms:_________ Isotope“m”“s”“p” Number of atoms Percent abundance Mass of atoms Mass of the isotope The atomic mass of candium is______________. Show your work under the data table!