# Relative atomic mass Atoms are too small to be weighed individually, even with the best balance! We can, however, compare the masses of different atoms.

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Relative atomic mass Atoms are too small to be weighed individually, even with the best balance! We can, however, compare the masses of different atoms.

Relative atomic mass We give the lightest atom (a hydrogen-1 atom) a mass of 1. All other atoms are compared so many times heavier than this. We call this relative atomic mass (RAM).

Relative atomic mass E.g. a helium atom has a RAM of 4. This means a helium atom is four times heavier than a hydrogen atom.

Relative atomic mass A magnesium atom has a RAM of 24. How many times heavier is a magnesium atom than a helium atom? SIX times heavier! (He has a RAM of 4, 24/4 = 6)

Relative atomic mass Some elements have RAM that are not whole numbers. This is because the elements exist as a number of isotopes – each of which has atoms of different mass. Tin has 10 different isotopes!

The RAM of chlorine The RAM of Cl is 35.5, how do we explain this? Cl exists as two isotopes, Cl-35 & Cl- 37. In any sample of Cl, 75% of the atoms are Cl- 35 and 25% are Cl-37.

The RAM of chlorine The average mass, taking into consideration the different number of atoms of the two isotopes, is: (75% x 35) + (25% x 37) = 35.5

Atomic Mass Silver is found to have two stable isotopes, one has an atomic mass of 106.904 amu and the other weighs 108.905 amu. The first isotope represents 51.82 % of the mass of the element and the second represents 48.18 %. What is the atomic mass of the element silver? The equation to use is %X + % Y = average And remember to turn your percents into fractions before multiplying. (0.5182) 106.904 amu + (0.4818) 108.905 amu =? 55.398 amu + 52.470 amu =? 107.868 amu !! Now look at the periodic table to verify the answer.

PRACTICE PROBLEMS 1 A sample of neon contains three isotopes, neon-20 (with an isotopic mass of 19.9924 amu), neon-21 (20.9939 amu) and neon-22 (21.9914 amu). The natural abundances of these isotopes are 90.92%, 0.257 %, and 8.82 %. Calculate the atomic weight of neon. 2.There are only two naturally occuring isotopes of copper, 63 Cu and 65 Cu. Calculate the atomic mass of Cu given the ff. mass(amu) abundance(%) Cu-63 62.930 69.09 Cu-65 64.928 30.91

Quiz 1.Carbon occurs naturally as two isotopes. Calculate the atomic mass of C, given the mass and natural abundance of each isotopes. 2.Si the second most abundant element in the earths’ crust is used extensively in the electronic industry. Calculate the atomic mass of Si, gives the mass and abundance of its three natural isotopes

Isotope massabundance Si-28 27.977 92.21 Si-29 28.976 4.70 Si-30 29.974 3.09

Zn occurs naturally as five isotopes. Calculate the atomic mass for Zn, given the ff. isotopes; Isotopes mass(amu)(%) Zn-6463.929148.89 Zn-6665.926027.81 Zn-6766.9271 4.11 Zn-6867.924918.57 Zn-7069.9253 0.62

Given the ff. information about the isotopes of potassium (K). Calculate the RAM. Isotope abundance(%) amu K-3993.258138.963707 K-400.011739.963998 K-416.730240.961826

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