Isotopes and Average Atomic Mass

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Presentation transcript:

Isotopes and Average Atomic Mass

Isotopes The number of protons define the type of atom: 6 protons is carbon 1 proton is hydrogen 8 protons is oxygen 92 protons is uranium However the number of neutrons can vary within a given type of atom

Hydrogen Isotopes There are three main isotopes of hydrogen Hydrogen – 1p, 0n, 1e Deuterium – 1p, 1n, 1e Tritium – 1p, 2n, 1e

Relative Mass of an Atom The mass of an atom is expressed in atomic mass units (amu) Atomic mass is determined based upon carbon-12 as a standard with a mass exactly equal to 12amu Using this, the relative mass of every atom on the periodic table can be determined (i.e. oxygen 16 would have a mass of 16amu)

Isotopic Abundance Because elements as a combination of different isotopes, it is useful to consider the isotopic abundance For example, magnesium is composed of: Magnesium-24 – 79% Magnesium-25 – 10% Magnesium-26 – 11% Regardless of where you find magnesium, it will be composed of these percentages

Average Atomic Mass and the Periodic Table The average atomic mass that appears on the periodic table is the average of the mass of all the element’s isotopes Therefore, even carbon, which is used as the standard, does not have an average atomic mass of 12 but rather 12.01 due to the presence of carbon-14 Carbon-14 is about 1.1% of all carbon while carbon-12 is the remaining 98.1%

Average Atomic Mass Calculation

Average Atomic Mass Naturally occuring silver exists as two isotopes. From the mass of each isotope and the isotopic abundance listed below, calculate the average atomic mass of silver Isotope Atomic mass (u) Abundance (%) Silver-107 106.9 51.8 Silver-109 108.9 48.2

Average Atomic Mass

Why do we need to know Average atomic mass? To calculate formula mass and molecular mass: 3 Steps to calculate the formula mass/molecular mass Above all Double Check to make sure you have the correct formula Count the atoms of each element Multiply the number of atoms by the elements atomic mass (round mass to hundredths) Add all the masses together

Formula Mass This is the mass of one molecule and the unit is atomic mass units (amu) This is a very, very small value so it is not very useful in the laboratory. We use moles of molecules, which is a group of molecules.