1.2 Atomic Mass Vs Atomic Weight Flashcards
What are the different terms used to describe the heaviness of an element?
Atomic mass and atomic number (which are essentially synonymous) and atomic weight.
While atomic weight is a constant for a given element and is reported in the periodic table, the atomic mass or mass number
Varies from one isotope to another
The mass of one proton is approximately
One amu
Atomic number (Z)
Number of protons
Mass number (A)
Number of protons + number of neutrons
Number of protons
Number of electrons (in a neutral atom)
Electrons are not included in mass calculations because
They are much smaller
The atomic number of an atom (in amu) is nearly equal to its
Mass number
Atoms of the same element with varying mass numbers are called
Isotopes
Isotopes mean
Same place
Isotopes differ in their number of neutrons and are referred to by
the name of the element followed by the mass number (ex: carbon-12, iodine-131)
What are the three isotopes of hydrogen?
Protium, deuterium, and tritium
Protium
Greek: “first”, has one proton and an atomic mass of 1 amu
Deuterium
Greek: “second”, has one proton and one neutron and an atomic mass of 2 amu
Tritium
Greek: “third”, has one proton and two neutrons and an atomic mass of 3 amu.
Because isotopes have the same number of protons and electrons
They generally exhibit similar chemical properties
In nature, almost all elements exists as
Two or more isotopes, and these isotopes are usually present in the same proportions in any sample of a naturally occurring element
The weighted average of these different isotopes is referred to as the
Atomic weight and is the number reported on the periodic table. For example chlorine has two main naturally occurring isotopes: chlorine-35 and chlorine-37; therefore, the atomic weight of chlorine is closer to 35 then 37. On the periodic table it is listed as 35.5.
Half life corresponds with
Stability, it also helps determine the relative proportions of these different isotopes
When an element has two or more isotopes
No one isotope will have a mass exactly equal to the elements atomic weight. Bromine, for example, is listed in the periodic table as having a mass of 79.9 amu. This is an average of the two naturally occurring isotopes, bromine-79 and bromine-81, which occur in almost equal proportions. There are no bromine atoms with an actual mass of 79.9 amu.
The utility of the atomic weight is that it represents both
The mass of the “average” atom of that element, in amu, and the mass of one mole of the element, in grams
A mole is a number of
“Things” (atoms, ions molecules) equal to Avogadro’s number NA=6.02 x 10^23
For example, the atomic weight of carbon is 12.0 amu, which means that the average carbon atom has a mass of 12.0 amu (carbon-12 is far more abundant than carbon-13 or carbon-14) , and 6.02 x 10^23 carbon atoms have a combined mass of 12.0 grams
Atomic mass is nearly synonymous with
Mass number
Atomic weight is a weighted average of
Naturally occurring isotopes of that element
