Notesale: Turn your study into money

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Counting Atoms
Atomic Number
All atoms are composed of the same basic particles
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Atoms of different dements have different
numbers of protons
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The atomic number (Z) of an element is
the number of protons of each atom of that element
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If you want to know which
element has atomic number 47, it is element silver Ag, on the periodic table
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Because atoms
are neutral, we also know from the atomic number that all silver atoms must also have 47 electrons
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All hydrogen atoms have only one proton
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There are three types of hydrogen atoms known
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It accounts
for 99
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The nucleus of a protium atom consists of one proton only, and it has one
electron moving about it
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One is called deuterium, which accounts for 0
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Each deuterium atom has a nucleus with one proton and one neutron
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It exists in very small amounts in nature, but it can be prepared artificially
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Protium, deuterium, and tritium are isotopes of hydrogen
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The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons
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Most of
the elements consist of mixtures of isotopes
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Mass Number
Identifying an isotope requires knowing both the name or atomic number of the element and the mass of the isotope
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The three isotopes of hydrogen
described earlier have mass numbers of 1, 2, and 3
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Isotopes are usually identified by specifying their mass
number
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In the first method, the mass number is written with a hyphen after the
name of the element
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This is referred as the hyphen notation
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The second
method shows the composition of a nucleus as the isotope’s nuclear symbol
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The superscript indicates the mass number and the subscript indicates the atomic number
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The number of neutrons if found by subtracting the atomic number from
the mass number
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Nuclide is a general term for a specific isotope of an
element
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For most chemical calculations it is more convenient to use relative atomic
masses
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The masses of all other atoms are expressed in relation to this defined standard
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It has been arbitrarily assigned a mass of
exactly 12 atomic mass units, or 12 amu
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The
atomic mass of any atom is determined by comparing it with the mass of the carbon-12 atom
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The precise value of the atomic mass of a hydrogen-1
atom is 1
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An oxygen-16 atom has about 16/12 (or 4/3) the mass of a carbon-12 atom
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Its atomic mass is 23
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Isotopes of an element may occur naturally, or they may be made in the laboratory (artificial isotopes)
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The masses of subatomic particles can also be expressed on the atomic mass scale
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000 5486 amu,
that of the proton is 1
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008 665 amu
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Average Atomic Masses of Elements
Most elements occur naturally as mixtures of isotopes
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The percentage at which each of an element’s isotopes
occurs in nature is taken into account when calculating the element’s average atomic mass
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The following is a simple example of how to calculate a weighted average
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If 25% of the marbles have masses of 2
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00 g each, how is the weighted average
calculated? You could count the number of each type of marble, calculate the total mass of the mixture, and divide by the total
number of marbles
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25 marbles x 2
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00 g = 225 g
Adding these masses gives the total mass of the marbles
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75 g
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Then add the products
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For example, naturally occurring copper consists of 69
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929 601 amu, and
30
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927 794 am
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0
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929 621 amu + 0
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927 794 amu = 63
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55 amu
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Three very important concepts—the mole, Avogadro’s number, and molar mass—provide the basis
for relating masses grams to numbers of atoms
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A mole (abbreviated mol) is the amount of a substance that contains as many
particles as there are atoms in exactly 12 g of carbon-12
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Mole relates to
masses of atoms and compounds
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The best modern value is 6
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This means that exactly 12 g of carbon-12 contains 6
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The numer of particles in
a mole is known as Avogadro’s number, named for the nineteenth-century Italian scientist Amedeo Avogrado, whose ideas
were crucial in explaining the relationship between mass and numbers of atoms
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022 1415 x 1023—is
the number of particles in exactly one mole of a pure substance
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022 x
1023
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Can you figure out
the approximate mass of one mole of helium atoms? You know that a mole of carbon-12 atom has a mass of exactly 12 g and
that a carbon-12 atom has an atomic mass of 12 amu
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00 amu, which is about one-third
the mass of a carbon-12 atom
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Thus, one mole of helium has a mass of about 4
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The mass of one mole of a pure substance is called the molar mass of that substance
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The molar mass of an element is numerically equal to the atomic mass of the element in atomic mass units (which can
be found in the periodic table)
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94 g/mol, while the molar mass of mercury,
Hg, is 200
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The molar mass of an element contains one mole of atoms
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00 g of helium, 6
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59 g of
mercury all contain a mole of atoms
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For example, the molar mass of helium is 4
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To find how many grams of helium there are in two moles of helium, multiply by the molar mass
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00 mol He x 4
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00g He
Mass of
elements in
grams

= Molar mass of element/ 1 mol x

X 1 mol/ molar mass of element =

Amount of element
in moles

= 1 mol/6
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022 x 1023 atoms/1 mol =

Conversions with Avogadro’s number
Avogadro’s number can be used to find the number of atoms of an element from the amount in moles or to find the amount of
an element in moles from the number of atoms
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