Atoms
Atomic Number
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Identifies the element
Atomic # = #protons= # electrons
Is the number of protons in the nucleus of
each atom of the element
In a periodic table the atomic number is
above the symbol of the element
http://education.jlab.org/qa/pen_number.
html
Mass Number
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Is the total number of protons and
neutrons in the nucleus
Helps to identify an Isotope (atoms of the
same elements with different masses)
Mass # - Atomic # = # of neutrons
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Which will identify isotopes
Example: How many protons, electrons and
neutrons are there in an atom of chlorine-37?
37 – 17(atomic # =protons and electrons)
=20 neutrons
Isotopes
 Isotopes
are atoms of the same element that
have different masses.
 The
isotopes of a particular element all have the
same number of protons and electrons but
different numbers of neutrons.
 Most
of the elements consist of mixtures of
isotopes.
 Isotopes
 Worksheet to be worked on in class
Molar Mass
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The mass of one mole of a pure substance
is called the molar mass of that substance.
Molar mass is usually written in units of
g/mol.
The molar mass of an element is
numerically equal to the atomic mass of
the element in atomic mass units
The Mole
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.
 The mole is the SI unit for amount of
substance
Avogadro’s Number
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Avogadro’s number—6.022 1415 × 1023—is
the number of particles in exactly one mole of a
pure substance.
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.
In these calculations, Avogadro’s number is
expressed in units of atoms per mole.
Relating Mass to Numbers of Atoms
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How many moles of silver, Ag, are in
3.01x1023atoms of silver?
Ag atoms x mol Ag/Avogadro’s # of Ag atoms
= moles of Ag
3.01x1023Ag atoms x1 mol Ag/6.022x1023Ag
atom
=0.500 mol Ag
Solving Mole Problems
Gram/Mole Conversions
 Chemists
use molar mass as a conversion factor in
chemical calculations.
 For example, the molar mass of helium is 4.00 g
He/mol He.
 For example, how many grams of He are in 2.00
mol of He?
2.00 m ol He 
4.00 g He
1 m ol He
= 8.00 g He
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2.25 mol Fe x 55.85g Fe/1mol Fe= 126g Fe
0.375 mol K x 39.10g K/1 mol K= 14.7g K
0.0135 mol Na x 23.0g Na/1mol
Na=0.310gNa
16.3 mol Ni x 58.70gNi/1 mol Ni= 958g Ni
5.00g Ca x 1 mol Ca/40.08g Ca= 0.125 molCa
3.60x10-10gAu x 1 mol Au/196.97g Au=
1.83x10-12mol Au
Solving Mole
How many moles of lead (Pb) are in
1.50 x 1012 atoms of lead?
1.50 x 1012Pb atoms x 1mol Pb/6.022 x 1023 Pb atoms
= 2.49 x 10-12 mol Pb
Mol to Atom Conversions

amount of substance x 6.022x1023atoms
given in mol
1 mol
= answer in atoms

How many atoms of sulfur are in 4.00 g of
S?
How do we start?
What is given?
What are we solving?
Atom to Mol Conversions
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Amount of substance x 1 mol
given in atoms
6.022 x1023 atoms
= answer in mol
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How many moles of tin,Sn, are in 2500
atoms of tin?
How do we start?
What is given?
What are we solving?
Mol to Mass Conversions
Amount of substance x molar mass of element(g)
given in mol
1 mol
= answer in grams
Remember molar mass unit is g/mol and is the
atomic mass on the periodic table

What is the mass in grams of 0.0135 mol
of sodium?
Mass to Mol Conversions

Amount of substance x 1mol
given in mass (g)
molar mass (g)
=answer in mol
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How many moles of gold are in 3.60x10-10 g
of Au?
Mass to Atom Conversions
Amt of substance x 1mol
x 6.022x1023atom
given in mass (g)
molar mass
1 mol
= answer in atoms

How many atoms of sulfur are in 4.00g of
S?
Atom to Mass Conversions
Amt. of substance x 1mol
x molar mass(g)
given in atoms 6.022x1023atom
1 mol
=answer in mass (g)
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What is the mass in grams of
1.50x1023atoms of tin?