Atomic Mass Unit
Chemists of the nineteenth century realized that, in order to measure the mass of an atomic particle, it was useless to use the
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standard weight measure unites (e.g. Kilogram). !
Initially they begun to consider the mass of an hydrogen, so the proton mass, as relative measure.
However, experimental evidence demonstrated that this method was not reliable as attraction forces between nucleons influence the masses of the atomic particles.
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As a matter of fact, the stronger is number of nucleons in an atom the lower is the relative mass.
This mass change is termed mass deficit .
Δ m = (unbound system calculated mass) - (measured mass of system)

Atomic Mass Unit
As in all atoms (except hydrogen) the nucleons are more than one, chemists used as relative mass the mass of an atom of Carbon
Carbon exists in nature as stable element in its two isotopic forms: C 12 and C 13 .
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The most abundant isotope is 12 C with an abundance of 98,89% .
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An atom of 12 C has in its nucleus 12 nucleons bounded (6 e and 6 n).
Hence, the mass of a nucleon, also called Atomic Mass Unit (amu), is defined exactly as ⅟
12
of the mass of a atom of Carbon-12 .
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Bonded protons and neutrons have an approximate mass of 1 amu (about 1,66 x 10 -24 g).

Atomic Mass Unit

Atomic weight
Atomic Mass Unit
Each element exist in nature as a mix of isotopes with different percentages, independently on the place were it is found.
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E.g.
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Chlorine (Cl) has 24 isotopes with mass numbers ranging from 28 Cl to 51 Cl. !
There are two principal stable isotopes, 35 Cl (75.78%) and 37 Cl (24.22%).
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Hence, for every 1000 atoms of Chlorine, there are: !
• 758 35 Cl which weight 35 amu !
• 242 37 Cl which weight 37 amu !
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The total mass of those 1000 Cl atoms is: !
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(758 x 35) + (242 x 37) = 35484 !
Each atom of those 1000 Cl has an atomic weight of: !
35484/1000 = 35,484 amu

Atomic weight
Atomic Mass Unit
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Practice exercise !
• Natural nitrogen (N) consists of two stable isotopes, 14 N, which makes up the vast majority of naturally occurring nitrogen (about 99.6%), and 15 N.
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Calculate the atomic weight of and atom of N !
• Lead (Pb) has four stable isotopes: 204 Pb, 206 Pb, 207 Pb, 208 Pb. The percentages of each isotope are: !
204 Pb = 1.4
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206 Pb = 24.1
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207 Pb = 22.1
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208 Pb = 52.4
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Calculate the atomic weight of and atom of Pb

Atomic weight
Atomic Mass Unit
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Practice exercise !
• Natural nitrogen (N) consists of two stable isotopes, 14 N, which makes up the vast majority of naturally occurring nitrogen (about 99.6%), and 15 N.
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Calculate the atomic weight of and atom of N !
• Lead (Pb) has four stable isotopes: 204 Pb, 206 Pb, 207 Pb, 208 Pb. The percentages of each isotope are: !
204 Pb = 1.4
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206 Pb = 24.1
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207 Pb = 22.1
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208 Pb = 52.4
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Calculate the atomic weight of and atom of Pb

Molecular weight
Molecules are composed of atoms that have bonded together. As the formation of chemical bonds does not involve any variation of mass, the molecular weight is the sum of the weight of each constituent atom multiplied by the number of atoms of that molecule.
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The molecular weight (m) is measured in atomic mass units (amu) and sometimes is called
Formula weight (FW)
The molecular weight ( m ) of a molecule of
Glucose (C
6
H
12
O
6
) is calculated as the sum of the atomic weights of the single atoms:
C
(C
6
H
12
O
6
)
H
C
=(6x12,01)+(12x1,01)+(6x16,00)= 180,18 uma

The Mole
Even the smallest bit of matter can contain an enormous number of atoms. So chemists have to deal with large amounts of atoms in any real situations.
The Mole is a unit of measurement used in chemistry to express amounts of a chemical substance. !
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The Mole is defined as the amount of substance that contains as many entities (atoms, molecules, or other particles) as there are atoms in 12 grams of pure 12 C.
This corresponds to the Avogadro number, which has a value of 6.022
× 10 23
It is one of the base units in the International System of Units, and has the unit symbol mol

The Mole
The mole is used in chemistry instead of units of mass as a convenient way to express
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amounts of reactants or of products of chemical reactions. !
For example, the chemical equation !
2 H
2
+ O
2
→ 2 H
2
O !
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implies that 2   mol of di-hydrogen (H
2
) and 1   mol of di-oxygen (O2) react to form 2   mol of water (H
2
O).
A mole is simply a unit

The Mole
A dozen is a specific amount of objects.
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E.g. a dozen of eggs = 12 eggs = 680 grams
1 dozen = 680 g
1 dozen = 12 eggs
EGGS x 12
÷ 12
DOZEN x 680 g
GRAMS
÷ 680 g

The Mole
A mole is a specific amount of particles.
1 mol = Molecular mass (g)
1 mol = 6,022x10 23 particles x 6,022x10 23
PARTICLES MOLES x Mol. mass
GRAMS
÷ 6,022x10 23 ÷ Mol. mass

The Mole mol =
Mass (g)
Molar mass
23
The mass (in g) of one mole of atoms of a pure element is equal to the
Atomic weight in amu.
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This is also called the Molar mass of the element and its units are g/mol
PARTICLES x 6,022x10 23
MOLES x Mol. mass
GRAMS
÷ 6,022x10 23 ÷ Mol. mass

The Mole
Mass (g) mol =
Molar mass
Practice exercise: moles of Atoms !
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How many atoms does 136.9 g of Iron contain?
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The Atomic weight of Fe is 55.85 amu, This tells us that the molar mass
(mass of one mole of atoms) is 55.85 g/mol (or one mole of Fe atoms is in
55,85 g of Iron ). We can express this as: !
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55.85 g Fe !
1 mol Fe atoms
= Molar mass
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Because 1 mol of Fe atoms has a mass of 55.85 g, we expect that 136.9 g will be:
Mass of Fe (g) 136.9 g mol of Fe atoms = = = 2.45 mols
Molar mass of Fe 55.85 g

The Mole
Mass (g) mol =
Molar mass
Practice exercise: Numbers of Atoms !
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How many atoms are contained in 2.451 mol of Fe?
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One mol of atoms of an element contains Avogadro’s number of atoms, or
6,022x10 23 atoms.
6,022 x 10 23 atoms !
1 mol of atoms
Fe atoms = 2.451 mol Fe x
6,022 x 10 23 atoms !
1 mol of atoms
= 1.48 x 10 24 Fe atoms