Download University Chemistry The Modern Atomic Theory Structure of Atoms

The Modern Atomic Theory
Modern Atomic theory has four assumptions:
1. Atoms make up all matter.
2. The atoms of one element are different from the
atoms of another element.
3. Atoms combine in definite ratios to make
compounds.
4. Combinations of atoms in compounds can change
only when a chemical reaction happens. This
means reactions alter atom combinations, but the
identity of the atoms themselves remain the same.
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Atomic number, Z:
Structure of Atoms
The identity of an element is controlled by the number
of protons in the nucleus.
• Atoms are made up of three main particles,
neutron, electron, and proton.
Particle
Symbol
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Charge
Mass
electron
e
-
-1
0.0005486 amu
proton
p+
+1
1.007276 amu
neutron
no
0
1.008665 amu
In the neutral atom: number of protons inside the
nucleus is the same number of electrons around the
nucleus.
Atomic number = # of Protons = # of Electrons
Every element has its own unique atomic number.
Example What is the atomic number for nitrogen, N?
Nitrogen is in the seventh position in the periodic table.
This means nitrogen atoms have 7 protons in the nucleus, 7 electrons
around the nucleus, and they have an atomic number of 7.
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1
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• Mass number, A:
Isotopes
It is equal to the sum of neutrons and protons inside the
nucleus, because the "massive" particles in the atom are
protons and neutrons.
Mass number = # of Protons + # of neutrons
# of neutrons = Mass number - # of Protons
# of neutrons = A – Z
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All the atoms of the same element have the same atomic
number, but they can have different:
numbers of neutrons and mass numbers
How many neutrons, electrons and protons are in an atom of Na?
Sodium, Na, has atomic number 11.
# of Protons =11 # of Electrons = 11
Number of neutrons = A - Z
Number of neutrons = 23 - 11 = 12
11
An atom with a mass number of 39 contains 20 neutrons. What is
the atomic number and identity of the element?
The atomic number is Z = 39- 20 =19.
The identity is potassium because K is element 19
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Atomic Mass
The atomic mass of an element is the mass average of the atomic masses of the
different isotopes of an element.
For example, naturally occurring carbon, for example, is a mixture of two
isotopes, 12C (98.89%) and 13C (1.11 %).
Individual carbon atoms therefore have a mass of either 12.000 or 13.03354 amu.
But the average mass of the different isotopes of carbon is 12.011 amu.
Isotope abundances
The isotopes of an element do not occur with equal frequency.
The relative abundance depends on the relative stability of the isotope.
The isotopes contribute to the average atomic mass based on their
abundance.
The atomic weights in the periodic table are weighted averages.
This means the tabulated value doesn't match any actual atom, but is closer to
the most common isotope.
Average weight = % First isotope abundance x its mass +
% Second isotope abundance x its mass
What is the average atomic mass for thallium, Tl, if there are two isotopes with the
following masses and abundances? (Tl-203 (203Tl) has a mass of 203.059 amu
with an abundance of 29.52 %, Tl-205 (205Tl) has a mass of 205.059 amu with an
abundance of 70.48 %)
Step 1: Convert percents to decimals 29.52 % to 0.2952 and 70.48 % to 0.7048
Step 2: Average weight = 0.2952 x ( 203.059 amu) + 0.7048 x ( 205.059 amu)
204.466 amu rounded off to 204.5 amu with 4 significant.
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Molecular Mass
•
•
It is the sum of the atomic masses of the atoms in the molecules that form
these compounds
For example, the molecular mass of the sugar molecule (C12H22O11) found
in cane sugar is the sum of the atomic masses of the 12 carbon atoms + 22
hydrogen atoms + and 11 oxygen atoms
12 C atoms = 12 x (12.011) amu =
144.132 amu
22 H atoms = 22 x (1.0079) amu =
22.174 amu
11 O atoms = 11 x (15.9994) amu =
175.993 amu
b) CH3OH
MW of CH3OH
= 4x H + 1xC +1xO = 4x1 + 1x12 + 1x16 = 32 g/mole
342.299 amu
Mole
Calculate the molecular weight of the following:
a) H2SO4
MW of H2SO4 = 2 x H + 1xS +4xO = 2x1 + 1x32 + 4x16 =
98 g/mole
One mole is defined as the number of carbon atoms in exactly
12.000000 grams of pure 12C.
From the sugar example, a mole of C12H22O11 would have a mass of
342.299 grams.
This quantity is known as the molar mass or molecular weight (MW),
a term that is often used in place of the terms atomic mass or
molecular mass.
c) Lauric acid C3H24O2
MW of C3H24O2 = 24 x H + 3xC +2xO = 24x1 + 3x12 +
2x16 = 92 g/mole
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Mass Percent
Chemical and structural Formulas
The Mass Percent of an element is defined as:
Mass Percent of an element =
Mass of the element
x100%
Total molar mass of the sample
What is the mass percent of carbon, hydrogen, and oxygen in pure
ethanol C2H6O?
-First: calculate the molar mass of C2H6O
MW of C2H6O = 2 x C + 6 x H + 1 x O
= 2 x 12.01 + 6 x 1.008 + 1 x 16.00
MW C2H6O = 46.07 g/mole
Mass % C = 100 x (
mass of C
) = 100 x ( 2 x 12.01 )= 52.14 %
total molar mass
46.07
Mass % H = 100 x (
mass of H
) = 100 x ( 6 x 1.008 )= 13.13 %
total molar mass
46.07
mass of O
) = 100 x ( 1 x 16.00 )= 34.72 %
total molar mass
46.07
Note that the mass percentages should add up to 100%.
Mass % = Mass % C + Mass % H + Mass % O
= 52.14 % + 13.13 % + 34.72 % = 99.99 %
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The chemical formula tells you how many of each type of atom are in a
molecule.
The structural formula tells you how many of each type of atoms are in a
molecule and also how they are connected.
For example, the chemical formula for ethanol is C2H6O and
The structural formula of ethanol is
Be carful, the chemical formula could be the same for different molecules, but
the structural formula is unique.
The chemical formula for dimethyl ether is C2H6O and
The structural formula of dimethyl ether is:
-Second: calculate the mass percents
Mass % O = 100 x (
10
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Empirical Formulas (simplest formula)
Combustion Analysis
• It shows the simplest whole number ratio
of atoms in a molecule.
• It is used to determine the mass % for
different elements in the compound.
• For example, hydrogen peroxide's chemical formula is
H2O2, but its empirical formula is HO
Molecular Formula = (
Molecular weight of unknown (g/mole)
)xEmperical formula
mass of Emperical formula
Write the different formulas for the glucose molecule
The chemical formula for glucose is C6H12O6, but its empirical
formula is CH2O, and its structural formula is
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• Ascorbic acid (vitamin C) contains only C, H, and O.
Combustion of 1.000 g of Ascorbic acid produced
40.9% C and 4.5% H. What is the empirical formula
for Ascorbic Acid?
What is the molecular formula if the molecular
mass of Ascorbic Acid was founded to be 176
g/mole?
First: calculate the mass percent of Oxygen.
Since the sample contains C, H, and O, then the remaining
100% - 40.9% - 4.5% = 54.6% is Oxygen
Molecular Formula = ( Molecular weight of unknown (g/mole) ) x empirical Formula
Second: Suppose 100 g of this substance
Steps
The sample is burned in the presence of excess oxygen which
converts all the carbon to carbon dioxide and all the hydrogen to
water. The CO2 and H2O produced are absorbed in two different
stages and their masses determined by measuring the increase in
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weight of the absorbers.
mass of emperical formula
C
H
O
40.9
4.5
54.6
Molecular Formula = (
1
Mass /g
mass
molar mass
2
No. of moles =
3
÷ smallest number (3.4)
4
x by a number to make step 3
integer numbers (x 3)
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Empirical formula C3H4O3
40.9
12
=3.4
4.5
1
=4.5
54.6
16 =3.4
1
1.3
1
1x3=3
1.3 x 3 = 4
1x3=3
3C
4H
3O
= 2 x C3H4O3 = C6H8O6
15
4
176 (g/mole)
3x12 + 4x1 + 3x16 ) x C3H4O3 =
16
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