University Chemistry
1 2The 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.
Structure of Atoms
• Atoms are made up of three main particles, neutron, electron, and proton.
1.008665 amu 0
no neutron
1.007276 amu +1
p+ proton
0.0005486 amu -1
e- electron
Mass Charge
Symbol Particle
Atomic number, Z:
The identity of an element is controlled by the number of protons in the nucleus.
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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• Mass number, A:
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
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
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
23 11
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Isotopes
All the atoms of the same element have the same atomic number, but they can have different:
numbers of neutrons and mass numbers
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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.
8 Atomic Mass
Theatomic 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.
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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
342.299 342.299 342.299 342.299 amuamuamuamu 175.993 175.993 175.993 175.993 amuamuamuamu 11 O atoms = 11 x (15.9994)
11 O atoms = 11 x (15.9994) 11 O atoms = 11 x (15.9994) 11 O atoms = 11 x (15.9994) amuamuamuamu = = = =
22.174 22.174 22.174 22.174 amuamuamuamu 22 H atoms = 22 x (1.0079)
22 H atoms = 22 x (1.0079) 22 H atoms = 22 x (1.0079) 22 H atoms = 22 x (1.0079) amuamuamuamu ====
144.132 144.132 144.132 144.132 amuamuamuamu 12 C atoms = 12 x (12.011)
12 C atoms = 12 x (12.011) 12 C atoms = 12 x (12.011) 12 C atoms = 12 x (12.011) amuamuamuamu ====
Mole
One mole is defined as the number of carbon atoms in exactly 12.000000 gramsof pure 12C.
From the sugar example, a mole of C12H22O11would 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.
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Calculate the molecular weight of the following:
a) H
2SO
4MW of H
2SO
4= 2 x H + 1xS +4xO = 2x1 + 1x32 + 4x16 = 98 g/mole
b) CH
3OH MW of CH
3OH
= 4x H + 1xC +1xO = 4x1 + 1x12 + 1x16 = 32 g/mole
c) Lauric acid C
3H
24O
2MW of C
3H
24O
2= 24 x H + 3xC +2xO = 24x1 + 3x12 + 2x16 = 92 g/mole
Mass Percent
The Mass Percent of an element is defined as:
% 100 samplex the of mass molar Total
element the of element Mass
an of Percent
Mass ====
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
-Second: calculate the mass percents
mass molar total
C of mass
46.07 12.01 x Mass % C = 100 x ( ) = 100 x (2 )= 52.14 %
mass molar total
H of mass
46.07 1.008 x
Mass % H = 100 x ( ) = 100 x (6 )= 13.13 %
mass molar total
O of mass
46.07 16.00 x Mass % O = 100 x ( ) = 100 x (1 )= 34.72 %
Note that the mass percentages should add up to 100%.
Chemical and structural Formulas
The chemical formulatells you how many of each type of atom are in a molecule.
The structural formulatells 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 C2H6Oand 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 C2H6Oand The structural formula of dimethyl ether is:
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Empirical Formulas (simplest formula)
• It shows the simplest whole number ratio of atoms in a molecule.
• For example, hydrogen peroxide's chemical formula is H
2O
2, but its empirical formula is HO
formula Emperical x formula ) Emperical of mass
(g/mole) unknown of weight Molecular ( Formula Molecular ====
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 is14
Combustion Analysis
• It is used to determine the mass % for different elements in the compound.
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 CO2and H2O produced are absorbed in two different stages and their masses determined by measuring the increase in weight of the absorbers.
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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?
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
Second: Suppose 100 g of this substance
mass molar
mass 12 40.9
1 4.5
16 No. of moles = 54.6
3 O 4 H 3 C
Empirical formula C3H4O3 5
1 x 3 = 3 1.3 x 3 = 4 1 x 3 = 3
x by a number to make step 3 integer numbers (x 3) 4
1 1.3 1
÷smallest number (3.4) 3
=3.4
=4.5
=3.4 2
54.6 4.5
40.9 Mass /g
1
O H C
Steps
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What is the molecular formula if the molecular mass of Ascorbic Acid was founded to be 176 g/mole?
formula emperical of mass
(g/mole) unknown of weight Molecular
16 x 3 1 4x 3x12
(g/mole) 176
+ + Molecular Formula = (
Molecular Formula = (
) x C3H4O3=
) x empirical Formula
= 2 x C3H4O3= C6H8O6
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