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Chapter 2
Topic 2
Atoms and the Periodic Table
Macroscopic, Microscopic and Symbolic
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Micro World
atoms & molecules
Atomic mass units
Macro World
Elements and Compounds
grams
Numbers vs. Mass
Is the atomic mass, the actual mass of the atom, the same as the mass number, A?
NO!!
The atomic number and mass number represent the number of particles, both protons or neutrons present in the atom. This is a whole number
The atomic mass is the actual mass of the atom either in atomic mass units. This number usually has a decimal
Chapter 2 Unit 1
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Atomic masses
Atoms are composed of protons, neutrons and electrons.
Almost all of the mass of an atom comes from the protons and neutrons which weigh approximately 1 mass unit each.
All atoms of the same element will have the same number of protons. The number of neutrons may vary. Therefore, the mass of a specific isotope of an element will depend upon the masses of protons and neutrons present.
Chapter 2 Unit 1
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Chapter 3 Unit 2
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A Mass Spectrometer is a device which allows the determination of atomic masses of the elements by separating the isotopes or molecular mass and structure by separation the mass of the fractions.
The Atomic Mass Scale and Average Atomic Mass
Atomic mass is the mass of an atom in atomic mass units (amu).
1 amu = 1/12 the mass of a carbon-12 atom
The atomic weight on the periodic table represents the average mass of the naturally occurring mixture of isotopes.
Isotope | Isotopic mass (amu) | Natural abundance (%) |
12C | 12.00000 | 98.93 |
13C | 13.003355 | 1.07 |
The Atomic Mass Scale and Average Atomic Mass
Average mass (C) = (12.00000 amu) x (0.9893)
+ (13.003355 amu) x (0.0107)
Average mass (C) = 12.01 amu
Average atomic masses
Example.
Silicon exists as a mixture of three isotopes. The atomic mass given on the Periodic Table is 28.0855 u. This mass is based on the following data.
Isotope Mass (u) Abundance
28Si 27.976 9265 92.23 %
29Si 28.976 4947 4.67 %
30Si 29.973 7702 3.10 %
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Average atomic masses
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92.23
100
(27.976 9265 u) = 25.80 u
4.67
100
(28.976 4947 u) = 1.35 u
3.10
100
(29.973 7702 u) = 0.929 u
28Si
29Si
30Si
Average atomic mass for silicon = 28.08 u
% abundance x isotopic mass = weighted mass
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Average atomic mass (6.941)
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The mole (mol) is the amount of a substance that contains as many elementary entities as there are atoms in exactly 12.00 grams of 12C. The Mole allows us to make comparisons �between substances that have different masses. The mole is just a conversion factor that allows us to accurately work between the microscopic, atoms and molecules, and the macroscopic.
1 mol = NA = 6.0221367 x 1023
Avogadro’s number (NA)
Dozen = 12
Pair = 2
The Mole (mol): A unit to count numbers of particles
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The Mole and Molar Masses
The Mole
12
1 “pair” objects | 2 objects |
1 “dozen” objects | 12 objects |
1 “gross” objects | 144 objects |
1 “million” objects | 1,000,000 objects |
1 “trillion” objects | 1012 objects |
1 “mole” objects | 6.023 × 1023 objects |
People use a variety of counting groups to conveniently indicate the number of objects in some set:
Dozen = 12
Pair = 2
If the jar contained Avogadro’s number of pennies, you would have 6.0225 x 1023 pennies.
If I had a mole of pennies?
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The pennies would occupy a cube that is 205 miles on each side, an area the size of Virginia and West Virginia.
If a penny weighs 1.2 grams, then a mole of pennies would weigh approximately 1.59 x 1021 pounds or the weight of about 80 moons.
I could pay off the national debt, give every person in the world a billion dollars and still have
$ 6,000,000,000,000,000,000,000.00
The Mole and Molar Mass
The molar mass of a substance is the mass in grams of 1 mole of the substance.
sulfur (left) 32.07 g/mol
copper (middle) 63.55 g/mol
mercury (right) 200.6 g/mol
helium (in balloon) 4.003 g/mol
NA = 6.0221415 x 1023 atoms
The Mole and Molar Mass
The mole is defined as the amount of a substance that contains as many elementary entities as there are atoms in exactly 12 g of carbon-12.�
The experimentally determined number is called Avogadro’s number (NA)
NA = 6.0221415 x 1023
Molar mass is the mass of 1 mole of in grams
1 mole 12C atoms = 6.022 x 1023 atoms = 12.00 g
1 12C atom = 12.00 amu
1 mole 12C atoms = 12.00 g 12C
1 mole lithium atoms = 6.941 g of Li
For any element
atomic mass (amu) = molar mass (grams)
Atoms
Molecules
Particles
Element | Average atomic�mass in amu or u of a single atom | Average atomic �mass in grams of a mole of atoms | Molar mass in Grams�mole |
Bromine | 79.9 amu | 79.9 g | 79.9 g/mol |
Phosphorus | | | |
Oxygen | | | |
The Mole and Molar Mass
It is important to be able to convert between mass, moles, and the number of particles.
Gram to mole conversions
How many moles are in 20.0 grams of lead?
Molar mass = 207.2 grams/mol
Chapter 3 Unit 2
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moles = 20.0 g x = 0.0965 mol
1 mol
207.2 g
Mole-gram conversion
How many grams are in 2.50 moles of N2 ?
molar mass
= 2 N x 14.01 g/mol
= 28.02 g /mol
mass N2
= 2.50 moles x 28.02 g / 1 mol
= 70.1 grams
Chapter 3 Unit 2
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Moles to number of particles
How many atoms are in 20.0 grams of lead?
Molar mass = 207.2 grams/mol
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moles = 20.0 g x = 0.0965 mol
1 mol
207.2 g
Atoms = 0.0965 mol x 6.02 x1023
atoms
mol
= 5.81 x1022 atoms