Download Atomic mass

AS Chemistry Unit 1
Weighing and Counting Atoms
Atoms: The Building Blocks of
Matter
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Dalton’s Atomic Theory – English school
teacher John Dalton was the first to
recognize that these laws and many
experimental results could be explained by
the existence of atoms.
Bell Work, 8/15/12
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Answer Question # 1 on pg. 21.
Statements of the Atomic Theory
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All matter is composed of extremely small particle
called atoms.
Atoms of a given element are identical in size, mass
and other properties; atoms of different elements
differ in size, mass, and other properties.
Atoms cannot be subdivided, created or destroyed.
Atoms of different elements can combine in simple,
whole-number ratios to form chemical compounds.
In chemical reactions, atoms are combined, separated,
or rearranged.
Dalton’s Atomic Theory Cont’d
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Dalton’s theory has since been discarded it
was the basis for further experiments because
it successfully explained the Law of
Conservation of Mass, Law of Definite
Composition and other observations.
Theory Updated
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We now know that atoms are divisible.
We also know that an element can have atoms
with differing masses; these atoms are called
isotopes.
Theory Laws
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Law of Definite Composition by Mass states
that a chemical compound contains the same
elements in exactly the same proportions by
mass regardless of the size of the sample.
Law of Multiple Proportions states that If two
or more different compounds are composed
of the same two elements, the masses of the
second element combined with a certain mass
of the first element can be expressed as ratios
of small whole numbers.
Example:
Carbon Monoxide
Carbon Dioxide
Oxygen
Ozone
CO
CO2
O2
O3
Atomic Numbers and Mass
Numbers
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Atomic Number – (Z) the number of protons
in the nucleus of each atom of that element.
Mass Number – total number of protons and
neutrons in the nucleus of an isotope.
Nuclide – the general term for any isotope of
any element.
Average Atomic Masses of Elements
Average Atomic Mass – the weighted average
of the atomic masses of the naturally occurring
isotopes of an element.
Example: Copper consists of
69.17% Cu-63 atomic mass 62.939 598 u
30.83% Cu-65
64.927 793 u
(.6917)(62.939 598u) + (.3083)(64.927 793u)
= 63.55 u (as seen on the periodic table)
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Average atomic mass equation
aam = (fa1)(m1) + (fa2)(m2) + etc……
aam – average atomic mass
fa – fraction abundance of that specific isotope
m – mass of specific isotope
Atomic Structure
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Protons – subatomic particles that have a positive
charge equal in magnitude to the negative charge of
an electron and are present in atomic nuclei. Protons
have a mass of 1.673 x 10-24.
Neutrons are electrically neutral subatomic particles
found in atomic nuclei. The mass of a neutron is
1.675 x 10-24 g.
Physicists have also found many other subatomic
particles. However particles other than electrons,
protons, and neutrons play little role in the properties
of matter that are of interest in chemistry.
Scientists

Look up the following Scientists in your
textbook and write what they were responsible
for discovering and how they did it.
Ernest Rutherford
 Sir John Joseph Thomson
 Robert Millikan (Oil Drop Experiment)
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Atomic mass – the relative atomic
mass of atoms of that nuclide.
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The Mole, Avogadro’s Number, and Molar Mass
mole – the amount of a substance that contains the
same number of particles as the number of atoms in
exactly 12 g of carbon-12. Abbreviation is mol.
Avogadro’s number – 6.022 x 1023 – the number of
particles in exactly one mole of a pure substance.
Molar mass – the mass in grams of one mole of a pure
substance.
Molecules and Ions
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Molecule – a collection of atoms held together
by forces called covalent chemical bonds.
Covalent chemical bonds – caused by sharing
electrons between atoms.
Chemical formula – a method of using
symbols and subscripts to represent molecules.
Structural formula – symbols and lines are
used to represent the shape of molecules
Ions
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Ions – an atom or groups of atoms that have a
net positive or negative charge.
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Cation – positively charged ion.
Anion – negatively charged ion.
Ionic bonding – force of attraction between
oppositely charged ions.
Copy the following questions and
answer. Turn in at the end of class
1.
2.
The element Boron, has an atomic mass of
10.81 u according to the periodic table.
However, no single atom of boron has a mass
of exactly 10.81 u. How can you explain the
difference?
How did the outcome of Rutherford’s goldfoil experiment indicate the existence of a
nucleus?
Bell Work, 8/16/12
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Complete the Average Atomic Mass Worksheet
for a grade.
3.
4.
5.
List the number of protons, electrons and
neutrons in the following
a. Si-30
b. Cr-52
c. Sr – 88
d. Be-11
FeO, Fe2O3, Fe3O4, represent what Law of
matter?
Tell which of Dalton’s principles of his
Atomic Theory have been changed and why.
6.
Convert the following to grams
a.
b.
c.
d.
7.
1.22 mol sodium
14.5 mol copper
1.204 x1024 atoms lithium
6.046 x 1023 atoms bismuth
Convert the following to atoms
a. 64.1 grams aluminum
b. 0.255 g sulfur
c. 0.366 mol zinc
d. 9.37 x 10-3 mol magnesium
8/17/12
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Work on Exam style question 2 on pages 21 and
22.