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Chapter 2: Atoms, Molecules and Ions
CHEMICAL LAWS
2.1 Conservation of Mass and the Law of Definite Proportions
2.2 Dalton s Atomic Theory and the Law of Multiple Proportions
THE ATOMIC STRUCTURE
2.3 The Structure of Atoms: Electrons
2.4 The Structure of Atoms: Protons and Neutrons
THE ELEMENTS AND THEIR ATOMS
2.5 Atomic Number
2.6 Atomic Mass
COMPOUNDS AND MOLECULES
2.7 Compounds and Mixtures
2.8 Molecules, Ions and Chemical Bonds
2.9 Acids and Bases
2.10 Naming Compounds
Conservation of mass and Law of Definite
Proportions
Law of Mass Conservation: Mass is neither created
nor destroyed in chemical reactions.
Law of Definite Proportions: Different samples of a
pure chemical substance always contain the same
proportion of elements by mass.
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Conservation of mass
Dalton s Atomic Theory
Elements are made of tiny particles called atoms
Atoms of the same element have the same
mass. Atoms of different elements have different
masses.
Chemical combination of elements to make
different substances occurs when atoms join
together in simple whole numbers.
Atoms are chemically indestructible.
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Dalton s Atomic Theory
Law of Multiple Proportions
When two elements form two different compounds, the mass ratios are
related by small whole numbers.
Examples
Nitrogen & oxygen combine to form NO or NO2:
In NO the N:O mass ratio is 7:8
In NO2 the N:O mass ratio is 7:16
Hydrogen & oxygen combine to form H2O or H2O2:
In H2O the H:O mass ratio is 1:8
In H2O2 the H:O mass ratio is 1:16
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The Structure of Atoms
Thomson calculated the electron s charge to
mass ratio as 1.758820 x 108 Coulombs per
gram.
Voltage on plates place 1.602176 x 10-19 C of
charge on each oil drop.
Millikan calculated the electron s mass as
9.109382 x 10-28 grams.
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The Structure of Atoms
Discovery of Nucleus (Rutherford,
1871 1937):
Rutherford irradiated gold foil with a beam
of alpha ( ) particles to search for positive
charged particles.
Atom must be mostly empty space except
for a central positive mass concentration.
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The Structure of Atoms
6
Atomic numbers, mass numbers
& isotopes
Atomic number, Z = # of protons & determines
the element
Mass number, A = # of protons + # of neutrons
In a neutral atom # of electrons =
# of protons = Atomic Number, Z
Atoms with the same # of protons but with a
different # of neutrons are called ISOTOPES
One isotope of Carbon; Carbon -12
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In Periodic Table of Elements
Carbon is shown as :
Atomic Mass: A weighted average of the
isotopic masses of an element s
naturally occurring isotopes. Carbon has
two natural isotopes, C-12 and C-13
Isotopes
Examples
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Isotopes
Q ? The isotope
75
34Se
is used medically for
diagnosis of pancreatic disorders. How many
protons, neutrons, and electrons does an atom of
this isotope have?
Q? An atom of element X contains 47 protons and
62 neutrons. Identify the element, and write the
symbol for the isotope in the standard format.
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