Transcript II. Units of Measurement

Chapter 3, Section 1 - Atoms
Suggested Reading pgs.. 67 - 71
Pages 66-70
STM (scanning tunneling microscope) image
of a single zigzag chain of cesium atoms
(red) on a gallium-arsenide surface (blue)
Early Models of the Atom
450 B.C. Greek - Democritus
“All matter is composed of tiny,
indivisible particles”
Atomos means Indivisible
This idea is not related to a measurable
property … it can’t be experimentally tested
Chemical Reaction
 The transformation of a
substance into one or more
new substance. With new
chemical and physical
properties.
 Reactants  Products
Law of Conservation of Mass
 States that mass is neither
created nor destroyed during
ordinary chemical reactions or
physical changes.
Law of Conservation of Mass
 total mass stays the same
 atoms can only rearrange
4H
4H
36 g
2O
4g
32 g
2O
Law of Definite Proportions:
 A chemical contains the same
elements in exactly the same
proportions by mass,
regardless of the size of the
sample, or the source of the
compound.
Law of Definite Proportions:
Salt - NaCl
is always 39.34% Sodium and 60.66% Chlorine
Law of Multiple Proportions:
 When elements combine, they
do so in small whole number
ratios. (non-metals)
Atom
 The smallest unit of an element
that retains the chemical
identity of that element.
 There are about 110 different
kinds of atoms that combine to
form all matter.
Dalton’s Atomic Theory –
Early 1800’s
 All matter is composed of atoms.
 Atoms of a given element are
identical, but are different from
atoms of other elements.
 Atoms cannot be subdivided,
created, or destroyed.
Dalton’s Atomic Theory –
Early 1800’s
 Atoms of different elements
combine in simple whole-number
ratios to form chemical
compounds.
 In chemical reactions, atoms are
combined, separated, or
rearranged.
Dalton’s Atomic Theory –
Early 1800’s
Revisions to Dalton’s Atomic Theory
 Atoms are divisible into smaller
particles called subatomic
particles.
 A given element can have atoms
with different masses, called
isotopes.
Scanning Tunneling Microscope
 STM – provides “close” to a 3-D
picture of atoms – that’s the best
we can see, even today.
STM picture of Nickel atoms