Download The Atom

Atoms & Their
Structure
Chapter 2 Section 1
Part 1
Objectives
How have historic experiments led to the
development of the modern model of the
atom?
How is the modern model of the atom
different from previous models?
What information is available in an element
block of the periodic table?
Important Vocabulary
Atom
Electron cloud
Atomic theory
Isotope
Law of conservation
of mass/matter
Neutron
Law of definite
proportions
Atomic number
Electron
Proton
Nucleus
Mass number
Atomic mass unit
Atomic mass
The Atom
An atom is the smallest particle of an
element that retains its identity in a
chemical change
The concept of the atom intrigued a
number of early scientists and
philosophers
However, they could not observe
individual atoms
History of Atoms
 The idea of the atom first came from a Greek
philosopher Democritus nearly 2500 years ago
 It started with a set of simple questions, “If
you take a piece of chalk and break it in half,
are both halves still chalk? If you continue to
divide each piece, is there a limit to the
division before it becomes something else?”
 Democritus thought there was a limit which he
called atomos, meaning “unable to be cut”
 This idea that matter is made of the
fundamental particles called atoms is known as
the atomic theory of matter
Democritus' Theory of Atoms
1. All matter consists of invisible particles
called atoms.
2. Atoms are indestructible.
3. Atoms are solid but invisible.
4. Atoms are homogenous.
5. Atoms differ in size, shape, mass,
position, and arrangement.
He believed that the movements of atoms
caused the changes in matter he observed
However, he had no way of proving the
existence of atoms
Contributors to Atomic Theory
Democritus
Erwin Schrodinger
Antoine Lavoisier
Werner
Heisenberg
Joseph Proust
John Dalton
J.J Thomson
Hantaro Nagaoka
Ernest Rutherford
Niels Bohr
Robert Millikan
Eugene Goldstein
James Chadwick
Irene Joliot-Curie
Antoine Lavoisier
In 1782, the French taxman Lavoisier made
measurements of chemical reactions in
sealed containers
He observed that the mass of reactants
before the reaction was equal to the mass
of the products after the reaction
He concluded that when an chemical
reaction occurs, matter is neither created
nor destroyed but only changed
This idea became known as the Law of
Conservation of Mass/Matter
Joseph Proust
In 1799, a French chemist Joseph Proust
observed that the compostion of water is
always 11.2% hydrogen and 88.8% oxygen
by mass
He studied many other compounds and
observed that compounds were always in
certain proportions by mass
This principle is referred to as the Law of
Definite Proportions
John Dalton
Was a teacher, chemist,
meteorologist, &
physicist
He is best known for his
development of the
modern atomic theory
He also did research into
color blindness
(sometimes called
Daltonism)
Dalton’s Atomic Theory
Was established in the early 1800’s
before the Periodic Table
Dalton believed that a few kinds of atoms
made up all matter
He also believed that elements were
composed of only one kind of atom and
compounds were made from two or more
kinds of atoms
He reasoned that only whole numbers of
atoms could combine to form compounds
Five Principles of Dalton’s Theory
1. All matter is composed of atoms which
cannot be subdivided, created, or
destroyed
2. Atoms of a given element are identical in
their physical and chemical properties
3. Atoms of different elements differ in
their physical and chemical properties
4. Atoms of different elements combine in
simple, whole-number ratios to form
compounds
5. In chemical reactions, atoms are
combined, separated, or rearranged
The Problem with Dalton’s Theory
It explained most of the chemical data
that existed during his time
However, it doesn’t all hold true today
Atoms can be divided into particles
Atoms can be created and
destroyed
It doesn’t account for molecules &
allotropes
Thus, it changed