Download Chapter 4 “Atomic Structure”

Chapter 4
“Atomic Structure”
Section 4.1 Defining the Atom
■ OBJECTIVES:
■ Describe
Democritus’s ideas
about atoms.
Section 4.1 Defining the Atom
■ OBJECTIVES:
■ Explain
theory.
Dalton’s atomic
Section 4.1 Defining the Atom
■ OBJECTIVES:
■ Identify
what instrument is
used to observe individual
atoms.
Section 4.1 Defining the Atom
Have you ever been asked to believe in
something that you can’t see? Yes, frequently!
•
The unaided eye cannot see the tiny fundamental
particles that make-up all matter.
•
Yet all matter is composed of such particles… they’re
called atoms.
•
An atom is the smallest particle of an element that
retains its identity in a chemical reaction.
The concept of the atom intrigued a
number of early scholars…
Although these philosophers and scientists could
not see and observe individual atoms, they were
still able to propose ideas (hypotheses) about the
structure of atoms.
Section 4.1 Defining the Atom
The Atomic Model of Democritus
•
The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among
the first to suggest the existence of atoms. He said matter
consisted of tiny, indivisible, unchangeable particles called
atoms.
•
Democritus believed that atoms were indivisible and
indestructible
•
Although his ideas did agree with later scientific theory, they did
not explain chemical behavior
•
They also lacked supporting evidence because his approach was
not based on the scientific method, just philosophy
•
His ideas were later challenged by the famed Greek
philosophers, Plato and Aristotle
Section 4.1 Defining the Atom
The Atomic Model of John Dalton
Remember: A model is used to study and explain things that are too large,
too small, or too complex to easily see (common models include globes,
subway maps, and weather maps)
John Dalton
(1766 – 1844)
• The real nature of atoms and the connection between
observable changes and events at the atomic level were
not established for more than 2000 years after Democritus.
• John Dalton, an English Chemist and school teacher, used
experimental methods to transform the ideas of Democritus
into a scientific theory.
Section 4.1 Defining the Atom
The Atomic Model of John Dalton
• Dalton studied the ratios in which elements combined during chemical
reactions (his experiments showed that the ratios in which elements
combined were whole numbers)
• Based on the data from his experiments, he then formulated additional
hypotheses and theories to explain his observations and tested these
• The result was Dalton’s Atomic Theory – based on experimental results
and able to explain chemical behavior
• Dalton’s Atomic Theory consists of four parts:
Section 4.1 Defining the Atom
The Atomic Model of John Dalton
1.All elements are composed of tiny indivisible particles called
atoms.
2.Atoms of the same element are identical. The atoms of any one
element are different from those of any other element.
3.Atoms of different elements can physically mix together or can
chemically combine in simple whole-number ratios to form
compounds.
4.Chemical reactions occur when atoms are separated, joined, or
rearranged. Atoms of one element, however, are never changed
into atoms of another element as a result of a chemical reaction.
Section 4.1 Defining the Atom
The Atomic Model of John Dalton
Section 4.1 Defining the Atom
Sizing up the Atom
Let’s use something we’re all familiar with, a US penny, to further
understand Dalton’s Atomic Theory…
• Pre-1857 pennies were made of pure copper (Cu)
• Imagine grinding the copper penny into dust
• Each speck of shiny red dust still has the properties of
copper
• If we had a way to make each speck even smaller, we would
eventually come upon a particle of copper that could no
longer be divided and still have the properties of copper
• This indivisible speck of copper is an atom
Section 4.1 Defining the Atom
Sizing up the Atom
• Copper atoms are very small
• A pure copper penny contains about 2.4 x 1022 atoms or
24,000,000,000,000,000,000,000 atoms
• For the sake of comparison, the Earth’s population is only
about 7.4 x 109 humans (7,400,000,000 humans)
• In other words, there are many more atoms in the coin than
there are humans on the Earth!
• If you could line up 100 million copper atoms side-by-side in a
line, it would only be 1 cm long!
Section 4.1 Defining the Atom
Sizing up the Atom
• Though atoms of different elements do vary in size, the radius of
a single atom usually falls within the range of:
5 x 10-11 m to 2 x 10-10 m
• It may seem impossible, but despite their small size, individual
atoms are observable with scientific instruments such as a
scanning tunneling microscope (STM)
• The designers of the STM won the 1986 Nobel Prize in Physics for
the design of this amazing tool
Section 4.1 Defining the Atom
Sizing up the Atom
An STM works by holding a one-atom sized tip over a sample of matter that
conducts electricity. Electric current flows between the tip and the sample.
The current is stronger when the tip and the sample are close and weaker when
they are farther apart. This change in voltage is interpreted to show surface
topography.
Section 4.1 Defining the Atom
Sizing up the Atom
Section 4.1 Defining the Atom
Actual STM images of atoms – note the arrangements
Above:
Iron (Fe) atoms on a
Copper (Cu) surface
Above:
Nickel (Ni) atoms
Left:
Silicon (Si) atoms
Section 4.1 Defining the Atom
Sizing up the Atom
• What are we actually seeing in the STM images? Essentially, the
images show the distribution range for the electrons of each atom -more on that in the future, but pretty neat, huh?
• Individual atoms can not only be seen, but actually moved around and
arranged in patterns.
• The ability to move individual atoms holds future promise for the
creation of atom-sized electronic devices such as circuits and
computer chips!
• This atomic-scale, “nanoscale,” technology has many potential uses
and could one day become essential in medicine, communications,
solar energy, and space exploration.
Section 4.1 Defining the Atom
Your assignments for today (due tomorrow)
1.
Complete the Reading Guide/Review WS for 4.1
2.
Complete the 4.1 Section Assessment # 1-7 on p. 103 (write the
questions and the answers – type or write, your choice)