Download 4.1 Studying Atoms PDF

CHAPTER 4:
ATOMIC STRUCTURE
4.1 Studying Atoms
4.2 The Structure of an Atom
4.3 Modern Atomic Theory
4.1 STUDYING ATOMS
Ø  VOCABULARY:
Ø  nucleus
Ø  OBJECTIVES:
Ø  Describe ancient Greek models of matter.
Ø  List the main points of Dalton’s atomic
theory and describe his evidence for the
existence of atoms.
Ø  Explain how Thomson and Rutherford used
data from experiments to produce their
atomic models.
1/30/14
4.2 The Structure of an Atom
2
ATOMS
Ø  extremely small
Ø  can’t even see structure using microscope
Ø  ~6
1/30/14
sextillion (6 x 1021) atoms in 1 water drop
4.2 The Structure of an Atom
3
ANCIENT GREEK MODELS
OF ATOMS: DEMOCRITUS
Ø  All matter consists of extremely
small particles (atoms) that cannot be
divided or destroyed.
Ø  Called particles atoms (atomos - “uncut” or “invisible”)
Ø  Different types of atoms had different properties
*Now know atoms can be divided - splitting of atom is basis
for nuclear or atomic energy
1/30/14
4.2 The Structure of an Atom
4
ANCIENT GREEK MODELS
OF ATOMS: ARISTOTLE
Ø  No limit to # of times matter could
be divided
Ø  Model used to describe matter:
Ø  All substances built up from
only 4 elements (earth, air,
fire, & water) which are
combinations of 4 qualities
(hot, cold, dry, & wet)
Ø  Accepted by most for centuries
1/30/14
4.2 The Structure of an Atom
5
DALTON’S ATOMIC THEORY
Ø  John Dalton (1766 - 1844)
Ø  English teacher & scientist
Ø  Interest in predicting weather à
studied behavior of gases in air
Ø  Based on way gases exert pressure, correctly
concluded that gas consists of individual particles
1/30/14
4.2 The Structure of an Atom
6
DALTON’S ATOMIC THEORY:
EVIDENCE FOR ATOMS
Ø  Measured masses of elements in compounds
Ø  All compounds have something in common
Ø  Ratio of masses of elements in compound is
always the same. Compounds have fixed
compositions.
Ø  i.e. ratio of Mg to O, by mass,
in magnesium oxide is
always ~ 3:2
1/30/14
4.2 The Structure of an Atom
7
DALTON’S ATOMIC THEORY:
DALTON’S THEORY
Ø  Explained why elements in a compound always join
in same way.
Ø  Proposed theory (model) stating that
Ø  All matter is made up of individual particles
(atoms) which cannot be divided.
Ø  Atoms look like solid spheres
1/30/14
4.2 The Structure of an Atom
8
DALTON’S ATOMIC THEORY:
MAIN POINTS
Ø  All elements are composed of atoms.
Ø  All atoms of same element have same mass Atoms
of different elements have different masses.
Ø  Compounds contain atoms of more than 1 element.
Ø  In a particular compound, atoms of different
elements always combine in same way.
*Over time scientists found that not all his ideas about atoms were
completely correct à revised theory to take into account new discoveries.
1/30/14
4.2 The Structure of an Atom
9
THOMSON’S MODEL OF THE ATOM
Ø  When some materials are rubbed, they gain ability
to attract or repel other materials
Ø  positive (+) or negative (-) charges
Ø  Fundamental concept in science:
Ø  Objects with like (similar) charges repel
Ø  Objects with opposite charges attract
Ø  electric current - flow of charged particles from one
location to another
1/30/14
4.2 The Structure of an Atom
10
THOMSON’S MODEL OF THE ATOM
Ø  Joseph John Thomson used an electric current
to learn more about atoms
Joseph John Thomson (1856-1940).
Photo courtesy of The Cavendish
Laboratory.
1/30/14
4.2 The Structure of an Atom
11
THOMSON’S MODEL OF THE ATOM:
THOMSON’S EXPERIMENTS
When current is on, disk becomes charged (1 negative & 1 positive)
and glowing beam appears in tube
1/30/14
4.2 The Structure of an Atom
12
THOMSON’S MODEL OF THE ATOM:
THOMSON’S EXPERIMENTS
Ø  Hypothesis
Ø  Beam was a stream of charged particles that
interacted with the air in tube & caused air to glow.
Ø  Experiment:
Ø  Placed charged metal plates on either side of glass
tube (B) à caused beam to deflect (bend) from its
straight path.
Ø  Observations
Ø  Beam was repelled by negatively charged plate &
attracted by positively charged plate.
1/30/14
4.2 The Structure of an Atom
13
THOMSON’S MODEL OF THE ATOM:
THOMSON’S EXPERIMENTS
Beam bends toward
positively charged plate
placed outside tube
1/30/14
4.2 The Structure of an Atom
14
THOMSON’S MODEL OF THE ATOM:
EVIDENCE FOR SUBATOMIC PARTICLES
Ø  Beam attracted to + plate
à negatively charged particles
Ø  Particles always same, with mass 1/2000 of an H
atom mass
à particles came from inside atom
Ø  Provided first evidence that atoms are made of even
smaller particles
1/30/14
4.2 The Structure of an Atom
15
THOMSON’S MODEL OF THE ATOM:
THOMSON’S MODEL
Ø  “Plum Pudding Model”
Ø  Atom is neutral (neither - or + charge)
Ø  Negative charge evenly scattered throughout atom filled
with positively charged mass of matter
1/30/14
4.2 The Structure of an Atom
16
RUTHERORD’S ATOMIC THEORY
§  Ernest Rutherford
§  British physicist
§  Once a student of Thomson
Ernest Rutherford in his Laboratory at
McGill University ca. 1903.
1/30/14
4.2 The Structure of an Atom
17
RUTHERORD’S ATOMIC THEORY:
RUTHERFORD’S HYPOTHESIS
Ø  1899, discovered that uranium emits alpha
particles (fast-moving particles that have +
charge)
Ø  1909, asked student Ernest Marsden to find out
what happens to alpha particles when they pass
through a thin sheet of gold
1/30/14
4.2 The Structure of an Atom
18
RUTHERORD’S ATOMIC THEORY:
RUTHERFORD’S HYPOTHESIS
Ø  Based on Thomson’s model, Rutherford
hypothesized that the mass and charge at any
location in gold would be too small to change path
of an alpha particle.
Ø  Predicted particles would travel in straight path
(source à screen)
1/30/14
4.2 The Structure of an Atom
19
RUTHERORD’S ATOMIC THEORY: THE
GOLD FOIL EXPERIMENT
Screen around gold - made of material that would produce
a flash light when struck by fast moving alpha particle.
1/30/14
4.2 The Structure of an Atom
More particles were deflected than
expected.
20
RUTHERORD’S ATOMIC THEORY:
THE GOLD FOIL EXPERIMENT
1/30/14
4.2 The Structure of an Atom
21
RUTHERORD’S ATOMIC THEORY:
DISCOVERY OF THE NUCLEUS
Ø  Evidence:
Ø  Alpha particles deflected when
passing through gold foil
à must have come close to another
positively charged object
1/30/14
4.2 The Structure of an Atom
22
RUTHERORD’S ATOMIC THEORY:
DISCOVERY OF THE NUCLEUS
Ø  Model:
Ø  Atom’s positive charge is concentrated in its
nucleus
Ø  Positively charged nucleus surrounded by a great
deal of "empty space" through which the
electrons moved.
1/30/14
4.2 The Structure of an Atom
23