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The Atom
Chapter 8
Great Idea:All of the matter around us is made of atoms, the chemical building blocks of
our world
Chapter Outline
• The Smallest Pieces• The Structure of the Atom• When Matter Meets Light• The Periodic Table of the Elements
The Smallest Pieces
The Smallest Pieces
• Democritus– Cut matter to reach smallest piece– Called “the atom” or “uncuttable”– All material formed from atoms
Dividing a Bar of Gold
Elements
• John Dalton– Father of modern atomic theory– Cannot break down elements– Elements composed of atoms
•Example: Water molecule is 1 oxygen atom and 2 hydrogen atoms
Envisioning Atoms
Are Atoms Real?
• Evidence for the reality of atoms– Behavior of a gas– Chemical combinations– Radioactivity– Brownian motion– X-ray crystallography– Atomic-scale microscopy
The Behavior of a Gas
• Bernoulli – Atoms have mass and velocity and thus kinetic energy
• Decreasing volume increases pressure
• Increasing temperature increases pressure
The Behavior of Gases
Relationship between Volume and Pressure
Volume
0 2 4 6 8 10 12 14
Pre
ssu
re (
mm
Hg
)
10
20
30
40
50
60
70
80
90
Relationship between Temperature and Pressure
Temperature
220 240 260 280 300 320 340 360 380 400 420 440
Pre
ssu
re
30
35
40
45
50
55
Chemical Combinations
• Dalton applied law of definite proportions– Elements combine in a specific ratio of weights•Ex: Water is 8 parts oxygen to 1 part hydrogen
Chemical Combinations (cont.)
• Ratio of weights is a small whole number– Ex: 12 lbs carbon can combine with either 16 lbs or 32 lbs of oxygen
• Implication: some units of elements are fundamentally indivisible
Radioactivity
• Radioactivity– Discovered in 1896
• Phosphors flash when hit by radiation
• 1903 demonstration of the twinkling caused by this effect
Brownian Motion
• Brownian motion: erratic, jiggling motion– Pollen grains suspended in water
• Einstein: motion is random collisions of atoms– Predicted particle movement
• Perrin tested and confirmed Einstein’s predictions
X-ray Crystallography
• Developed in 1912• Demonstrates arrangement of atoms in crystals
Discovering Chemical Elements
• Electrolysis• Mendeleev• Current periodic table
– 112 elements•92 found in nature
• Helium, carbon, aluminum, copper, gold
Everyday Elements
The Structure of the Atom
The Structure of the Atom
• Joseph Thomson identified electron– Negatively charged– Smaller and lighter than smallest atom
• Atoms are NOT fundamental building blocks, but are made up of smaller more fundamental particles
The Atomic Nucleus
• Ernest Rutherford determined atomic structure
• Concluded– Atom has nucleus at center surrounded by electrons
• Later discoveries found nucleus is composed of protons and neutrons
Rutherford’s Experiment
Important Terms Related to Atoms
Why the Rutherford Atom Couldn’t Work
• Why?– Object in circular orbit is accelerating
– Accelerated electrical charge emits electromagnetic radiation
– Electrons giving off energy while orbiting•Result: electrons spin toward nucleus and eventually atom ceases to exist
• Rutherford atom exists <1min
The Nuclear Regulatory Commission
• Logo: highly stylized atomic model
When Matter Meets Light
Niels Bohr
The Bohr Atom
• First working model of atom • Energy levels for electrons
– Specific distances from nucleus– Electrons exist with no radiation– Electrons cannot exist between allowed distances (energy levels)
Bohr Atom
Electron Energy Levels
Atomic Model
Photons: Particles of Light
• Photon absorbed– Used to move electron to a higher energy state
• Photon emitted as electron moves to lower energy state
Photons: Particles of Light
Photons - cont.
• Quantum leap or jump -electron disappears from original location and reappears in final location-never at positions in between
Energy in Bohr Atom
• Energy required to leave ground state– Absorb photon– Heat
•Increases collisions
Spectroscopy
• Atoms emit and absorb different photons
• Depends on differences between energy levels
• Each atom has distinct set of photons
• Spectrum: all photons emitted by an atom– Used for identification
Na, K, and Li – Give Distinctive Colors to a Flame
Spectroscopy – cont.
• Glass prism
Spectroscopy – cont.
• Line spectra for H, Na, and Ne
The Periodic Table of the Elements
The Science of Life
• Spectra of life’s chemical reactions– Used to determine how enzymes work– Used to identify pollutants emitted from automobile exhaust pipes
Science in the Making
• Helium– Discovery– Properties
Technology
• Lasers– Produce a beam of light
Action of a Laser
Lasers Have Many Uses
Periodic Table of the Elements
• Systematizes elements• Columns contain similar elements– Alkali metals: soft, silvery metals
– Noble gases: colorless, odorless, and inert
Periodic Table of the Elements
Why the Periodic Table Works: Electron Shells
• Patterns mirror arrangement of electrons in shells
• Pauli exclusion principle • First shell 2 electrons, second shell 8, etc.
Electron Representation