Atomic StructureChapter 4
4.1 Defining the Atom
• Democritus (460 BC – 370 BC)
• Had an atomic philosophy
• Matter was made of small invisible particles called atoms
• No scientific evidence, but it was logical
Dalton’s Atomic Theory
• John Dalton (1766-1844) was an English chemist and school teacher
• Dalton’s theory was the first scientific theory of the atom
John Dalton looks like another famous John
Dalton’s Atomic Theory
Dalton’s Atomic Theory
1. Matter is composed of tiny indivisible atoms
2. All atoms of a given element are exactly the same
3. Different elements are made of different atoms
4. Atoms of different elements can combine in simple whole number ratios to form compounds
5. Chemical reactions involve separating, combining, or rearranging atoms.
How big are atoms?
• Pretty small
• Just 4 grams of helium (He) contains 6.02 x 1023 atoms!
• Most atoms have a radius in the range of 5 x 10-11 m to 2 x 10-10 m
• Atomic sizes are in the pico- and nanometer range
Can we see atoms?
• Amazingly, yes!• STM• Scanning tunneling
microscope has made it possible to see atoms.
• These are gold atoms• They are only 2.3 Å
apart!
Here is a picture of a mixture of tin, lead, and silicon atoms!
Custance's team demonstrated their "fingerprinting" technique by using an atomic force microscope (AFM) to distinguish atoms of tin (blue) and lead (green) deposited on a silicon substrate (red). (Credit: Oscar Custance) physicsworld.com/cws/article/news/27208
The colors are added to help distinguish the atoms
4.2 Structure of the Nuclear Atom
When do I know I have learned this section? When…
• I can describe 3 types of subatomic particles• I can describe the structure of the nuclear atom• I can explain Thomson’s discovery
– experiment and model of the atom• I can explain Millikan’s discovery• I can explain Rutherford’s discovery
– experiment and model of the atom
Subatomic Particles• A cathode ray is deflected by electrically charged
plates.
4.2
Subatomic Particles
• In Dalton’s theory, atoms had no charge
• Thomson studied cathode rays (the cathode ray experiment)
• Resulted in discovery of the electron
• Electrons are negatively charged particles
• Discovered the charge to mass ratio of electron
JJ Thomson’s Plum Pudding Model
• Atom was a sphere
• Atoms were electrically neutral
• (-) charged electrons embedded in a (+) charged matrix
• Like a chocolate chip cookie
Millikan’s Oil Drop Experiment
• Discovered the actual quantity of charge for the electron
• Using Thompson’s charge/mass ratio, Millikan was also able to determine the mass of the electron
• Mass of electron is 1/1840 the mass of a hydrogen atom.
Protons
• If H atoms are neutral, and they give off negatively charged electrons, what must be left?
• Positive charge
• Proton discovered by Eugen Goldstein noticed a ray traveling in opposite direction of the electrons in the cathode ray tube
• Toward the negative end (cathode)
Neutrons
• Discovered by James Chadwick
Quarks
• Quarks are subnuclear particles
• Protons and neutrons are composed of quarks
The Atomic Nucleus• Ernest Rutherford discovered the nucleus• Gold foil experiment
Simulation of Rutherford’s Gold Foil Experiment
Interpretation of Rutherford’s Findings
Rutherford’s Conclusions
• All positive charge and nearly all the mass of the atom are concentrated in a very small nucleus
• Atom contains mostly empty space
• Rutherford’s model of the atom was like a mini solar system
Summary of Atomic Models
• DemocritusInvisible particles• Dalton Tiny homogeneous sphere
5 (4) statements
• Thomson Plum pudding• Rutherford Nuclear atom
Thomson’s Model of the Atom
• Plum Pudding• Negatively charged
electrons embedded in a positive matrix
Rutherford’s Model
• Nuclear Atom• Most of the volume of the
atom is empty space• All positive charge & most
mass in a small nucleus• Nucleus contains protons
and neutrons• Electrons occupy space
outside the nucleus
Distinguishing Among Atoms
• Atomic Number= number of protons in nucleus
Each element has unique atomic number
• Mass Number= number of protons and neutrons in nucleus
# neutrons = mass # - atomic #
Practice
Symbols of Elements
Elements are symbolized by one or two letters.
First letter is always capitalized.
Second letter, if there is one, is always lower case.
Atomic Number
All atoms of the same element have the same number of protons:
The atomic number (Z)
Mass Number
The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom.
Isotopes
• Isotopes are atoms of the same element with different masses.
• Isotopes have different numbers of neutrons.
116C
126C
136C
146C
Different Ways of Indicating Isotopes
• Chemical symbols16
8O 188O
• Symbol and mass number
O-16 O-18
• Name and mass number
oxygen-16 oxygen-18
Isotopes of Neon
Atomic #
Mass #
Atomic Mass
• Measured in amu’s (atomic mass units) because their mass is so small
• Definition: one atom of carbon-12 equals exactly 12 amu’s
• 1 amu = exactly 1/12 the mass of a C-12 atom
• 1 amu = 1.6605 x 10-24g
Different Isotopes Have Different Abundance & Atomic Masses
Isotope AbundanceAtomic mass (amu)
Chlorine-35 75.77% 34.969
Chlorine-37 24.23% 36.966
What is the atomic mass of chlorine on the periodic table?
Average Atomic Mass
Isotope Atomic Mass
Abundance
R-125 125 ________ ________
R-120 120 ________ ________
124
•Determining abundance of isotopes when average atomic mass is known
•Multiply the mass of each isotope by its relative abundance (decimal equivalent) and add the products
Periodic Table - Preview
• Allows you to easily compare and predict properties of elements
• Groups = Columns– Numbered 1-18 or– 1A-8A (representative elements)– 1B-8B (transition metals)
• Periods = Rows– Numbered 1-7