1. Chapter 4 Section 1
Introduction to Atoms

2. Development of Atomic Models
Why would it be difficult to figure out what atoms are made up of?
Atoms are very, very small. In fact, it takes a special electron microscope to see an entire atom, let alone look inside!
Around 5 hundred quadrillion atoms are in a speck of dust.

3. Ancient Greeks: Democritus
The first people to think about atoms were the ancient Greeks.
430BC the Greek philosopher named Democritus proposed that matter is formed of small pieces
These small pieces were “uncuttable” or could not be cut into smaller parts.
He named these parts atomos which means uncuttable.
Today, we call them ATOMS.

4. Dalton’s Atomic Theory 1803
English Chemist : “All atoms have certain characteristics”
ALL ELEMENTS COMPOSED OF ATOMS THAT CAN’T BE DIVIDED
ALL ATOMS OF THE SAME ELEMENTS ARE ALIKE & HAVE SAME MASS.
ATOMS CAN’T BE CREATED OR DESTROYED, ONLY REARRANGED
EVERY COMPOUND IS COMPOSED OF ATOMS OF DIFFERENT ELEMENTS IN SPECIFIC RATIO
Theory still accepted today with only a few changes!

5. Thomson & Smaller Parts of the Atom
1897 British scientist J.J. Thomson found negative particles in atoms and called them ELECTRONS.
Since he knew atoms have NO ELECTRICAL CHARGE, he reasoned that they must also have POSITIVE PARTICLES.
He proposed that atoms had negative electrons embedded in a positive sphere. (berries in a muffin)

6. 1911 Thomas Rutherford & Nucleus
Found evidence that disproved Thomson’s model that electrons are embedded in positive sphere
GOLD FOIL EXPERIMENT:
Fired positive charged particles through gold foil. Most particles passed straight through; however, some were deflected
Like charges repel like charges
He found that positive charges in atoms are located in a central region of atom which he named the NUCLEUS

7. Rutherford continued…
He also knew that electrons have almost no mass, so he reasoned that the mass of an atom must be located in the neucleus where the protons existed.
Rutherford suggested that most of the atom was empty space
With the electrons floating around.

8. 1913 Neils Bohr, Danish Student of both Thomson and Rutherford
Revised Atomic Model again
Found electrons have specific amounts of energy
This causes them to move in certain orbits like planets orbiting the sun.

9. 1920’s Bohr’s Theory Disproved
Scientists determined that electrons do not orbit nucleus like planets.
Instead, electrons can be anywhere in a cloudlike region around nucleus.
We call this the ELECTRON CLOUD
An electron’s movement is related to its ENERGY LEVEL
OR the specific amount of energy it has
Electrons of different energy levels are found in different places

10. 1930’s Chadwick: Modern Atomic Model
James Chadwick discovers the NEUTRON
He found that it has neutral charge
He found that it is located in nucleus
He found that its mass is almost the same as that of the PROTON
The Atomic Model has not changed much since 1930’s

11. Particle Charges Protons + Electrons – Neutrons have NO charge
Why does every atom have a neutral charge?
# of protons = # of electrons (they cancel out)
NEUTRONS have NO effect on the electrical charge of the atom.

12. Scale and Size of Atoms
Most of an atoms volume is located in the electron cloud.
Atoms are so small that one speck of dust contains 10 million billion atoms!!!
Must use very high power microscopes to study.

13. Atomic Number – Atom Identity
Every atom of the SAME element has the SAME number of PROTONS
The nucleus of every Carbon atom has 6 protons, every Oxygen atom has 8 protons, every Iron atom has 26 protons.
The number of protons in an atoms nucleus is it’s ATOMIC NUMBER
Atomic

14. Isotopes
All atoms of same element ALWAYS have the same number of protons; HOWEVER the number of NEUTRONS can differ.

15. Why are Isotopes Important?
Used in medical imaging to diagnose & treat cancer
Used in Smoke Detectors
Batteries in Nasa Satellites
To calibrate detectors in shipping ports to keep them safe from nuclear terrorism.
To enable new sources of energy – nuclear fusion
Used to date ancient remains – radioactive decay

16. Isotopes
There are 90 naturally occurring elements with roughly 250 stable isotopes – stable = reliable reactions
There are 3200 UNSTABLE isotopes – unreliable reactions
Different isotopes often have different properties than the original element
Production of isotopes is very expensive and time consuming