1. Atomic Structure
“Jiminy Jillikers!”

2. What are Atoms?
Atoms are the smallest units of matter that contains all of the properties of a substance
-- remember that elements are substances that cannot be broken down into any simpler substance by ordinary physical or chemical means
-- all elements are made of atoms, all atoms are of a specific kind of element
You can basically think of atoms as the object, and element as the specific type of object

3. Early History of the Atom
The earliest theory about atoms came in 460 B.C. from the Greek philosopher Democritus, who asked:
If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further?
-- Democritus called this smallest piece of matter the atom
However, because the leading scientists/philosophers of the time (including Aristotle) didn’t believe in this theory, the properties of atoms remained largely undiscovered until the early 19th century. . .

4. John Dalton’s Discoveries
Dalton did experiments with chemicals and discovered several different properties of atoms. They are:
-- Atoms can’t be broken into smaller pieces – they are like marbles or spheres that are impossible to break
-- In any element, all the atoms are alike
-- Atoms of different elements are different
-- Atoms of two or more elements combine to form compounds
-- Atoms of each element have a unique mass
-- the masses of the elements in a compound always have a constant ratio

5. J.J. Thomson’s Discoveries
Even though Dalton discovered much of the properties of atoms, he did not discover what atoms were made of
In the late 19th century, a scientist named J.J. Thomson discovered a unique property when electrifying gases in a cathode-ray tube
-- the beams of his particles were negatively-charged because they were attracted to a positive charge
-- about 2000 of these particles equaled the mass of one hydrogen atom
Thomson concluded that these particles must come from inside the atom, and were released when the gas was electrified, thus the atom was not indestructible
Thomson called his negatively-charged particles electrons

6. The Thomson Model
From his experiments, Thomson proposed a model of the atom, which he called the “plum pudding model”.
Thomson’s “Plum-Pudding” Model said:
-- tiny, negatively-charged electrons are embedded in a cloud of positive charge
-- much like the plums in a plum pudding
-- there must be a cloud of positive charge to balance out the negative charge of electrons, because atoms contain no electric charge

7. Rutherford’s Discoveries
In 1911, Ernest Rutherford performed his famous gold-foil scattering experiment, which disproved Thomson’s plum-pudding model. . .
Rutherford took a very thin piece of pure gold foil and bombarded it with positively-charged alpha particles
-- if Thomson’s model were true, all of the alpha particles would pass through the diffuse positive charge freely
HOWEVER, what Rutherford found was that MOST of the alpha particles passed through freely as expected, but a few bounced straight back and a few were deflected at other angles

8. Rutherford’s Conclusion
From his experiment, Rutherford concluded that atoms must be made up of a small, very dense positively-charged area called a nucleus, around which the electrons orbit

9. Neils Bohr
Rutherford’s work lasted only 3 years, until 1914, when Neils Bohr discovered that atoms, when heated, emit specific amounts of energy in the form of light
-- he theorized that electrons release or absorb energy depending on their distance from the positively-charged nucleus
From this, he developed the Bohr Model of the atom, which said:
-- Electrons orbit the nucleus at specific distances, which are known as energy levels or shells
-- When atoms gain energy, electrons move to higher shells, and the energy is released when the electrons move back down to a lower energy level

10. Image of the Bohr Model

11. More Modern Discoveries
In 1932, British physicist James Chadwick discovered a particle that had about the same mass as a proton, but with no charge. He called this particle the neutron
Scientists have continued to modify the Bohr model of the atom, specifically:
-- electrons orbit the nucleus in clouds, not perfectly discrete circular energy levels
-- it is impossible to know the exact location of an electron at any moment in time