1. Models of the Atom: A Historical perspective

2. Early Greek Theories Democritus
400 B.C. – His theory: thought matter could not be divided indefinitely
There was a limit to how far you could divide matter. You would eventually end up with a piece of matter that could not be cut
He named the smallest piece of matter “atomos,” meaning “not to be cut”
He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?
Democritus: you would eventually arrive at individual pieces that could not be cut. To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes. Atoms were infinite in number, always moving and capable of joining together.

3. Early greek theories Aristotle Very famous Greek philosopher
Believed matter could be divided smaller and smaller pieces forever
Believed that matter was made of a combination of four “elements”: earth, fire, water, air
Aristotle was wrong. However, his theory persisted for 2000 years.
He held a very strong influence on popular belief and his views on this were accepted for two thousand years.

4. What is an atom?
PIECES OF CARBON
An atom is the smallest particle that an element can be divided and still be that element.
For example the smallest particle of carbon is a single atom of carbon. If you divide it is no longer carbon anymore.
CARBON ATOM

5. John Dalton All matter is made of atoms
1800 -Dalton proposed a modern atomic model based on experimentation not on pure reason
His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).
All matter is made of atoms
Atoms of the same element are identical
Each element has different atoms
Atoms of different elements combine in constant ratios to form compounds.
Atoms are rearranged in reactions.
2,000 years later, Dalton brings back Democritus’s idea of the atom. He performed many experiments to study how elements join together to form new substances.

6. William crookes Accidental discovery of the cathode ray
Discovery led to the development of the television

7. History of the atom ELECTRON Joseph John Thompson
found that atoms could sometimes eject a far smaller negative particle which he called an
What particle did Thompson discover?
ELECTRON

8. History of the atom PLUM PUDDING MODEL
Thompson develops the idea that an atom was made up of electrons scattered unevenly within an elastic sphere
This sphere contained a soup of positive charge to balance the electron's charge
electrons similar to plums in a pudding or chocolate chips in ice cream.
PLUM PUDDING
MODEL
Negative electron
Positive charge spread over
sphere

9. Robert Millikan
Developed the charge of an atom… e-

10. Ernest Rutherford Rutherford shot alpha () particles at gold foil
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive substance
path of invisible -particles
Most particles passed through
So, atoms are mostly empty space
Some positive -particles deflected or bounced back!
Thus, a “nucleus” is positive & holds most of an atom’s mass
Rutherford disagreed with the plum pudding model. Devised an experiment to investigate the structure of positive and negative charges in the atom.

11. Bohr’s model Electrons orbit the nucleus in “shells”
Electrons can be bumped up to a higher shell if hit by an electron or a photon of light

12. What is in an Atom? There are 3 types of subatomic particles.
electrons (e–)
protons (p+)
neutrons (n0) They have: no charge
Elements are often symbolized with their mass number and atomic number
The periodic table give you all this information
For now, round the mass # to a whole number
# of protons = # of electrons = atomic number
Mass number – atomic number = # of protons
E.g. Oxygen: O 16 8
Nuclear Symbol

13. Atomic structure Particle Charge Mass Proton + charge 1 Neutron
No charge
Electron
nil

14. He 4 2 ATOMIC STRUCTURE EXAMPLE Atomic mass
the number of protons and neutrons in an atom
Atomic number 2
the number of protons in an atom
All of these pieces of information (numbers) are obtained from your periodic table
Atomic number = number of electrons = number of protons 14

15. Practice
Atomic
Mass p+ n0 e– Ca Ar Br

16. Assignment-
Copy the table right below your notes and fill in the information that is required of you. Use the periodic table to fill in this information.
Symbol
Atomic number
Mass p+ n0 e-
Calcium
Neon
Magnesium
Sodium
Lithium
Oxygen
Carbon
Potassium
Silicon
Chlorine

17. Questions??