Unit 10: Atoms and Periodicity

An atom is the smallest particle in which matter can be divided and still be the same substance. The same type of atoms are elements. Elements can be chemically combined to form compounds.

The atom was first seen in 1981 with the invention of the scanning tunneling microscope, but atomic theory(the ideas of the existence of atoms) has been around for over 2000 years!

Democritus (440 BC)--Greek PhilosopherCame up with the idea of the atomHe thought that if you were to cut up a material you would eventually come to a piece that is "uncuttable". He named this particle atom, which comes from the Greek word atomos for "indivisible"

John Dalton ( late 1700s)-- English School teacher

Developed the 3-part atomic theory based on his observations and published it in 1803

His theory states:1)All substances are made of matter

2)Atoms of the same element are exactly alike*, atoms of different elements are different.

3)Atoms join with other atoms to make new substances--______________

The Dalton model of the atom

J.J. Thomson (1897)--

An atom has NO overall charge if it contains the same number of protons and electrons.

http://my.hrw.com/sh/hp1/0030724929/student/ch11/sec01/qc02/hp111_01_q02fs.htm

His experiment used a Cathode Ray Tube to figure out what is inside the atom.

discovered the electron which shows that the atom is, in fact, divisible. There are smaller particles within the atom itself.

http://www.youtube.com/watch?v=eO2Z_2d9UJc&feature=related&safety_mode=true&persist_safety_mode=1&safe=active

What happened when the magnets were brought close to the tube?

What does that suggest about the particles that make up the ray?

In his model, the atom is a blob of positively charged material with negatively charged bits of electrons embedded in it.

http://www.youtube.com/watch?v=IdTxGJjA4Jw&safety_mode=true&persist_safety_mode=1&safe=active

http://www.youtube.com/watch?v=JUJPyQtoB5E&safety_mode=true&persist_safety_mode=1&safe=active

Thomson's model

Thomson called this the "plum-pudding model". The bits of plum represents the negative corpuscles (electrons).

This is like chocolate chip ice cream

Ernest Rutherford (1909-1911)-- The Gold foil experiment

Discovers that the atom is mostly empty space with a small dense and positively charged center called the nucleus. It is many times smaller than the diameter of the atom. In this model, the electrons move randomly around the nucleus

http://my.hrw.com/sh/hp1/0030724929/student/ch11/sec01/qc03/hp111_01_q03fs.htm

He discovers particles larger than electrons. He discovered the nucleus by shooting alpha particles at a piece of gold foil.

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An alpha Particle Alpha particles are emitted during

radioactive decay, along with gamma rays. Rutherford used them as postitively charged bullets to fire at the gold foil.

In Rutherford's experiment, he used the positively charge alpha particles and observed:

1. most of the particles passed straight through the foil

2. some particles were slightly deflected

3. a few particles bounce back.

Rutherford proposes that atoms have a densely packed, positively-charged center which is surrounded by the negatively charged electrons.

http://www.youtube.com/watch?v=wzALbzTdnc8&feature=related&safety_mode=true&persist_safety_mode=1&safe=active

·How can we use indirect evidence to figure how something is constructed or works without directly seeing it?

·How did scientists build our understanding of atoms?

·What is plum-pudding atomic model and what evidence was used to refute it?

·What is the Rutherford atomic model and what evidence is there to support it?

Niels Bohr (1913)-- suggested that electrons travel around the nucleus in definite paths.

http://my.hrw.com/sh/hp1/0030724929/student/ch11/sec01/qc05/hp111_01_q05fs.htm

Neutron

Proton

Electron

Atoms

Hydrogen

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Neutron

Proton

Electron

Atoms

Helium

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Neutron

Proton

Electron

Atoms

Carbon

Neutron

Proton

Electron

Atoms

Oxygen

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Eat your skittles, but do not leave a mess.

The Electron Cloud ModelOur modern model of the atom suggests that there are clouds of electrons surrounding the nucleus and that the exact location of the electron cannot be predicted.

http://my.hrw.com/sh/hp1/0030724929/student/ch11/sec01/qc07/hp111_01_q07fs.htm

Compare:

http://my.hrw.com/sh/hp1/0030724929/student/ch11/sec01/qc06/hp111_01_q06fs.htm

The Electron Cloud Model