Atomic Theory: A Timeline• More than 2000 years ago

ancient philosophers had theories on matter.

• 450-360 BC - Democritus believed that matter was made of small invisible particles he named “atoms”.

• Aristotle believed in 4 elements : Earth, Air, Fire, and Water.

John Dalton 1803

• “Father of Modern Atomic Theory” – he proposed the first useful atomic theory of matter

John Dalton – Atomic Theory • He believed that:1.All Matter is made of small particles called

Atoms

2.The atoms of an element are identical in their masses

3.Atoms of different elements have different masses

4.Compounds are created when atoms of different elements link together in definite proportions.

5.Atoms can neither be created or destroyed

J.J. Thompson - 1897

• Atoms were thought to be the smallest division of matter

• In 1897 Thompson discovered the electron through experiments using cathode ray tubes.

• He concluded that electrons are smaller than atoms, and are one of the components that make up atoms.

Two Videos of Thompson’s Discovery

• http://www.youtube.com/watch?v=XU8nMKkzbT8

• http://www.youtube.com/watch?v=IdTxGJjA4Jw

Thompson Cont…

• His discovery led him to propose the “plum pudding” or “muffin model” of the atom,

• This states that an atom is composed of negative charges (electrons) evenly distributed on positive matter that makes up the rest of the atom.

Ernest Rutherford - 1911• He discovered the

Nucleus of an Atom in an experiment known as the “gold foil alpha particle experiment”.

• http://www.youtube.com/watch?v=5pZj0u_XMbc&feature=related

• http://www.youtube.com/watch?v=wzALbzTdnc8&feature=related

Rutherford Cont…

• Through this experiment he discovered the nucleus.

• Because the Alpha particles are positive, some went through the atom (passing the negative electrons).

• A small fraction either bounced back, or bent as they went past.

Rutherford• He observed that this small dense area in the

centre of an Atom was positively charged and called it the Nucleus

• The negative charges (electrons) exist around the nucleus in a much larger space

• He also proposed the idea of a Neutron when he noticed the weights of the atom of gold did not equal its 79 protons and 79 electrons. There was missing mass. He concluded this missing mass must be neutral. James Chadwick would be the first person to prove this in 1932.

Niels Bohr - 1913

• He proposed the planetary model of the atom.

Bohr• Proposed that electrons traveled in fixed paths

around the nucleus• The atom has a tiny dense central core called the

nucleus, where positive and neutral charges are held

• The remainder of the atom is empty space• Electrons travel in orbits around the nucleus.

There are up to seven different levels of orbit within an atom.

• Electrons always travel in the lowest orbit in which there is space

• The electrons furthest from the nucleus play a role in chemical bonding.

Orbits and Electrons!

Level How Many Electrons it can hold

1 2

2 8

3 8

4 32

5 32

6 18

7 8

Current Atomic Model • In 1920s Werner Heisenberg and Erwin Schrodinger argued that it is impossible to know where an electron is at any given time. •The Electron Cloud Theory was developed as a result of their discoveries in quantum mechanics•The electron cloud model says that we can't know exactly where an electron is, but the electrons are more likely to be in specific areas of an atom.

Electron Cloud Model • An electron tends to exist more frequently in certain regions around the nucleus than others depending on its energy level. • These regions are referred as its atomic orbital. • Orbitals come in a variety of shapes and sizes: from the simple sphere of the full helium orbital, to the dumbbell shape of the full neon orbital.

Atomic Theory

http://videos.howstuffworks.com/discovery/29430-assignment-discovery-origin-of-the-atom-theory-video.htm

• Atom – The basic building block of all matter.

• Most of the mass is in the central nucleus made of protons and neutrons, and is surrounded by a cloud of electrons.

• An atom is classified according to the number of protons and neutrons in its nucleus. For Example Hydrogen has one proton and has the atomic number of 1 on the periodic table.

The Atom Song

http://www.youtube.com/watch?v=vUzTQWn-wfE

Now You TRY!• Using Bohr’s Diagram, draw 8 models of the

atom from the first 18 elements (first three periods) – showing how many protons are in the nucleus, how many orbital rings it has, and place the electrons on their respectful rings.

• Remember – the Atomic Number equals the number of protons and the number of protons equals the number of electrons – EX: Hydrogen has the Atomic Number 1 and has 1 proton and 1 electron.

• See page 562-563 for periodic table