Models of the Atom a Historical Perspective

Ancient Greeks were the first to come up with the idea of atoms. Democritus suggested that all matter was made of tiny indivisible particles called atoms. (Greek “atomos”) This led to the idea of atoms in a void. Democritus

What is an atom? Definition: An atom is the smallest particle of an element that still retains the identity and properties of the element. The average size of an atom is 10-10 m in diameter

Early Greek Theories Aristotle fire air water earth 350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. Aristotle was wrong. However, his theory persisted for 2000 years.

ATOMS? What’s that? OFF with your HEAD! In the dark ages, the idea of atoms was frowned upon. Not much progress was made. ATOMS? What’s that? OFF with your HEAD!

In the early 1800’s, John Dalton came up with the ATOMIC THEORY. (1800-1900)  

Dalton’s Atomic Theory All matter is made of atoms. Atoms cannot be created nor destroyed into smaller particles. The atoms of one element cannot be converted into the atoms of another element. 3) All the atoms of one element have the same properties (mass and size). These properties are different from atoms of another element. 4) Atoms of different elements combine in specific proportions to form compounds.

Ha Ha! You can’t break me!!!! An indestructible “Dalton” atom

J.J. Thomson's Experiments “Plum pudding” model (1900) Materials, when rubbed, can develop a charge difference. This electricity is called “cathode rays” when passed through an evacuated tube. These rays have a small mass and are negative. Thomson noted that these negative subatomic particles were a fundamental part of all atoms. Later, “e” was found and the mass of an electron was found to be 9.11 × 10-28 g (much lighter than H)

Thomson’s Plum Pudding Model

Ernest Rutherford

Rutherford’s Scattering Experiment

Ernest Rutherford (movie: 10 min.) Rutherford shot alpha () particles at gold foil. ZnS screen Thin gold foil Lead block Radioactive substance path of invisible -particles Most particles passed through. So, atoms are mostly empty. Some positive -particles deflected or bounced back! Thus, a “nucleus” is positive & holds most of an atom’s mass.

An Interactive Model of Rutherford’s Gold Foil Experiment http://micro.magnet.fsu.edu/electromag/java/rutherford Click here Rutherford Gold Foil Experiment - Backstage Science http://www.youtube.com/watch?v=XBqHkraf8iE

What did most of the particles shot at the gold foil do? Most of the particles traveled straight through the gold foil What was the surprising behavior of a few of the particles? A few of the particles were deflected and some even bounced back

Rutherford’s Revised Atomic Theory (1911) Result: Most of the positively charged particles went straight through the gold foil. Atomic Theory: Most of the matter of the atom is found in a very small part of the atom. This is called the nucleus of the atom. It is very tiny and extremely dense. Result: Some of the positively charged particles were deflected or even bounced back. Atomic Theory: Like charges repel so the nucleus must have a positive charge. If electrons have a negative charge they could not be in a positively charged nucleus. Electrons must surround the nucleus at a distance. Result: The diameter of the nucleus is 100,000 times smaller than the diameter of the entire gold atom. Atomic Theory: Atoms are mostly empty space with a tiny, massive nucleus at the center .

Why is the head of a pin compared to the diameter of a stadium like an atom? The diameter of a pinhead is 100,000 times smaller than the diameter of a stadium. Likewise the diameter of the nucleus of an atom is 100,000 times smaller than the diameter of an atom.

Rutherford could not explain why the electron didn’t fall into the nucleus and destroy the atom. ?

Neils Bohr I think I can help! Electrons can be bumped up to a higher shell if hit by an electron or a photon of light.

Each “jump” would give off light of a particular wavelength or colour Each “jump” would give off light of a particular wavelength or colour. This gave rise to hydrogen’s spectrum.

According to Bohr, each energy “level” corresponded to a different “orbit” of an electron around the atom. (Like planets around the sun.) I’m so SMART & Handsome!!

The Bohr Model of the Atom What did Bohr learn about electron movement? Bohr proposed that electrons move in paths at certain distances around the nucleus. Electrons can jump from a path on one level to a path on another level.

Bohr's Atom: Quantum Behavior in Hydrogen (ONLINE) http://www.colorado.edu/physics/2000/quantumzone/bohr.html SPECTRAL LINES: (ONLINE) http://www.colorado.edu/physics/2000/quantumzone/index.html

How can bookshelves help you understand the movement of electrons? Each shelf represents an energy level Each book represents an electron You can move a book to a higher or lower shelf with the correct amount of energy. A book cannot be between shelves (An electron can move by gaining or losing energy but can never be between energy levels)

Element 107 on the Periodic Table is called Bohrium (Bh)

In the 1920’s things changed! Although Bohr’s idea of energy levels was still accepted, his idea of planetary orbits for electrons was rejected! REJECT ! !

The Modern Theory of the Atom Electrons travel in regions called “electron clouds” You cannot predict exactly where an electron will be found http://www.fearofphysics.com/Atom/atom3.html

How small are atoms? THERE ARE 2 X 1022 ATOMS IN A PENNY. If all the atoms in a penny were blown up to the size of a grain of sand they would cover the entire state of California!