Atomic Structure Timeline History of Atomic Theory

Essential Questions What does it mean when my science teacher says stuff cannot be created or destroyed? What is matter made up of? And who figured it out?

Evolution of Modern Atomic Theory

Democritus (400 B.C.) Proposed that matter was composed of tiny indivisible particles Not based on experimental data Greek: atomos

The Early Atom Aristotle succeeded Democritus and did not believe in atoms. Instead, he thought that all matter was continuous. It was his theory that was accepted for the next 2000 years.

Alchemy (next 2000 years) Mixture of science and mysticism. Lab procedures were developed, but alchemists did not perform controlled experiments like true scientists.

Lavoisier 1777 French chemist, who is considered the founder of modern chemistry. Lavoisier clarified the concept of an element as a simple substance that could not be broken down by any known method of chemical analysis He devised the Law of Conservation of Mass

Basic Laws of Matter Law of Conservation of Mass- mass is neither created nor destroyed during ordinary chemical reactions or physical changes. CH4 + 2O2 → 2H2O + CO2 16g + 64g → 36g + 44g Antoine Lavoisier stated this about 1785

Alka Seltzer in Water Ziploc bag Alka seltzer tablet Water Using the reaction between the tablet and the water, prove that the Law of Conservation of matter is true.

Antoine Lavoisier and his wife, Marie-Anne "It took them only an instant to cut off that head, and a hundred years may not produce another like it." Joseph-Louis Lagrange

Proust (1794) Proust stated the Law of Definite Proportions. The law states that the ratio of elements in a compound is always the same.    

Basic Laws of Matter Law of Definite Proportions – no matter how much salt you have, it is always 39.34% Na and 60.66% Cl by mass. Example: Sodium chloride always contains 39.34% Na and 60.66% Cl by mass. 2NaCl → 2Na + Cl2 100g → 39.34g + 60.66g 116.88g → ? + ? Joseph Louis Proust stated this in 1794.

I was a school teacher at the age of 12! John Dalton (1808) Dalton add his own - the Law of multiple proportions He proposed a theory to summarize and explain these three laws I was a school teacher at the age of 12!

John Dalton proposed this in 1803. Basic Laws of Matter Law of Multiple Proportions- Two or more elements can combine to form different compounds in whole-number ratios. Example John Dalton proposed this in 1803.

John Dalton Dalton’s Four Postulates 1. Elements are composed of small indivisible particles called atoms. 2. Atoms of the same element are identical. Atoms of different elements are different. 3. Atoms of different elements combine together in simple whole number ratios to create compounds. 4. In a chemical reaction, atoms are rearranged, but not changed.

John Dalton’s Elements

Was Dalton right? Elements are composed of small indivisible particles called atoms. Subatomic particles – electrons, protons, neutrons, and more Atoms of the same element are identical. Atoms of different elements are different. No, isotopes are atoms that have the same number of protons but a different number of neutrons

Was Dalton right? Atoms of different elements combine together in simple whole number ratios to create a compound. Yes! He was right! In a chemical reaction, atoms are rearranged, but not changed. Yes! He was right!

Essential Questions So, Dalton figured out the atom, how did we figure out what the inside of an atom looks like?

How Big (Small) Is the Nucleus of an Atom? If the total atom was the size of a football field, then the nucleus would be the size of an apple in the middle of the field.

The width of a human hair = 1,000,000 carbon atoms across How Small is an Atom? The width of a human hair = 1,000,000 carbon atoms across One bucket full of water contains more atoms than there are bucketfuls of water in the Atlantic ocean

Discovery of the Electron In the late 1870’s many experiments were performed in which electric current was passed through gases at low pressures due to the fact that gases at atmospheric pressure don’t conduct electricity well. These experiments were carried out in glass tubes called cathode-ray tubes or Crookes tubes for the man that developed them.

Crookes Tube CRT

J. J. Thomson (1903) Cathode Ray Tube Experiments beam of negative particles Discovered particles smaller than the atom!

J.J. Thomson (1903) Proved that the atom was divisible and that all atoms contain electrons. This contradicted Dalton’s Atomic Theory. This allowed a new model of the atom.

J. J. Thomson (1903) Plum-pudding Model positive sphere (pudding) with negative electrons (plums) dispersed throughout http://www.youtube.com/watch?v=GzMh4q-2HjM

Robert Millikan (1910’s) American physicist Determined the mass and charge of an electron Oil drop experiment http://www.youtube.com/watch?v=XMfYHag7Liw

Oil Drop Experiment Millikan dropped negatively charged microscopic oil particles into a chamber containing metallic plates and viewed them with a microscope. By applying voltage to the metallic plates, Millikan created an electric field. He was able to suspend the oil droplets by adjusting the electric field to the appropriate strength and direction to overcome gravity.

Oil Drop Experiment Knowing the mass of the droplets and the strength of the electric field necessary to suspend them, he was able to calculate the charge of the electron. He noticed that the charge was always a whole-number multiple of 1.602 X10-19 Coulombs.

Ernest Rutherford (1911) Gold Foil Experiment Discovered the nucleus dense, positive charge in the center of the atom Nuclear Model

Gold Foil Experiment

Gold Foil Experiment As expected, most of the alpha particles passed straight through with little or no deflection. However, 1/8000 of the positively charged alpha particles were deflected, some back at the source.

Gold Foil Experiment

Gold Foil Experiment From this experiment, Rutherford discovered that there must be a very densely packed positively charged bundle of matter within the atom which caused the deflections. He called this positive bundle the nucleus. He tried this experiment with other metals and found the same results.

Gold Foil Experiment The volume of the nucleus was very small compared to the volume of the atom. Therefore, most of the atom was composed of empty space.

“It was about as believable as if you had fired a 15-inch shell at a piece of tissue paper, and it came back and hit you.” -Ernest Rutherford

Ernest Rutherford (1911) Nuclear Model http://www.youtube.com/watch?v=XBqHkraf8iE

Evolution of Modern Atomic Theory Almost there! - - - - - - - + -

Niels Bohr (1913) Met with J.J. Thomson but didn’t impress him Worked with Rutherford and liked his model of the atom Incorporated idea of quantum mechanics into the Rutherford model

Niels Bohr (1913) Introduced the idea of electrons traveling around the nucleus in orbits or energy levels. Like the planets, there are multiple orbits like concentric circles around the nucleus Planetary Model

Niels Bohr (1913) He used Bright-Line Spectrum tried to explain presence of specific colors in hydrogen’s spectrum Built upon Max Planck’s concepts of quantized energy

Erwin Schrödinger (1926) Treats electrons as waves Tells us the probability of finding an electron at any given location at any given moment Electron cloud model Atomic orbital: region around the nucleus where electrons are likely to be found

Electron Cloud Model (orbital) Erwin Schrödinger (1926) Electron Cloud Model (orbital) dots represent probability of finding an e- not actual electrons

James Chadwick (1932) Studied under Rutherford and built upon his model Discovered neutrons neutral particles in the nucleus of an atom

He projected alpha particles (from a decaying polonium) towards beryllium target. The particles released are uncharged and have the same mass as protons. He called them neutrons.

revision of Rutherford’s Nuclear Model James Chadwick (1932) Neutron Model revision of Rutherford’s Nuclear Model

Models of the Atom Timeline Let’s recap with a video (10:52) http://www.youtube.com/watch?v=NSAgLvKOPLQ

Dogs teaching Atoms http://www.youtube.com/watch?v=_M9khs87xQ8&list=FLTWKpM0RNaHUX5DRoXcf7qw&index=2