Atom Models. A History of Atomic Theory. (by Dr. Bob Drake)

Transcription

1 Atom Models A History of Atomic Theory (by Dr. Bob Drake)

2 What is a model? MODEL: A detailed, three-dimensional representation of an object, typically on a smaller scale than the original. model airplane model car clothing model

3 It Started with the Greeks DEMOCRITUS Lived in Greece 2500 years ago. Father of modern science He said: All matter is made of atoms that are tiny, indestructible and indivisible.

4 Democritus Greek banknote with Democritus and drawing of atom Where did Democritus get his ideas for small, indestructible, indivisible atoms?

5 Atoms are small new wedding rings old wedding ring EVIDENCE: Old wedding rings get worn away slowly, and get thinner and thinner, but you never see the gold atoms on your finger, so gold atoms must be very small!

6 Atoms are indestructible EVIDENCE: The Earth has been around a long time. Even though mountains get washed away, the rocks continue to exist. New plants grow where old plants die. Atoms are indivisible EVIDENCE: None. This was a HYPOTHESIS made by Democritus to explain nature as he saw it.

7 What did Democritus think atoms looked like? Appearance of atoms assumed based on their behavior. Liquids pour. Solids are hard and rigid.

8 Liquids pour. Why? Liquids require a container. If liquid atoms were like little balls they would roll out when you tip the container. Greek idea of liquid atom

9 Solids are rigid. Why? cocklebur plant cockleburs stuck on clothing and each other little hooks on cockleburs Velcro fastener Velcro hooks and loops. Greek idea of solid atom.

10 Aristotle had other ideas Greek banknote and coin picturing Aristotle ARISTOTLE: Famous Greek philosopher, born 384 BCE. He was a student of Plato (another famous philosopher) and the teacher of Alexander the Great, who later conquered the world.

11 Aristotle s idea of matter Aristotle did not believe that Democritus s idea of atoms was correct. He believed all matter was made from four elements: Earth Air Fire Water

12 Aristotle s idea of matter Aristotle was more famous than Democritus, so people believed him, even though he was wrong. So Democritus s idea of atomos (atoms) was lost for nearly 2000 years until John Dalton brought it back in 1803.

13 Atom idea lost for ~2000 years

14 John Dalton, New Atom,1803 Born in England, Studied chemistry, physics, and color blindness. Brought back Democritus s idea of an indivisible atom.

15 Color blindness Can you see a number in this gray box? If not, you may be color blind. (More boys are color blind than girls.)

16 Dalton s Theory of Atoms Five parts to Dalton s modern atomic theory: 1.Elements are made of extremely small particles called atoms. 1. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements are different in size, mass, and other properties. 3.Atoms cannot be subdivided, created, or destroyed. (Later this was proved wrong.)

17 Dalton s Theory of Atoms 4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. (H 2 O, 2:1 ratio) 5. In chemical reactions, atoms are combined, separated or rearranged. 2H 2 O + 2Na 2NaOH + H 2

18 Dalton s Chemical Symbols We use different symbols today.

19 Matter is electrical! Michael Faraday, English chemist, born In 1834 he learned that matter would interact with electricity. He realized that electricity had to me made up of particles that could be counted, but he did not know what they were. (They were electrons.)

20 J.J. Thomson discovers Electrons English physicist, born 1856, won Nobel Prize in Discovered the electron in 1897 by studying cathode rays using a Crooke s tube, meaning that atoms were divisible. New model of atom: plum pudding model.

21 Crooke s tube cathode cross-shaped anode cross-shaped shadow Cathode rays (a stream of electrons) move from the metal cathode (on the left) to the cross-shaped anode on the right and cast a cross-shaped shadow on the glass on the right. Electrons are particles with a negative charge.

22 What do we mean by charge? ELECTRICAL CHARGE: is a property of matter that expresses the extent to which it has either more or fewer electrons than protons. batteries have (+) and (-) ends ELECTRONS are negatively charged (-1). PROTONS are positively charged (+1). MATTER IS NEUTRAL (no charge) because (+) charges cancel (-) charges.

23 More About Charge Opposite charges attract, alike charges repel (push away).

24 Thomson s Plum pudding atom ENGLISH PLUM PUDDING electron raisin RULE: Matter is NEUTRAL, so for every electron (one negative charge) they must also have a proton (one positive charge) to balance the math to zero charge ( neutral ).

25 Ernest Rutherford Nuclear Atom, 1911 Born in New Zealand, 1871, moved to England, then to Canada. Won Nobel Prize in chemistry in Discovered that most of the mass of an atom is in its nucleus.

26 Rutherford gold foil experiment To test the plum pudding model of the atom, Rutherford sent tiny radioactive particles called a (alpha) particles, like bullets, towards a thin sheet of gold foil, expecting that all would pass through it. Most did, but some bounced in other directions, which was unexpected! So he had to invent a new atom model.

27 Rutherford gold foil experiment Rutherford expected the a particles to pass straight through, like this But what he got was most a particles went straight through, but a few were deflected, like this

28 Rutherford s Nuclear Atom Model The scattering results of his experiment meant that the plum pudding model of the atom was INCORRECT, so he had to think of a new model that fit what he saw. He concluded that all atoms have a positive (+) nucleus that contains most of the mass of the atom, and that most of the atom is empty space except for the very tiny ( ) electrons. electrons ( ) / nucleus / [protons (+)] The NUCLEAR atom model

29 Problem with Rutherford s Model But there also was a problem with Rutherford s nuclear atom! To prevent the negative (-) electrons from being attracted to the positive (+) nucleus, the electrons would have to orbit the nucleus like the Earth orbits the sun. But electrons need energy to orbit the nucleus, and these is no source of energy to do this. Every Rutherford atom would die, Real atoms do not die.

30 Bohr s Atom Model, 1915 Niels Bohr was born in Denmark, Created quantized atom model, Won a Nobel Prize in In Bohr s model, electrons can only move between energy levels in the atom, emitting energy when they jump from higher to lower levels, absorbing energy when they jump from lower to higher levels.

31 Bohr s Quantized Energy Levels Bohr s new atomic model had quantized energy levels, meaning the electrons could only move by jumping between levels (numbered n = 1, n = 2, n = 3, etc.). They absorb energy when they jump out, and emit (send out) energy when they jump in.

32 Erwin Schrödinger Wave Model Erwin Schrödinger was born in Vienna, Austria, in He won the Nobel Prize in physics in He calculated the wave model of a hydrogen atom in 1926 (also called the cloud model of the atom), the atom model we still use today.

33 Wave/Cloud Model of the Atom In Schrödinger s wave model of the atom, an electron behaves as an energy wave as well as a matter particle. (Light also behaves as a particle and a wave.) Einstein had predicted that energy and matter were related in his equation E = mc 2. If we could see an electron it might look like this cloud.

34 A Missing Particle The Neutron James Chadwick discovered the last major piece of the atom when he discovered the neutron, which has no charge ( neutral ), in He was born in England, 1891, and won the 1935 Nobel Prize in physics. A neutron weighs about as much as a proton. (An electron is much smaller.)

35 The Complete Modern Atom An atom is mostly empty space with a nucleus containing protons and neutrons which contain most of the mass of an atom. Electrons are in energy levels around the nucleus. Electrons jump between levels, emitting and absorbing energy as they jump.

36 Comparison of Atom Models Atom Model Indivisible Has Electron Has Nucleus Orbits/Or bitals Electron Cloud Democritus X Dalton X Thomson X Rutherford X X X Bohr X X X Wave X X X